Analytical balance question

[Sean Breheny]

Hi all,

I have a question that I hope someone here can answer - someone who is
familiar with high-accuracy analytical balances.

I recently bought an as-is Scientech SA410 analytical balance on eBay
for about $60. This is a 410g capacity unit with a repeatability to
0.1milligram. It has a wind screen with sliding doors. This model is
still sold and goes for about $3500 USD new.

It did not work when I received it but I discovered that the only
apparent problem was that a TVS device inside was shorted. I replaced
it and it seems to work.

I don't have the proper weights to calibrate it. However, I do have a
150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
balance, I get 150g to within about 2mg. However, removing and
replacing the weight causes a variation of about 0.5milligram up to as
much as 1mg. In other words, the overall calibration seems close if
not very close, but the repeatability is off by a factor of 5 or 10
from the spec. The unit always returns to exactly zero when the weight
is removed, and lower value weights have less variation in repeated
readings. I handle the weight using a piece of sheet plastic to avoid
getting oils from my hand on it, so I don't think that is the reason
for the discrepancy. I also have been operating the unit on a large
granite-slab table we have at work so vibration does not seem to be
affecting it much.

If I place the weight on different parts of the weighing pan, I can
make the reading vary by 10s of milligrams. I am wondering if perhaps
the repeatability problem I am seeing is due to inexact placement of
the weight.

Can anyone tell me if it is normal for weight positioning to make such
a difference? It almost seems like the repeatability spec is rather
meaningless if so. I do notice that the weighing pan seems to have
been modified on this unit: instead of being a concave circular pan as
shown in the manual, it looks like someone attached (welded? glued?) a
flat plate to the top of it.

For my own purposes, its present capabilities are more than enough. I
am toying with the idea, though, of re-selling it, stating that it
does work, hoping to get more money for it. If I were to do this, I
would probably spend the $50 or so to get some more accurate weights
to perform the proper calibration on it.

Thanks,

Sean
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[Cris Wilson]

The manual I have with my SA410 says that the repeatability is 15mg,
so your measurements seem about right to me.

As for the flat plate, mine does have a concave plate on it, and a stainless
steel flat plate that fits on top of that to help with whatever you want the
mass of  (rezeroing required obviously). The plate should lift up easily,
but it is possible that someone glued it in place to keep it from getting
stolen - or they glued a flat plate over an acid hole in the concave one.

The SA410s are not very accurate when get down to milligrams. I don't
trust mine below a gram. If I need something below a gram I use a different
scale. Or as one of my assistants just said, "It's meant for massing marijuana,
not cocaine"


_______________________________________________________________________________
Cris Wilson
Information Resource Consultant
College of Architecture, Arts, & Humanities
Clemson University
Report computer problems to aah_computers@...


-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
Sent: Sunday, September 27, 2009 11:43 PM
To: Microcontroller discussion list - Public.
Subject: [OT] Analytical balance question

Hi all,

I have a question that I hope someone here can answer - someone who is
familiar with high-accuracy analytical balances.

I recently bought an as-is Scientech SA410 analytical balance on eBay
for about $60. This is a 410g capacity unit with a repeatability to
0.1milligram. It has a wind screen with sliding doors. This model is
still sold and goes for about $3500 USD new.

It did not work when I received it but I discovered that the only
apparent problem was that a TVS device inside was shorted. I replaced
it and it seems to work.

I don't have the proper weights to calibrate it. However, I do have a
150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
balance, I get 150g to within about 2mg. However, removing and
replacing the weight causes a variation of about 0.5milligram up to as
much as 1mg. In other words, the overall calibration seems close if
not very close, but the repeatability is off by a factor of 5 or 10
from the spec. The unit always returns to exactly zero when the weight
is removed, and lower value weights have less variation in repeated
readings. I handle the weight using a piece of sheet plastic to avoid
getting oils from my hand on it, so I don't think that is the reason
for the discrepancy. I also have been operating the unit on a large
granite-slab table we have at work so vibration does not seem to be
affecting it much.

If I place the weight on different parts of the weighing pan, I can
make the reading vary by 10s of milligrams. I am wondering if perhaps
the repeatability problem I am seeing is due to inexact placement of
the weight.

Can anyone tell me if it is normal for weight positioning to make such
a difference? It almost seems like the repeatability spec is rather
meaningless if so. I do notice that the weighing pan seems to have
been modified on this unit: instead of being a concave circular pan as
shown in the manual, it looks like someone attached (welded? glued?) a
flat plate to the top of it.

For my own purposes, its present capabilities are more than enough. I
am toying with the idea, though, of re-selling it, stating that it
does work, hoping to get more money for it. If I were to do this, I
would probably spend the $50 or so to get some more accurate weights
to perform the proper calibration on it.

Thanks,

Sean
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[Sean Breheny]

Are you sure you are reading that spec correctly? I just checked my
manual again and it definitely says 0.00015g for the repeatability.
That's 0.15mg (I had said 0.1mg before so I was a little off but not
by much).

Sean

On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:

> The manual I have with my SA410 says that the repeatability is 15mg,
> so your measurements seem about right to me.
>
> As for the flat plate, mine does have a concave plate on it, and a stainless
> steel flat plate that fits on top of that to help with whatever you want the
> mass of  (rezeroing required obviously). The plate should lift up easily,
> but it is possible that someone glued it in place to keep it from getting
> stolen - or they glued a flat plate over an acid hole in the concave one.
>
> The SA410s are not very accurate when get down to milligrams. I don't
> trust mine below a gram. If I need something below a gram I use a different
> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
> not cocaine"
>
>
> _______________________________________________________________________________
> Cris Wilson
> Information Resource Consultant
> College of Architecture, Arts, & Humanities
> Clemson University
> Report computer problems to aah_computers@...
>
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
> Sent: Sunday, September 27, 2009 11:43 PM
> To: Microcontroller discussion list - Public.
> Subject: [OT] Analytical balance question
>
> Hi all,
>
> I have a question that I hope someone here can answer - someone who is
> familiar with high-accuracy analytical balances.
>
> I recently bought an as-is Scientech SA410 analytical balance on eBay
> for about $60. This is a 410g capacity unit with a repeatability to
> 0.1milligram. It has a wind screen with sliding doors. This model is
> still sold and goes for about $3500 USD new.
>
> It did not work when I received it but I discovered that the only
> apparent problem was that a TVS device inside was shorted. I replaced
> it and it seems to work.
>
> I don't have the proper weights to calibrate it. However, I do have a
> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
> balance, I get 150g to within about 2mg. However, removing and
> replacing the weight causes a variation of about 0.5milligram up to as
> much as 1mg. In other words, the overall calibration seems close if
> not very close, but the repeatability is off by a factor of 5 or 10
> from the spec. The unit always returns to exactly zero when the weight
> is removed, and lower value weights have less variation in repeated
> readings. I handle the weight using a piece of sheet plastic to avoid
> getting oils from my hand on it, so I don't think that is the reason
> for the discrepancy. I also have been operating the unit on a large
> granite-slab table we have at work so vibration does not seem to be
> affecting it much.
>
> If I place the weight on different parts of the weighing pan, I can
> make the reading vary by 10s of milligrams. I am wondering if perhaps
> the repeatability problem I am seeing is due to inexact placement of
> the weight.
>
> Can anyone tell me if it is normal for weight positioning to make such
> a difference? It almost seems like the repeatability spec is rather
> meaningless if so. I do notice that the weighing pan seems to have
> been modified on this unit: instead of being a concave circular pan as
> shown in the manual, it looks like someone attached (welded? glued?) a
> flat plate to the top of it.
>
> For my own purposes, its present capabilities are more than enough. I
> am toying with the idea, though, of re-selling it, stating that it
> does work, hoping to get more money for it. If I were to do this, I
> would probably spend the $50 or so to get some more accurate weights
> to perform the proper calibration on it.
>
> Thanks,
>
> Sean
> --
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> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
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>

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[William "Chops" Westfield]

On Sep 28, 2009, at 1:27 PM, Sean Breheny wrote:

>> Can anyone tell me if it is normal for weight positioning to make  
>> such
>> a difference?

It sounds vaguely like the sort of damage that would be caused by  
physically abusing the weighing pan.  "At one time, repeatability was  
0.15mg, but after a couple years of having students slap their samples  
on the pan and then shipping it across the country, that spec is toast."

(one assumes that some pretty careful mechanical design is needed to  
get measurements that are not dependent on position, and that might  
not survive having the flat plate glued to the concave plate, or  
subsequent attempts to remove the flat plate, or ... whatever.  Maybe  
the spec is a marketing number for the sensor and electronics, and  
never was accurate for the full assembly...)

