Count By Weight Scale - Calibration within specification, but not linear

gpainter

Quite Involved in Discussions
#1
We have a count by weight scale and had it calibrated. It was within specifications, but they said it was not linear. I am not that in-tune with calibration and would like to know what they meant by their statement in relation to the scale not being linear? Thanks in advance!
 
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BradM

Staff member
Admin
#2
Re: Count By Weight Scale

Without seeing exactly what they reported on, I'll just take a stab at it.

Say you have a device with a calibrated range of 0-1000, with a tolerance of 1. at 0, the error is measured at .4, at 200, .38, at 500, .2, at 800, .4, etc. I am within tolerance, but my error has no linear characteristics. The error reflects a non-linear curve. This is especially an issue with instruments that should reflect a linear scale.

While most instruments employ a zero/span type adjustment setup, when you adjust at low and high, it linearizes the new settings. Thus, you have confidence that you instrument will operate on a point other than the exact point you calibrated at. In the above example, while my instrument is in calibration, what can I reasonable assume is my error at 940? What about 593?

Just something you might want to keep an eye on.
 

gpainter

Quite Involved in Discussions
#3
Re: Count By Weight Scale

Brad
So this would explain why at the following points that my count was off
100 we had 99
300 we had 298
500 we had 497
700 we had 696
1000 we had 995

So, I will assume that for a more accurate count by weight scale a linear scale should have been bought. This is critical as we send produt KD and must have the correct amount of hardware.
 
C

Craig H.

#4
Re: Count By Weight Scale

If the scale is used to count to the same number of the same item every time, then it might not be an issue. Just calibrate to that one point (and zero). Remember to tare any container used. It might be helpful in this case to maintain a traceable check weight close to the weight of the final count, so that you can quickly verify the accuracy.
 
C

Craig H.

#5
Re: Count By Weight Scale

Brad
So this would explain why at the following points that my count was off
100 we had 99
300 we had 298
500 we had 497
700 we had 696
1000 we had 995

So, I will assume that for a more accurate count by weight scale a linear scale should have been bought. This is critical as we send produt KD and must have the correct amount of hardware.
In this case, yes, it seems it is time to consider a new scale.
 

gpainter

Quite Involved in Discussions
#6
Re: Count By Weight Scale

Craig
We use it for approximately 1oo + items of 100 different weights and it is costly to fed-ex 30-40 bolts over-night. I did consider the variation in weight of each item.
 
Last edited:
C

Craig H.

#7
Re: Count By Weight Scale

Craig
We use it for approximately 1oo + items of 100 different weights. I did consider the variation in weight of each item.
In that case absolutely disregard my workaround. We went through something similar here a few years ago and it really messed up our calibration system, when it did not have to - each scale saw a very limited range of weight, as part of repetitive analysis schemes.
 
J

jfgunn

#8
Re: Count By Weight Scale

I am not sure that your inaccurate counts would be due to the scale being non-linear. Your incorrect counts are actually linear (the error gets worse as your count gets higher).

The error shown in your counts look like one that is more likely due to an error in the average part weight. This error can be caused by one of the following:

1.) Sample size that is too small. ie. setting the scale up by placing two parts on the scale and having it determine the average based upon two pieces that may not be representative.

2.) Placing the wrong number of parts on the scale when performing the sample process. This often happens when you try to use a high sample.

Check with your scale vendor. They should be able to explain all of these better. If they can't let me know and I will find someone in your area who can.
 
C

crendfrey

#9
Greetings gpainter,

By all means check on user error (sample size etc.)

To what specifications are you calibrating?
Manufacturer specs, user ranges…….
Who is doing your calibrations? Is it strictly a calibration lab or do they do repairs?
Sounds to me, and I am no technician, that you may need a new load cell.
A qualified scale technician should be able to narrow down the true problem with the unit. Give them a call……It is cheaper than buying a new scale.
 
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