Calibration of Small Scales

[duff999]

Looking for calibration advice.

We are looking at performing in house calibration for our small scales. We do have a set of weights that we intend to use. How is it best to document the calibration using these set of weights? What sort of documentation should I require from the manufacturer of the weight set we intend to use?

Any help is appreciated, thank you.

 

[QChas]

Make sure your weights are calibrated (we have our set done every 5 years). I've attached a simple sheet we use to document. Systems been in place for 15 years, no problems.

 

[Mike S.]

Hi QChas,

How does what you are doing tell you if the scale was in or out of calibration as-found?

 

[duff999]

Thanks for the info QChas, very helpful

 

[duff999]

Make sure your weights are calibrated (we have our set done every 5 years). I've attached a simple sheet we use to document. Systems been in place for 15 years, no problems.

Another question regarding the weight set. Can these also be calibrated in house, if I am using a scale/balnce that is currently up to date with its cal, or dies the weight set actually need to be sent out for calibration

 

[Mike S.]

I am not a metrologist or calibration expert. Having said that, if the weight set is used to calibrate the scale, then the scale is used to calibrate the weight set, that seems to me like circular calibration and would not be acceptable.

 

[dwperron]

I am not a metrologist or calibration expert. Having said that, if the weight set is used to calibrate the scale, then the scale is used to calibrate the weight set, that seems to me like circular calibration and would not be acceptable.

You are correct. You need to break such a circular calibration by maintaining traceability to an outside organization.
There is also the question of accuracies - if the weight set is good enough to calibrate the scale then the scale is not good enough
to calibrate the weight set.

 

[dwperron]

Looking for calibration advice.

We are looking at performing in house calibration for our small scales. We do have a set of weights that we intend to use. How is it best to document the calibration using these set of weights? What sort of documentation should I require from the manufacturer of the weight set we intend to use?

Any help is appreciated, thank you.

It depends on what you mean by "calibrate" - and this is a touchy question regarding scales.
Most come with a "Calibrate" function, where you set a test weight on it and the scale adjusts itself to that value.

But is that "Calibration"? I would say not.
You have set the scale to measure a single value accurately. You have done nothing to verify that the scale is accurate throughout its full range (linearity), you have done nothing to verify that it reads the same on any point on the pan (cross pan balance), and that it can tare a weight value accurately. You would also be keeping results, determining if they were in or out of tolerance. That would be a "calibration".

Do you really want to "calibrate" these scales, or just perform a daily check on them using a test weight?
Calibrating your own scales means you are setting up all the infrastructure that a calibration lab needs to withstand audits.
Will that cost more to your organization than having someone else calibrate the scales?

 

[Philip B]

We have our scales supplier come in and perform an annual calibration. The cost is reasonable (around £350 for three sets of scales), the calibration is independent, we don't have to faff about with test weights and we get a UKAS accredited calibration certificate.

 

[duff999]

It depends on what you mean by "calibrate" - and this is a touchy question regarding scales.
Most come with a "Calibrate" function, where you set a test weight on it and the scale adjusts itself to that value.

But is that "Calibration"? I would say not.
You have set the scale to measure a single value accurately. You have done nothing to verify that the scale is accurate throughout its full range (linearity), you have done nothing to verify that it reads the same on any point on the pan (cross pan balance), and that it can tare a weight value accurately. You would also be keeping results, determining if they were in or out of tolerance. That would be a "calibration".

Do you really want to "calibrate" these scales, or just perform a daily check on them using a test weight?
Calibrating your own scales means you are setting up all the infrastructure that a calibration lab needs to withstand audits.
Will that cost more to your organization than having someone else calibrate the scales?

Very good points, thank you. It sounds like this weight set would be good for our daily checks, but for our routine calibration (6 mo, year) where would would get a calibration cert, we stick with an outside firm that we are currently using. I thought I would be able to use this set as my primary calibration for my 6 mo. or yearly calibration.