BillW

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[Cris Wilson]

The 0.15 mg is the repeatability. You are correct. I didn't  move
the decimal correctly.  
I still don't trust the sa410s below a gram though. The 5 of them that
I routinely work with swing all over the place with milligrams.

 

________________________________________
From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
Sent: Monday, September 28, 2009 4:27 PM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Analytical balance question

Are you sure you are reading that spec correctly? I just checked my
manual again and it definitely says 0.00015g for the repeatability.
That's 0.15mg (I had said 0.1mg before so I was a little off but not
by much).

Sean

On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:

> The manual I have with my SA410 says that the repeatability is 15mg,
> so your measurements seem about right to me.
>
> As for the flat plate, mine does have a concave plate on it, and a stainless
> steel flat plate that fits on top of that to help with whatever you want the
> mass of  (rezeroing required obviously). The plate should lift up easily,
> but it is possible that someone glued it in place to keep it from getting
> stolen - or they glued a flat plate over an acid hole in the concave one.
>
> The SA410s are not very accurate when get down to milligrams. I don't
> trust mine below a gram. If I need something below a gram I use a different
> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
> not cocaine"
>
>
> _______________________________________________________________________________
> Cris Wilson
> Information Resource Consultant
> College of Architecture, Arts, & Humanities
> Clemson University
> Report computer problems to aah_computers@...
>
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
> Sent: Sunday, September 27, 2009 11:43 PM
> To: Microcontroller discussion list - Public.
> Subject: [OT] Analytical balance question
>
> Hi all,
>
> I have a question that I hope someone here can answer - someone who is
> familiar with high-accuracy analytical balances.
>
> I recently bought an as-is Scientech SA410 analytical balance on eBay
> for about $60. This is a 410g capacity unit with a repeatability to
> 0.1milligram. It has a wind screen with sliding doors. This model is
> still sold and goes for about $3500 USD new.
>
> It did not work when I received it but I discovered that the only
> apparent problem was that a TVS device inside was shorted. I replaced
> it and it seems to work.
>
> I don't have the proper weights to calibrate it. However, I do have a
> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
> balance, I get 150g to within about 2mg. However, removing and
> replacing the weight causes a variation of about 0.5milligram up to as
> much as 1mg. In other words, the overall calibration seems close if
> not very close, but the repeatability is off by a factor of 5 or 10
> from the spec. The unit always returns to exactly zero when the weight
> is removed, and lower value weights have less variation in repeated
> readings. I handle the weight using a piece of sheet plastic to avoid
> getting oils from my hand on it, so I don't think that is the reason
> for the discrepancy. I also have been operating the unit on a large
> granite-slab table we have at work so vibration does not seem to be
> affecting it much.
>
> If I place the weight on different parts of the weighing pan, I can
> make the reading vary by 10s of milligrams. I am wondering if perhaps
> the repeatability problem I am seeing is due to inexact placement of
> the weight.
>
> Can anyone tell me if it is normal for weight positioning to make such
> a difference? It almost seems like the repeatability spec is rather
> meaningless if so. I do notice that the weighing pan seems to have
> been modified on this unit: instead of being a concave circular pan as
> shown in the manual, it looks like someone attached (welded? glued?) a
> flat plate to the top of it.
>
> For my own purposes, its present capabilities are more than enough. I
> am toying with the idea, though, of re-selling it, stating that it
> does work, hoping to get more money for it. If I were to do this, I
> would probably spend the $50 or so to get some more accurate weights
> to perform the proper calibration on it.
>
> Thanks,
>
> Sean
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
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[Sean Breheny]

OK, thanks for the input. Do you have your SA410 on a very solid
table? I found that moving it from an ordinary table to a granite-top
measuring table made a big difference in repeatability and stability.
I think the vibration really affects it. This thing is so sensitive
that if you leave one of the doors open and blow into it from across
the room, you see several digits of the display move when the "wave"
of air reaches it. Also, do you keep it plugged in all the time?

Sean


On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:

> The 0.15 mg is the repeatability. You are correct. I didn't  move
> the decimal correctly.
> I still don't trust the sa410s below a gram though. The 5 of them that
> I routinely work with swing all over the place with milligrams.
>
>
>
> ________________________________________
> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
> Sent: Monday, September 28, 2009 4:27 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> Are you sure you are reading that spec correctly? I just checked my
> manual again and it definitely says 0.00015g for the repeatability.
> That's 0.15mg (I had said 0.1mg before so I was a little off but not
> by much).
>
> Sean
>
> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>> The manual I have with my SA410 says that the repeatability is 15mg,
>> so your measurements seem about right to me.
>>
>> As for the flat plate, mine does have a concave plate on it, and a stainless
>> steel flat plate that fits on top of that to help with whatever you want the
>> mass of  (rezeroing required obviously). The plate should lift up easily,
>> but it is possible that someone glued it in place to keep it from getting
>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>
>> The SA410s are not very accurate when get down to milligrams. I don't
>> trust mine below a gram. If I need something below a gram I use a different
>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>> not cocaine"
>>
>>
>> _______________________________________________________________________________
>> Cris Wilson
>> Information Resource Consultant
>> College of Architecture, Arts, & Humanities
>> Clemson University
>> Report computer problems to aah_computers@...
>>
>>
>> -----Original Message-----
>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>> Sent: Sunday, September 27, 2009 11:43 PM
>> To: Microcontroller discussion list - Public.
>> Subject: [OT] Analytical balance question
>>
>> Hi all,
>>
>> I have a question that I hope someone here can answer - someone who is
>> familiar with high-accuracy analytical balances.
>>
>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>> for about $60. This is a 410g capacity unit with a repeatability to
>> 0.1milligram. It has a wind screen with sliding doors. This model is
>> still sold and goes for about $3500 USD new.
>>
>> It did not work when I received it but I discovered that the only
>> apparent problem was that a TVS device inside was shorted. I replaced
>> it and it seems to work.
>>
>> I don't have the proper weights to calibrate it. However, I do have a
>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>> balance, I get 150g to within about 2mg. However, removing and
>> replacing the weight causes a variation of about 0.5milligram up to as
>> much as 1mg. In other words, the overall calibration seems close if
>> not very close, but the repeatability is off by a factor of 5 or 10
>> from the spec. The unit always returns to exactly zero when the weight
>> is removed, and lower value weights have less variation in repeated
>> readings. I handle the weight using a piece of sheet plastic to avoid
>> getting oils from my hand on it, so I don't think that is the reason
>> for the discrepancy. I also have been operating the unit on a large
>> granite-slab table we have at work so vibration does not seem to be
>> affecting it much.
>>
>> If I place the weight on different parts of the weighing pan, I can
>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>> the repeatability problem I am seeing is due to inexact placement of
>> the weight.
>>
>> Can anyone tell me if it is normal for weight positioning to make such
>> a difference? It almost seems like the repeatability spec is rather
>> meaningless if so. I do notice that the weighing pan seems to have
>> been modified on this unit: instead of being a concave circular pan as
>> shown in the manual, it looks like someone attached (welded? glued?) a
>> flat plate to the top of it.
>>
>> For my own purposes, its present capabilities are more than enough. I
>> am toying with the idea, though, of re-selling it, stating that it
>> does work, hoping to get more money for it. If I were to do this, I
>> would probably spend the $50 or so to get some more accurate weights
>> to perform the proper calibration on it.
>>
>> Thanks,
>>
>> Sean
>> --
>> http://www.piclist.com PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>
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>
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[Jinx-4]

> I found that moving it from an ordinary table to a granite-top
> measuring table made a big difference in repeatability and stability.

Sean, our lab's fine balances were always on stone benches in a
temperature-controlled room and levelled. And treated with kid
gloves too. Dropping anything on the pan was very much frowned
upon. On the lighter steel or wooden benches they were only as
good as the general purpose scales

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[Cris Wilson]

We keep our balances and other sensitive equipment on lab tables.
They are not granite, but they are some kind of thick burn proof, acid
proof, scratch proof, shock proof, minimal vibration carrying, hard black
plastic material. (I can't remember what it is called).

We do keep it plugged in all of the time to a surge protecting power strip;
although, the power strip is more for convenience to turn on/off  several
things.

It is sensitive to wind, temperature, and humidity. We had to move it
away from an air duct because of the air flow and local temperature change
when the AC would come on.  For the humidity problem (I'm in the south east US)
we have just decided not to trust the last digit of resolution - not that I trust it
below a gram anyway.

_______________________________________________________________________________
Cris Wilson
Information Resource Consultant
College of Architecture, Arts, & Humanities
Clemson University
Report computer problems to aah_computers@...

-----Original Message-----
From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
Sent: Tuesday, September 29, 2009 1:32 AM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Analytical balance question

OK, thanks for the input. Do you have your SA410 on a very solid
table? I found that moving it from an ordinary table to a granite-top
measuring table made a big difference in repeatability and stability.
I think the vibration really affects it. This thing is so sensitive
that if you leave one of the doors open and blow into it from across
the room, you see several digits of the display move when the "wave"
of air reaches it. Also, do you keep it plugged in all the time?

Sean


On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:

> The 0.15 mg is the repeatability. You are correct. I didn't  move
> the decimal correctly.
> I still don't trust the sa410s below a gram though. The 5 of them that
> I routinely work with swing all over the place with milligrams.
>
>
>
> ________________________________________
> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
> Sent: Monday, September 28, 2009 4:27 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> Are you sure you are reading that spec correctly? I just checked my
> manual again and it definitely says 0.00015g for the repeatability.
> That's 0.15mg (I had said 0.1mg before so I was a little off but not
> by much).
>
> Sean
>
> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>> The manual I have with my SA410 says that the repeatability is 15mg,
>> so your measurements seem about right to me.
>>
>> As for the flat plate, mine does have a concave plate on it, and a stainless
>> steel flat plate that fits on top of that to help with whatever you want the
>> mass of  (rezeroing required obviously). The plate should lift up easily,
>> but it is possible that someone glued it in place to keep it from getting
>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>
>> The SA410s are not very accurate when get down to milligrams. I don't
>> trust mine below a gram. If I need something below a gram I use a different
>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>> not cocaine"
>>
>>
>> _______________________________________________________________________________
>> Cris Wilson
>> Information Resource Consultant
>> College of Architecture, Arts, & Humanities
>> Clemson University
>> Report computer problems to aah_computers@...
>>
>>
>> -----Original Message-----
>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>> Sent: Sunday, September 27, 2009 11:43 PM
>> To: Microcontroller discussion list - Public.
>> Subject: [OT] Analytical balance question
>>
>> Hi all,
>>
>> I have a question that I hope someone here can answer - someone who is
>> familiar with high-accuracy analytical balances.
>>
>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>> for about $60. This is a 410g capacity unit with a repeatability to
>> 0.1milligram. It has a wind screen with sliding doors. This model is
>> still sold and goes for about $3500 USD new.
>>
>> It did not work when I received it but I discovered that the only
>> apparent problem was that a TVS device inside was shorted. I replaced
>> it and it seems to work.
>>
>> I don't have the proper weights to calibrate it. However, I do have a
>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>> balance, I get 150g to within about 2mg. However, removing and
>> replacing the weight causes a variation of about 0.5milligram up to as
>> much as 1mg. In other words, the overall calibration seems close if
>> not very close, but the repeatability is off by a factor of 5 or 10
>> from the spec. The unit always returns to exactly zero when the weight
>> is removed, and lower value weights have less variation in repeated
>> readings. I handle the weight using a piece of sheet plastic to avoid
>> getting oils from my hand on it, so I don't think that is the reason
>> for the discrepancy. I also have been operating the unit on a large
>> granite-slab table we have at work so vibration does not seem to be
>> affecting it much.
>>
>> If I place the weight on different parts of the weighing pan, I can
>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>> the repeatability problem I am seeing is due to inexact placement of
>> the weight.
>>
>> Can anyone tell me if it is normal for weight positioning to make such
>> a difference? It almost seems like the repeatability spec is rather
>> meaningless if so. I do notice that the weighing pan seems to have
>> been modified on this unit: instead of being a concave circular pan as
>> shown in the manual, it looks like someone attached (welded? glued?) a
>> flat plate to the top of it.
>>
>> For my own purposes, its present capabilities are more than enough. I
>> am toying with the idea, though, of re-selling it, stating that it
>> does work, hoping to get more money for it. If I were to do this, I
>> would probably spend the $50 or so to get some more accurate weights
>> to perform the proper calibration on it.
>>
>> Thanks,
>>
>> Sean
>> --
>> http://www.piclist.com PIC/SX FAQ & list archive
>> View/change your membership options at
>> http://mailman.mit.edu/mailman/listinfo/piclist
>>
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>
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[Sean Breheny]

We all agree that lots of factors can affect it, but it does puzzle me
why you "don't trust it below a gram." To me, that sounds like saying
"I don't trust my car for trips longer than 1 mile"

With my SA410, even though it is in questionable condition at this
point, I was able to weigh one nylon 4-40 screw (about 200mg if I
remember correctly), then weigh a bag's worth of them (100), and when
I divided the total by the individual weight, I obtained the screw
count to within +/- 1 out of 100. To me that says that it is clearly
usable well below 1 gram. My experience so far would be trust all but
the last digit (100s of micrograms).

Sean


On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:

> We keep our balances and other sensitive equipment on lab tables.
> They are not granite, but they are some kind of thick burn proof, acid
> proof, scratch proof, shock proof, minimal vibration carrying, hard black
> plastic material. (I can't remember what it is called).
>
> We do keep it plugged in all of the time to a surge protecting power strip;
> although, the power strip is more for convenience to turn on/off  several
> things.
>
> It is sensitive to wind, temperature, and humidity. We had to move it
> away from an air duct because of the air flow and local temperature change
> when the AC would come on.  For the humidity problem (I'm in the south east US)
> we have just decided not to trust the last digit of resolution - not that I trust it
> below a gram anyway.
>
> _______________________________________________________________________________
> Cris Wilson
> Information Resource Consultant
> College of Architecture, Arts, & Humanities
> Clemson University
> Report computer problems to aah_computers@...
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
> Sent: Tuesday, September 29, 2009 1:32 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> OK, thanks for the input. Do you have your SA410 on a very solid
> table? I found that moving it from an ordinary table to a granite-top
> measuring table made a big difference in repeatability and stability.
> I think the vibration really affects it. This thing is so sensitive
> that if you leave one of the doors open and blow into it from across
> the room, you see several digits of the display move when the "wave"
> of air reaches it. Also, do you keep it plugged in all the time?
>
> Sean
>
>
> On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:
>> The 0.15 mg is the repeatability. You are correct. I didn't  move
>> the decimal correctly.
>> I still don't trust the sa410s below a gram though. The 5 of them that
>> I routinely work with swing all over the place with milligrams.
>>
>>
>>
>> ________________________________________
>> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
>> Sent: Monday, September 28, 2009 4:27 PM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT] Analytical balance question
>>
>> Are you sure you are reading that spec correctly? I just checked my
>> manual again and it definitely says 0.00015g for the repeatability.
>> That's 0.15mg (I had said 0.1mg before so I was a little off but not
>> by much).
>>
>> Sean
>>
>> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>>> The manual I have with my SA410 says that the repeatability is 15mg,
>>> so your measurements seem about right to me.
>>>
>>> As for the flat plate, mine does have a concave plate on it, and a stainless
>>> steel flat plate that fits on top of that to help with whatever you want the
>>> mass of  (rezeroing required obviously). The plate should lift up easily,
>>> but it is possible that someone glued it in place to keep it from getting
>>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>>
>>> The SA410s are not very accurate when get down to milligrams. I don't
>>> trust mine below a gram. If I need something below a gram I use a different
>>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>>> not cocaine"
>>>
>>>
>>> _______________________________________________________________________________
>>> Cris Wilson
>>> Information Resource Consultant
>>> College of Architecture, Arts, & Humanities
>>> Clemson University
>>> Report computer problems to aah_computers@...
>>>
>>>
>>> -----Original Message-----
>>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>>> Sent: Sunday, September 27, 2009 11:43 PM
>>> To: Microcontroller discussion list - Public.
>>> Subject: [OT] Analytical balance question
>>>
>>> Hi all,
>>>
>>> I have a question that I hope someone here can answer - someone who is
>>> familiar with high-accuracy analytical balances.
>>>
>>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>>> for about $60. This is a 410g capacity unit with a repeatability to
>>> 0.1milligram. It has a wind screen with sliding doors. This model is
>>> still sold and goes for about $3500 USD new.
>>>
>>> It did not work when I received it but I discovered that the only
>>> apparent problem was that a TVS device inside was shorted. I replaced
>>> it and it seems to work.
>>>
>>> I don't have the proper weights to calibrate it. However, I do have a
>>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>>> balance, I get 150g to within about 2mg. However, removing and
>>> replacing the weight causes a variation of about 0.5milligram up to as
>>> much as 1mg. In other words, the overall calibration seems close if
>>> not very close, but the repeatability is off by a factor of 5 or 10
>>> from the spec. The unit always returns to exactly zero when the weight
>>> is removed, and lower value weights have less variation in repeated
>>> readings. I handle the weight using a piece of sheet plastic to avoid
>>> getting oils from my hand on it, so I don't think that is the reason
>>> for the discrepancy. I also have been operating the unit on a large
>>> granite-slab table we have at work so vibration does not seem to be
>>> affecting it much.
>>>
>>> If I place the weight on different parts of the weighing pan, I can
>>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>>> the repeatability problem I am seeing is due to inexact placement of
>>> the weight.
>>>
>>> Can anyone tell me if it is normal for weight positioning to make such
>>> a difference? It almost seems like the repeatability spec is rather
>>> meaningless if so. I do notice that the weighing pan seems to have
>>> been modified on this unit: instead of being a concave circular pan as
>>> shown in the manual, it looks like someone attached (welded? glued?) a
>>> flat plate to the top of it.
>>>
>>> For my own purposes, its present capabilities are more than enough. I
>>> am toying with the idea, though, of re-selling it, stating that it
>>> does work, hoping to get more money for it. If I were to do this, I
>>> would probably spend the $50 or so to get some more accurate weights
>>> to perform the proper calibration on it.
>>>
>>> Thanks,
>>>
>>> Sean
>>> --
>>> http://www.piclist.com PIC/SX FAQ & list archive
>>> View/change your membership options at
>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>
>>> --
>>> http://www.piclist.com PIC/SX FAQ & list archive
>>> View/change your membership options at
>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>
>>
>> --
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>> http://mailman.mit.edu/mailman/listinfo/piclist
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>
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[Tamas Rudnai]

Out of curiosity, what are you using that scale for? Are you doing some
chemistry or pharmacy?

Anyway, bit of hijacking the topic, but recently I was thinking of buying
some digital scale of precision of 0.01g for my airplane modelling hobby but
those scales are not in the same level as a 0.15mg precise ones :-) Also I
would like to use that scale for my other hobby which is numismatics so I
could weight my coins.

Any suggestion of a scale which is good enough for these but moderate in
price?

Thanks
Tamas




On Wed, Sep 30, 2009 at 4:56 AM, Sean Breheny <shb7@...> wrote:

> We all agree that lots of factors can affect it, but it does puzzle me
> why you "don't trust it below a gram." To me, that sounds like saying
> "I don't trust my car for trips longer than 1 mile"
>
> With my SA410, even though it is in questionable condition at this
> point, I was able to weigh one nylon 4-40 screw (about 200mg if I
> remember correctly), then weigh a bag's worth of them (100), and when
> I divided the total by the individual weight, I obtained the screw
> count to within +/- 1 out of 100. To me that says that it is clearly
> usable well below 1 gram. My experience so far would be trust all but
> the last digit (100s of micrograms).
>
> Sean
>
>
> On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:
> > We keep our balances and other sensitive equipment on lab tables.
> > They are not granite, but they are some kind of thick burn proof, acid
> > proof, scratch proof, shock proof, minimal vibration carrying, hard black
> > plastic material. (I can't remember what it is called).
> >
> > We do keep it plugged in all of the time to a surge protecting power
> strip;
> > although, the power strip is more for convenience to turn on/off  several
> > things.
> >
> > It is sensitive to wind, temperature, and humidity. We had to move it
> > away from an air duct because of the air flow and local temperature
> change
> > when the AC would come on.  For the humidity problem (I'm in the south
> east US)
> > we have just decided not to trust the last digit of resolution - not that
> I trust it
> > below a gram anyway.
> >
> >
> _______________________________________________________________________________
> > Cris Wilson
> > Information Resource Consultant
> > College of Architecture, Arts, & Humanities
> > Clemson University
> > Report computer problems to aah_computers@...
> >
> > -----Original Message-----
> > From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
> Of Sean Breheny
> > Sent: Tuesday, September 29, 2009 1:32 AM
> > To: Microcontroller discussion list - Public.
> > Subject: Re: [OT] Analytical balance question
> >
> > OK, thanks for the input. Do you have your SA410 on a very solid
> > table? I found that moving it from an ordinary table to a granite-top
> > measuring table made a big difference in repeatability and stability.
> > I think the vibration really affects it. This thing is so sensitive
> > that if you leave one of the doors open and blow into it from across
> > the room, you see several digits of the display move when the "wave"
> > of air reaches it. Also, do you keep it plugged in all the time?
> >
> > Sean
> >
> >
> > On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...>
> wrote:
> >> The 0.15 mg is the repeatability. You are correct. I didn't  move
> >> the decimal correctly.
> >> I still don't trust the sa410s below a gram though. The 5 of them that
> >> I routinely work with swing all over the place with milligrams.
> >>
> >>
> >>
> >> ________________________________________
> >> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of
> Sean Breheny [shb7@...]
> >> Sent: Monday, September 28, 2009 4:27 PM
> >> To: Microcontroller discussion list - Public.
> >> Subject: Re: [OT] Analytical balance question
> >>
> >> Are you sure you are reading that spec correctly? I just checked my
> >> manual again and it definitely says 0.00015g for the repeatability.
> >> That's 0.15mg (I had said 0.1mg before so I was a little off but not
> >> by much).
> >>
> >> Sean
> >>
> >> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...>
> wrote:
> >>> The manual I have with my SA410 says that the repeatability is 15mg,
> >>> so your measurements seem about right to me.
> >>>
> >>> As for the flat plate, mine does have a concave plate on it, and a
> stainless
> >>> steel flat plate that fits on top of that to help with whatever you
> want the
> >>> mass of  (rezeroing required obviously). The plate should lift up
> easily,
> >>> but it is possible that someone glued it in place to keep it from
> getting
> >>> stolen - or they glued a flat plate over an acid hole in the concave
> one.
> >>>
> >>> The SA410s are not very accurate when get down to milligrams. I don't
> >>> trust mine below a gram. If I need something below a gram I use a
> different
> >>> scale. Or as one of my assistants just said, "It's meant for massing
> marijuana,
> >>> not cocaine"
> >>>
> >>>
> >>>
> _______________________________________________________________________________
> >>> Cris Wilson
> >>> Information Resource Consultant
> >>> College of Architecture, Arts, & Humanities
> >>> Clemson University
> >>> Report computer problems to aah_computers@...
> >>>
> >>>
> >>> -----Original Message-----
> >>> From: piclist-bounces@... [mailto:piclist-bounces@...] On
> Behalf Of Sean Breheny
> >>> Sent: Sunday, September 27, 2009 11:43 PM
> >>> To: Microcontroller discussion list - Public.
> >>> Subject: [OT] Analytical balance question
> >>>
> >>> Hi all,
> >>>
> >>> I have a question that I hope someone here can answer - someone who is
> >>> familiar with high-accuracy analytical balances.
> >>>
> >>> I recently bought an as-is Scientech SA410 analytical balance on eBay
> >>> for about $60. This is a 410g capacity unit with a repeatability to
> >>> 0.1milligram. It has a wind screen with sliding doors. This model is
> >>> still sold and goes for about $3500 USD new.
> >>>
> >>> It did not work when I received it but I discovered that the only
> >>> apparent problem was that a TVS device inside was shorted. I replaced
> >>> it and it seems to work.
> >>>
> >>> I don't have the proper weights to calibrate it. However, I do have a
> >>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
> >>> balance, I get 150g to within about 2mg. However, removing and
> >>> replacing the weight causes a variation of about 0.5milligram up to as
> >>> much as 1mg. In other words, the overall calibration seems close if
> >>> not very close, but the repeatability is off by a factor of 5 or 10
> >>> from the spec. The unit always returns to exactly zero when the weight
> >>> is removed, and lower value weights have less variation in repeated
> >>> readings. I handle the weight using a piece of sheet plastic to avoid
> >>> getting oils from my hand on it, so I don't think that is the reason
> >>> for the discrepancy. I also have been operating the unit on a large
> >>> granite-slab table we have at work so vibration does not seem to be
> >>> affecting it much.
> >>>
> >>> If I place the weight on different parts of the weighing pan, I can
> >>> make the reading vary by 10s of milligrams. I am wondering if perhaps
> >>> the repeatability problem I am seeing is due to inexact placement of
> >>> the weight.
> >>>
> >>> Can anyone tell me if it is normal for weight positioning to make such
> >>> a difference? It almost seems like the repeatability spec is rather
> >>> meaningless if so. I do notice that the weighing pan seems to have
> >>> been modified on this unit: instead of being a concave circular pan as
> >>> shown in the manual, it looks like someone attached (welded? glued?) a
> >>> flat plate to the top of it.
> >>>
> >>> For my own purposes, its present capabilities are more than enough. I
> >>> am toying with the idea, though, of re-selling it, stating that it
> >>> does work, hoping to get more money for it. If I were to do this, I
> >>> would probably spend the $50 or so to get some more accurate weights
> >>> to perform the proper calibration on it.
> >>>
> >>> Thanks,
> >>>
> >>> Sean
> >>> --
> >>> http://www.piclist.com PIC/SX FAQ & list archive
> >>> View/change your membership options at
> >>> http://mailman.mit.edu/mailman/listinfo/piclist
> >>>
> >>> --
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> >>> View/change your membership options at
> >>> http://mailman.mit.edu/mailman/listinfo/piclist
> >>>
> >>
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[Cris Wilson]

My mistrust of the SA410s below a gram is based on using them.
When running repeatability checks with a 250g calibration block,
ours usually have an error of +/- 0.75g.  Thus my distrust of them
below a gram.

Out of curiosity, I decided to run a repeatability check this morning
using a 5 mg calibration block. Much to my surprise, the SA410 did
come in at 5.00 mg every time. So then I tried increasing mass calibration
blocks and the inaccuracy got larger with more mass. I haven't curve fit
the data, but it looks roughly like the inaccuracy changes parabolically with
the mass.  

So your trust in the values that the SA410s depends on what you are massing
with the inaccuracy increasing with mass. On the ones I checked this morning
the repeatability was +/- 1.27g at 395g, +/- 0.5mg at 100g, +/-0.2mg at 50g,
and in the expected error range below 50g.

For me, I typically mass things in the 300g range so my data says don't trust it
to one gram.  But the data also shows that I should trust it to one gram below
100 grams.

Results may vary with the phase of the moon and the number of dead fish you wave.


________________________________________
From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
Sent: Tuesday, September 29, 2009 11:56 PM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Analytical balance question

We all agree that lots of factors can affect it, but it does puzzle me
why you "don't trust it below a gram." To me, that sounds like saying
"I don't trust my car for trips longer than 1 mile"

With my SA410, even though it is in questionable condition at this
point, I was able to weigh one nylon 4-40 screw (about 200mg if I
remember correctly), then weigh a bag's worth of them (100), and when
I divided the total by the individual weight, I obtained the screw
count to within +/- 1 out of 100. To me that says that it is clearly
usable well below 1 gram. My experience so far would be trust all but
the last digit (100s of micrograms).

Sean


On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:

> We keep our balances and other sensitive equipment on lab tables.
> They are not granite, but they are some kind of thick burn proof, acid
> proof, scratch proof, shock proof, minimal vibration carrying, hard black
> plastic material. (I can't remember what it is called).
>
> We do keep it plugged in all of the time to a surge protecting power strip;
> although, the power strip is more for convenience to turn on/off  several
> things.
>
> It is sensitive to wind, temperature, and humidity. We had to move it
> away from an air duct because of the air flow and local temperature change
> when the AC would come on.  For the humidity problem (I'm in the south east US)
> we have just decided not to trust the last digit of resolution - not that I trust it
> below a gram anyway.
>
> _______________________________________________________________________________
> Cris Wilson
> Information Resource Consultant
> College of Architecture, Arts, & Humanities
> Clemson University
> Report computer problems to aah_computers@...
>
> -----Original Message-----
> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
> Sent: Tuesday, September 29, 2009 1:32 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> OK, thanks for the input. Do you have your SA410 on a very solid
> table? I found that moving it from an ordinary table to a granite-top
> measuring table made a big difference in repeatability and stability.
> I think the vibration really affects it. This thing is so sensitive
> that if you leave one of the doors open and blow into it from across
> the room, you see several digits of the display move when the "wave"
> of air reaches it. Also, do you keep it plugged in all the time?
>
> Sean
>
>
> On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:
>> The 0.15 mg is the repeatability. You are correct. I didn't  move
>> the decimal correctly.
>> I still don't trust the sa410s below a gram though. The 5 of them that
>> I routinely work with swing all over the place with milligrams.
>>
>>
>>
>> ________________________________________
>> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
>> Sent: Monday, September 28, 2009 4:27 PM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT] Analytical balance question
>>
>> Are you sure you are reading that spec correctly? I just checked my
>> manual again and it definitely says 0.00015g for the repeatability.
>> That's 0.15mg (I had said 0.1mg before so I was a little off but not
>> by much).
>>
>> Sean
>>
>> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>>> The manual I have with my SA410 says that the repeatability is 15mg,
>>> so your measurements seem about right to me.
>>>
>>> As for the flat plate, mine does have a concave plate on it, and a stainless
>>> steel flat plate that fits on top of that to help with whatever you want the
>>> mass of  (rezeroing required obviously). The plate should lift up easily,
>>> but it is possible that someone glued it in place to keep it from getting
>>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>>
>>> The SA410s are not very accurate when get down to milligrams. I don't
>>> trust mine below a gram. If I need something below a gram I use a different
>>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>>> not cocaine"
>>>
>>>
>>> _______________________________________________________________________________
>>> Cris Wilson
>>> Information Resource Consultant
>>> College of Architecture, Arts, & Humanities
>>> Clemson University
>>> Report computer problems to aah_computers@...
>>>
>>>
>>> -----Original Message-----
>>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>>> Sent: Sunday, September 27, 2009 11:43 PM
>>> To: Microcontroller discussion list - Public.
>>> Subject: [OT] Analytical balance question
>>>
>>> Hi all,
>>>
>>> I have a question that I hope someone here can answer - someone who is
>>> familiar with high-accuracy analytical balances.
>>>
>>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>>> for about $60. This is a 410g capacity unit with a repeatability to
>>> 0.1milligram. It has a wind screen with sliding doors. This model is
>>> still sold and goes for about $3500 USD new.
>>>
>>> It did not work when I received it but I discovered that the only
>>> apparent problem was that a TVS device inside was shorted. I replaced
>>> it and it seems to work.
>>>
>>> I don't have the proper weights to calibrate it. However, I do have a
>>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>>> balance, I get 150g to within about 2mg. However, removing and
>>> replacing the weight causes a variation of about 0.5milligram up to as
>>> much as 1mg. In other words, the overall calibration seems close if
>>> not very close, but the repeatability is off by a factor of 5 or 10
>>> from the spec. The unit always returns to exactly zero when the weight
>>> is removed, and lower value weights have less variation in repeated
>>> readings. I handle the weight using a piece of sheet plastic to avoid
>>> getting oils from my hand on it, so I don't think that is the reason
>>> for the discrepancy. I also have been operating the unit on a large
>>> granite-slab table we have at work so vibration does not seem to be
>>> affecting it much.
>>>
>>> If I place the weight on different parts of the weighing pan, I can
>>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>>> the repeatability problem I am seeing is due to inexact placement of
>>> the weight.
>>>
>>> Can anyone tell me if it is normal for weight positioning to make such
>>> a difference? It almost seems like the repeatability spec is rather
>>> meaningless if so. I do notice that the weighing pan seems to have
>>> been modified on this unit: instead of being a concave circular pan as
>>> shown in the manual, it looks like someone attached (welded? glued?) a
>>> flat plate to the top of it.
>>>
>>> For my own purposes, its present capabilities are more than enough. I
>>> am toying with the idea, though, of re-selling it, stating that it
>>> does work, hoping to get more money for it. If I were to do this, I
>>> would probably spend the $50 or so to get some more accurate weights
>>> to perform the proper calibration on it.
>>>
>>> Thanks,
>>>
>>> Sean
>>> --
>>> http://www.piclist.com PIC/SX FAQ & list archive
>>> View/change your membership options at
>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>
>>> --
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>>> View/change your membership options at
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>>>
>>
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[Cris Wilson]

For me, the SA410s we use are for biological tissue samples, usually around 300 g,
and a gram here or there doesn't really affect the outcome. For below a gram, or when
we need to be precise, we have a nice expensive Mettler in the building.

Your choice of scales depends on what kind of error you can live with. I've worked
with some expensive scales that had an obnoxious error fit curve and I've worked with
so cheap scales that surprisingly had a nice linear error. The rule of thumb is to use a
scale with at least one more digit of precision than you need and set up an error curve
for the device.
_______________________________________
From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Tamas Rudnai [tamas.rudnai@...]
Sent: Wednesday, September 30, 2009 7:54 AM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Analytical balance question

Out of curiosity, what are you using that scale for? Are you doing some
chemistry or pharmacy?

Anyway, bit of hijacking the topic, but recently I was thinking of buying
some digital scale of precision of 0.01g for my airplane modelling hobby but
those scales are not in the same level as a 0.15mg precise ones :-) Also I
would like to use that scale for my other hobby which is numismatics so I
could weight my coins.

Any suggestion of a scale which is good enough for these but moderate in
price?

Thanks
Tamas




On Wed, Sep 30, 2009 at 4:56 AM, Sean Breheny <shb7@...> wrote:

> We all agree that lots of factors can affect it, but it does puzzle me
> why you "don't trust it below a gram." To me, that sounds like saying
> "I don't trust my car for trips longer than 1 mile"
>
> With my SA410, even though it is in questionable condition at this
> point, I was able to weigh one nylon 4-40 screw (about 200mg if I
> remember correctly), then weigh a bag's worth of them (100), and when
> I divided the total by the individual weight, I obtained the screw
> count to within +/- 1 out of 100. To me that says that it is clearly
> usable well below 1 gram. My experience so far would be trust all but
> the last digit (100s of micrograms).
>
> Sean
>
>
> On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:
> > We keep our balances and other sensitive equipment on lab tables.
> > They are not granite, but they are some kind of thick burn proof, acid
> > proof, scratch proof, shock proof, minimal vibration carrying, hard black
> > plastic material. (I can't remember what it is called).
> >
> > We do keep it plugged in all of the time to a surge protecting power
> strip;
> > although, the power strip is more for convenience to turn on/off  several
> > things.
> >
> > It is sensitive to wind, temperature, and humidity. We had to move it
> > away from an air duct because of the air flow and local temperature
> change
> > when the AC would come on.  For the humidity problem (I'm in the south
> east US)
> > we have just decided not to trust the last digit of resolution - not that
> I trust it
> > below a gram anyway.
> >
> >
> _______________________________________________________________________________
> > Cris Wilson
> > Information Resource Consultant
> > College of Architecture, Arts, & Humanities
> > Clemson University
> > Report computer problems to aah_computers@...
> >
> > -----Original Message-----
> > From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf
> Of Sean Breheny
> > Sent: Tuesday, September 29, 2009 1:32 AM
> > To: Microcontroller discussion list - Public.
> > Subject: Re: [OT] Analytical balance question
> >
> > OK, thanks for the input. Do you have your SA410 on a very solid
> > table? I found that moving it from an ordinary table to a granite-top
> > measuring table made a big difference in repeatability and stability.
> > I think the vibration really affects it. This thing is so sensitive
> > that if you leave one of the doors open and blow into it from across
> > the room, you see several digits of the display move when the "wave"
> > of air reaches it. Also, do you keep it plugged in all the time?
> >
> > Sean
> >
> >
> > On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...>
> wrote:
> >> The 0.15 mg is the repeatability. You are correct. I didn't  move
> >> the decimal correctly.
> >> I still don't trust the sa410s below a gram though. The 5 of them that
> >> I routinely work with swing all over the place with milligrams.
> >>
> >>
> >>
> >> ________________________________________
> >> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of
> Sean Breheny [shb7@...]
> >> Sent: Monday, September 28, 2009 4:27 PM
> >> To: Microcontroller discussion list - Public.
> >> Subject: Re: [OT] Analytical balance question
> >>
> >> Are you sure you are reading that spec correctly? I just checked my
> >> manual again and it definitely says 0.00015g for the repeatability.
> >> That's 0.15mg (I had said 0.1mg before so I was a little off but not
> >> by much).
> >>
> >> Sean
> >>
> >> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...>
> wrote:
> >>> The manual I have with my SA410 says that the repeatability is 15mg,
> >>> so your measurements seem about right to me.
> >>>
> >>> As for the flat plate, mine does have a concave plate on it, and a
> stainless
> >>> steel flat plate that fits on top of that to help with whatever you
> want the
> >>> mass of  (rezeroing required obviously). The plate should lift up
> easily,
> >>> but it is possible that someone glued it in place to keep it from
> getting
> >>> stolen - or they glued a flat plate over an acid hole in the concave
> one.
> >>>
> >>> The SA410s are not very accurate when get down to milligrams. I don't
> >>> trust mine below a gram. If I need something below a gram I use a
> different
> >>> scale. Or as one of my assistants just said, "It's meant for massing
> marijuana,
> >>> not cocaine"
> >>>
> >>>
> >>>
> _______________________________________________________________________________
> >>> Cris Wilson
> >>> Information Resource Consultant
> >>> College of Architecture, Arts, & Humanities
> >>> Clemson University
> >>> Report computer problems to aah_computers@...
> >>>
> >>>
> >>> -----Original Message-----
> >>> From: piclist-bounces@... [mailto:piclist-bounces@...] On
> Behalf Of Sean Breheny
> >>> Sent: Sunday, September 27, 2009 11:43 PM
> >>> To: Microcontroller discussion list - Public.
> >>> Subject: [OT] Analytical balance question
> >>>
> >>> Hi all,
> >>>
> >>> I have a question that I hope someone here can answer - someone who is
> >>> familiar with high-accuracy analytical balances.
> >>>
> >>> I recently bought an as-is Scientech SA410 analytical balance on eBay
> >>> for about $60. This is a 410g capacity unit with a repeatability to
> >>> 0.1milligram. It has a wind screen with sliding doors. This model is
> >>> still sold and goes for about $3500 USD new.
> >>>
> >>> It did not work when I received it but I discovered that the only
> >>> apparent problem was that a TVS device inside was shorted. I replaced
> >>> it and it seems to work.
> >>>
> >>> I don't have the proper weights to calibrate it. However, I do have a
> >>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
> >>> balance, I get 150g to within about 2mg. However, removing and
> >>> replacing the weight causes a variation of about 0.5milligram up to as
> >>> much as 1mg. In other words, the overall calibration seems close if
> >>> not very close, but the repeatability is off by a factor of 5 or 10
> >>> from the spec. The unit always returns to exactly zero when the weight
> >>> is removed, and lower value weights have less variation in repeated
> >>> readings. I handle the weight using a piece of sheet plastic to avoid
> >>> getting oils from my hand on it, so I don't think that is the reason
> >>> for the discrepancy. I also have been operating the unit on a large
> >>> granite-slab table we have at work so vibration does not seem to be
> >>> affecting it much.
> >>>
> >>> If I place the weight on different parts of the weighing pan, I can
> >>> make the reading vary by 10s of milligrams. I am wondering if perhaps
> >>> the repeatability problem I am seeing is due to inexact placement of
> >>> the weight.
> >>>
> >>> Can anyone tell me if it is normal for weight positioning to make such
> >>> a difference? It almost seems like the repeatability spec is rather
> >>> meaningless if so. I do notice that the weighing pan seems to have
> >>> been modified on this unit: instead of being a concave circular pan as
> >>> shown in the manual, it looks like someone attached (welded? glued?) a
> >>> flat plate to the top of it.
> >>>
> >>> For my own purposes, its present capabilities are more than enough. I
> >>> am toying with the idea, though, of re-selling it, stating that it
> >>> does work, hoping to get more money for it. If I were to do this, I
> >>> would probably spend the $50 or so to get some more accurate weights
> >>> to perform the proper calibration on it.
> >>>
> >>> Thanks,
> >>>
> >>> Sean
> >>> --
> >>> http://www.piclist.com PIC/SX FAQ & list archive
> >>> View/change your membership options at
> >>> http://mailman.mit.edu/mailman/listinfo/piclist
> >>>
> >>> --
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> >>> View/change your membership options at
> >>> http://mailman.mit.edu/mailman/listinfo/piclist
> >>>
> >>
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> >> http://mailman.mit.edu/mailman/listinfo/piclist
> >>
> >
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> > View/change your membership options at
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> >
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> >
>
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[Sean Breheny]

That is very interesting but still puzzling to me. As I said before, I
get about 1 milligram repeatability with a 150g mass. I'm wondering
why mine seems to be so much better than yours?!

As for what I am using it for - nothing, really. I bought it because
the guy was selling it for only $60 and I knew that if it were in
working condition, it would be worth far more than that. I thought it
was likely that I could fix it since it didn't show any external
damage. That's why I am considering re-selling it, but instead of
as-is, offering a 7-day right of return or similar, along with some
proof that it works. I am trying to determine whether it is worth
spending the time and money to establish that it works to the factory
specs. I am also considering just keeping it and using it for weighing
parts for model aircraft, although it is still way overkill for that.

Sean


On Wed, Sep 30, 2009 at 1:30 PM, William Wilson <CRIS@...> wrote:

> My mistrust of the SA410s below a gram is based on using them.
> When running repeatability checks with a 250g calibration block,
> ours usually have an error of +/- 0.75g.  Thus my distrust of them
> below a gram.
>
> Out of curiosity, I decided to run a repeatability check this morning
> using a 5 mg calibration block. Much to my surprise, the SA410 did
> come in at 5.00 mg every time. So then I tried increasing mass calibration
> blocks and the inaccuracy got larger with more mass. I haven't curve fit
> the data, but it looks roughly like the inaccuracy changes parabolically with
> the mass.
>
> So your trust in the values that the SA410s depends on what you are massing
> with the inaccuracy increasing with mass. On the ones I checked this morning
> the repeatability was +/- 1.27g at 395g, +/- 0.5mg at 100g, +/-0.2mg at 50g,
> and in the expected error range below 50g.
>
> For me, I typically mass things in the 300g range so my data says don't trust it
> to one gram.  But the data also shows that I should trust it to one gram below
> 100 grams.
>
> Results may vary with the phase of the moon and the number of dead fish you wave.
>
>
> ________________________________________
> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
> Sent: Tuesday, September 29, 2009 11:56 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> We all agree that lots of factors can affect it, but it does puzzle me
> why you "don't trust it below a gram." To me, that sounds like saying
> "I don't trust my car for trips longer than 1 mile"
>
> With my SA410, even though it is in questionable condition at this
> point, I was able to weigh one nylon 4-40 screw (about 200mg if I
> remember correctly), then weigh a bag's worth of them (100), and when
> I divided the total by the individual weight, I obtained the screw
> count to within +/- 1 out of 100. To me that says that it is clearly
> usable well below 1 gram. My experience so far would be trust all but
> the last digit (100s of micrograms).
>
> Sean
>
>
> On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:
>> We keep our balances and other sensitive equipment on lab tables.
>> They are not granite, but they are some kind of thick burn proof, acid
>> proof, scratch proof, shock proof, minimal vibration carrying, hard black
>> plastic material. (I can't remember what it is called).
>>
>> We do keep it plugged in all of the time to a surge protecting power strip;
>> although, the power strip is more for convenience to turn on/off  several
>> things.
>>
>> It is sensitive to wind, temperature, and humidity. We had to move it
>> away from an air duct because of the air flow and local temperature change
>> when the AC would come on.  For the humidity problem (I'm in the south east US)
>> we have just decided not to trust the last digit of resolution - not that I trust it
>> below a gram anyway.
>>
>> _______________________________________________________________________________
>> Cris Wilson
>> Information Resource Consultant
>> College of Architecture, Arts, & Humanities
>> Clemson University
>> Report computer problems to aah_computers@...
>>
>> -----Original Message-----
>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>> Sent: Tuesday, September 29, 2009 1:32 AM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT] Analytical balance question
>>
>> OK, thanks for the input. Do you have your SA410 on a very solid
>> table? I found that moving it from an ordinary table to a granite-top
>> measuring table made a big difference in repeatability and stability.
>> I think the vibration really affects it. This thing is so sensitive
>> that if you leave one of the doors open and blow into it from across
>> the room, you see several digits of the display move when the "wave"
>> of air reaches it. Also, do you keep it plugged in all the time?
>>
>> Sean
>>
>>
>> On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:
>>> The 0.15 mg is the repeatability. You are correct. I didn't  move
>>> the decimal correctly.
>>> I still don't trust the sa410s below a gram though. The 5 of them that
>>> I routinely work with swing all over the place with milligrams.
>>>
>>>
>>>
>>> ________________________________________
>>> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
>>> Sent: Monday, September 28, 2009 4:27 PM
>>> To: Microcontroller discussion list - Public.
>>> Subject: Re: [OT] Analytical balance question
>>>
>>> Are you sure you are reading that spec correctly? I just checked my
>>> manual again and it definitely says 0.00015g for the repeatability.
>>> That's 0.15mg (I had said 0.1mg before so I was a little off but not
>>> by much).
>>>
>>> Sean
>>>
>>> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>>>> The manual I have with my SA410 says that the repeatability is 15mg,
>>>> so your measurements seem about right to me.
>>>>
>>>> As for the flat plate, mine does have a concave plate on it, and a stainless
>>>> steel flat plate that fits on top of that to help with whatever you want the
>>>> mass of  (rezeroing required obviously). The plate should lift up easily,
>>>> but it is possible that someone glued it in place to keep it from getting
>>>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>>>
>>>> The SA410s are not very accurate when get down to milligrams. I don't
>>>> trust mine below a gram. If I need something below a gram I use a different
>>>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>>>> not cocaine"
>>>>
>>>>
>>>> _______________________________________________________________________________
>>>> Cris Wilson
>>>> Information Resource Consultant
>>>> College of Architecture, Arts, & Humanities
>>>> Clemson University
>>>> Report computer problems to aah_computers@...
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>>>> Sent: Sunday, September 27, 2009 11:43 PM
>>>> To: Microcontroller discussion list - Public.
>>>> Subject: [OT] Analytical balance question
>>>>
>>>> Hi all,
>>>>
>>>> I have a question that I hope someone here can answer - someone who is
>>>> familiar with high-accuracy analytical balances.
>>>>
>>>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>>>> for about $60. This is a 410g capacity unit with a repeatability to
>>>> 0.1milligram. It has a wind screen with sliding doors. This model is
>>>> still sold and goes for about $3500 USD new.
>>>>
>>>> It did not work when I received it but I discovered that the only
>>>> apparent problem was that a TVS device inside was shorted. I replaced
>>>> it and it seems to work.
>>>>
>>>> I don't have the proper weights to calibrate it. However, I do have a
>>>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>>>> balance, I get 150g to within about 2mg. However, removing and
>>>> replacing the weight causes a variation of about 0.5milligram up to as
>>>> much as 1mg. In other words, the overall calibration seems close if
>>>> not very close, but the repeatability is off by a factor of 5 or 10
>>>> from the spec. The unit always returns to exactly zero when the weight
>>>> is removed, and lower value weights have less variation in repeated
>>>> readings. I handle the weight using a piece of sheet plastic to avoid
>>>> getting oils from my hand on it, so I don't think that is the reason
>>>> for the discrepancy. I also have been operating the unit on a large
>>>> granite-slab table we have at work so vibration does not seem to be
>>>> affecting it much.
>>>>
>>>> If I place the weight on different parts of the weighing pan, I can
>>>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>>>> the repeatability problem I am seeing is due to inexact placement of
>>>> the weight.
>>>>
>>>> Can anyone tell me if it is normal for weight positioning to make such
>>>> a difference? It almost seems like the repeatability spec is rather
>>>> meaningless if so. I do notice that the weighing pan seems to have
>>>> been modified on this unit: instead of being a concave circular pan as
>>>> shown in the manual, it looks like someone attached (welded? glued?) a
>>>> flat plate to the top of it.
>>>>
>>>> For my own purposes, its present capabilities are more than enough. I
>>>> am toying with the idea, though, of re-selling it, stating that it
>>>> does work, hoping to get more money for it. If I were to do this, I
>>>> would probably spend the $50 or so to get some more accurate weights
>>>> to perform the proper calibration on it.
>>>>
>>>> Thanks,
>>>>
>>>> Sean
>>>> --
>>>> http://www.piclist.com PIC/SX FAQ & list archive
>>>> View/change your membership options at
>>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>>
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>>>> http://www.piclist.com PIC/SX FAQ & list archive
>>>> View/change your membership options at
>>>> http://mailman.mit.edu/mailman/listinfo/piclist
>>>>
>>>
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>>
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[Cris Wilson]

The difference could be wear and tear on the equipment.
The ones I have came into the lab about 7 years ago from another part
of the university. No here remembers what they were used for before that.
For all I know, they were used in the freshman chemistry labs where things
seem to be constantly broken from misuse.  

________________________________________
From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
Sent: Wednesday, September 30, 2009 2:05 PM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Analytical balance question

That is very interesting but still puzzling to me. As I said before, I
get about 1 milligram repeatability with a 150g mass. I'm wondering
why mine seems to be so much better than yours?!

As for what I am using it for - nothing, really. I bought it because
the guy was selling it for only $60 and I knew that if it were in
working condition, it would be worth far more than that. I thought it
was likely that I could fix it since it didn't show any external
damage. That's why I am considering re-selling it, but instead of
as-is, offering a 7-day right of return or similar, along with some
proof that it works. I am trying to determine whether it is worth
spending the time and money to establish that it works to the factory
specs. I am also considering just keeping it and using it for weighing
parts for model aircraft, although it is still way overkill for that.

Sean


On Wed, Sep 30, 2009 at 1:30 PM, William Wilson <CRIS@...> wrote:

> My mistrust of the SA410s below a gram is based on using them.
> When running repeatability checks with a 250g calibration block,
> ours usually have an error of +/- 0.75g.  Thus my distrust of them
> below a gram.
>
> Out of curiosity, I decided to run a repeatability check this morning
> using a 5 mg calibration block. Much to my surprise, the SA410 did
> come in at 5.00 mg every time. So then I tried increasing mass calibration
> blocks and the inaccuracy got larger with more mass. I haven't curve fit
> the data, but it looks roughly like the inaccuracy changes parabolically with
> the mass.
>
> So your trust in the values that the SA410s depends on what you are massing
> with the inaccuracy increasing with mass. On the ones I checked this morning
> the repeatability was +/- 1.27g at 395g, +/- 0.5mg at 100g, +/-0.2mg at 50g,
> and in the expected error range below 50g.
>
> For me, I typically mass things in the 300g range so my data says don't trust it
> to one gram.  But the data also shows that I should trust it to one gram below
> 100 grams.
>
> Results may vary with the phase of the moon and the number of dead fish you wave.
>
>
> ________________________________________
> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
> Sent: Tuesday, September 29, 2009 11:56 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] Analytical balance question
>
> We all agree that lots of factors can affect it, but it does puzzle me
> why you "don't trust it below a gram." To me, that sounds like saying
> "I don't trust my car for trips longer than 1 mile"
>
> With my SA410, even though it is in questionable condition at this
> point, I was able to weigh one nylon 4-40 screw (about 200mg if I
> remember correctly), then weigh a bag's worth of them (100), and when
> I divided the total by the individual weight, I obtained the screw
> count to within +/- 1 out of 100. To me that says that it is clearly
> usable well below 1 gram. My experience so far would be trust all but
> the last digit (100s of micrograms).
>
> Sean
>
>
> On Tue, Sep 29, 2009 at 2:00 PM, William Wilson <CRIS@...> wrote:
>> We keep our balances and other sensitive equipment on lab tables.
>> They are not granite, but they are some kind of thick burn proof, acid
>> proof, scratch proof, shock proof, minimal vibration carrying, hard black
>> plastic material. (I can't remember what it is called).
>>
>> We do keep it plugged in all of the time to a surge protecting power strip;
>> although, the power strip is more for convenience to turn on/off  several
>> things.
>>
>> It is sensitive to wind, temperature, and humidity. We had to move it
>> away from an air duct because of the air flow and local temperature change
>> when the AC would come on.  For the humidity problem (I'm in the south east US)
>> we have just decided not to trust the last digit of resolution - not that I trust it
>> below a gram anyway.
>>
>> _______________________________________________________________________________
>> Cris Wilson
>> Information Resource Consultant
>> College of Architecture, Arts, & Humanities
>> Clemson University
>> Report computer problems to aah_computers@...
>>
>> -----Original Message-----
>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>> Sent: Tuesday, September 29, 2009 1:32 AM
>> To: Microcontroller discussion list - Public.
>> Subject: Re: [OT] Analytical balance question
>>
>> OK, thanks for the input. Do you have your SA410 on a very solid
>> table? I found that moving it from an ordinary table to a granite-top
>> measuring table made a big difference in repeatability and stability.
>> I think the vibration really affects it. This thing is so sensitive
>> that if you leave one of the doors open and blow into it from across
>> the room, you see several digits of the display move when the "wave"
>> of air reaches it. Also, do you keep it plugged in all the time?
>>
>> Sean
>>
>>
>> On Mon, Sep 28, 2009 at 9:04 PM, William Wilson <CRIS@...> wrote:
>>> The 0.15 mg is the repeatability. You are correct. I didn't  move
>>> the decimal correctly.
>>> I still don't trust the sa410s below a gram though. The 5 of them that
>>> I routinely work with swing all over the place with milligrams.
>>>
>>>
>>>
>>> ________________________________________
>>> From: piclist-bounces@... [piclist-bounces@...] On Behalf Of Sean Breheny [shb7@...]
>>> Sent: Monday, September 28, 2009 4:27 PM
>>> To: Microcontroller discussion list - Public.
>>> Subject: Re: [OT] Analytical balance question
>>>
>>> Are you sure you are reading that spec correctly? I just checked my
>>> manual again and it definitely says 0.00015g for the repeatability.
>>> That's 0.15mg (I had said 0.1mg before so I was a little off but not
>>> by much).
>>>
>>> Sean
>>>
>>> On Mon, Sep 28, 2009 at 2:38 PM, William Wilson <CRIS@...> wrote:
>>>> The manual I have with my SA410 says that the repeatability is 15mg,
>>>> so your measurements seem about right to me.
>>>>
>>>> As for the flat plate, mine does have a concave plate on it, and a stainless
>>>> steel flat plate that fits on top of that to help with whatever you want the
>>>> mass of  (rezeroing required obviously). The plate should lift up easily,
>>>> but it is possible that someone glued it in place to keep it from getting
>>>> stolen - or they glued a flat plate over an acid hole in the concave one.
>>>>
>>>> The SA410s are not very accurate when get down to milligrams. I don't
>>>> trust mine below a gram. If I need something below a gram I use a different
>>>> scale. Or as one of my assistants just said, "It's meant for massing marijuana,
>>>> not cocaine"
>>>>
>>>>
>>>> _______________________________________________________________________________
>>>> Cris Wilson
>>>> Information Resource Consultant
>>>> College of Architecture, Arts, & Humanities
>>>> Clemson University
>>>> Report computer problems to aah_computers@...
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: piclist-bounces@... [mailto:piclist-bounces@...] On Behalf Of Sean Breheny
>>>> Sent: Sunday, September 27, 2009 11:43 PM
>>>> To: Microcontroller discussion list - Public.
>>>> Subject: [OT] Analytical balance question
>>>>
>>>> Hi all,
>>>>
>>>> I have a question that I hope someone here can answer - someone who is
>>>> familiar with high-accuracy analytical balances.
>>>>
>>>> I recently bought an as-is Scientech SA410 analytical balance on eBay
>>>> for about $60. This is a 410g capacity unit with a repeatability to
>>>> 0.1milligram. It has a wind screen with sliding doors. This model is
>>>> still sold and goes for about $3500 USD new.
>>>>
>>>> It did not work when I received it but I discovered that the only
>>>> apparent problem was that a TVS device inside was shorted. I replaced
>>>> it and it seems to work.
>>>>
>>>> I don't have the proper weights to calibrate it. However, I do have a
>>>> 150g weight which is spec'd to +/- 7.5mg. When I weigh this on the
>>>> balance, I get 150g to within about 2mg. However, removing and
>>>> replacing the weight causes a variation of about 0.5milligram up to as
>>>> much as 1mg. In other words, the overall calibration seems close if
>>>> not very close, but the repeatability is off by a factor of 5 or 10
>>>> from the spec. The unit always returns to exactly zero when the weight
>>>> is removed, and lower value weights have less variation in repeated
>>>> readings. I handle the weight using a piece of sheet plastic to avoid
>>>> getting oils from my hand on it, so I don't think that is the reason
>>>> for the discrepancy. I also have been operating the unit on a large
>>>> granite-slab table we have at work so vibration does not seem to be
>>>> affecting it much.
>>>>
>>>> If I place the weight on different parts of the weighing pan, I can
>>>> make the reading vary by 10s of milligrams. I am wondering if perhaps
>>>> the repeatability problem I am seeing is due to inexact placement of
>>>> the weight.
>>>>
>>>> Can anyone tell me if it is normal for weight positioning to make such
>>>> a difference? It almost seems like the repeatability spec is rather
>>>> meaningless if so. I do notice that the weighing pan seems to have
>>>> been modified on this unit: instead of being a concave circular pan as
>>>> shown in the manual, it looks like someone attached (welded? glued?) a
>>>> flat plate to the top of it.
>>>>
>>>> For my own purposes, its present capabilities are more than enough. I
>>>> am toying with the idea, though, of re-selling it, stating that it
>>>> does work, hoping to get more money for it. If I were to do this, I
>>>> would probably spend the $50 or so to get some more accurate weights
>>>> to perform the proper calibration on it.
>>>>
>>>> Thanks,
>>>>
>>>> Sean
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>>>
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[Lee Jones]

> Any suggestion of a scale which is good enough for these
> but moderate in price?

A gun powder / bullet reloading scale.  Mechanical balance beam
units are around US$100 new and give 0.1 grain accuracy (~0.007g).
Examples are RCBS (rcbs.com) model 5-0-2 or 5-0-5 reloading scale.

                                                Lee Jones

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[Alan B. Pearce-2]

Sorry for late reply, now back at work after 9 weeks away, tending to ailing
father.

>OK, thanks for the input. Do you have your SA410 on a very solid
>table? I found that moving it from an ordinary table to a granite-top
>measuring table made a big difference in repeatability and stability.
>I think the vibration really affects it. This thing is so sensitive
>that if you leave one of the doors open and blow into it from across
>the room, you see several digits of the display move when the "wave"
>of air reaches it. Also, do you keep it plugged in all the time?

When I was at secondary school, the chemistry lab had a scale of similar
accuracy, and they had a seperate concrete table that had its own foundation
into the ground to minimize vibration effects from the building.

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