Balance Calibration Verification question

J

JodiB

I've searched the existing threads and don't have a clear handle on this yet. Am hoping someone can talk me through it... We are a test lab (mostly water samples but some small metal coupons and millipores also) for internal customers....Not accred to ISO17025 yet but trying to get there.

1) Not sure which Class of weights we should get for verifying the calibration of our balances. If our balance is accurate to ex. .001 gr , but we routinely weigh items that only require accuracy to the .01 gr, can we get by with Class 4 weights for the verification step? or must we step it up to Class 2 or 3? Is this a trick question? We don't require great accuracy with our measurements, we can be +/- 10%. Are the weights supposed to be as "precise" as the balance they are being used on, or can we select a class that provides "enough" accuracy for our needs?

2) After getting our weights, and assuming our balance has a current calibration sticker, are we then supposed to determine our measurement of uncertainty?

3) I've read a procedure that calls for a Drift Check each day at "one level within the range of operation".... what does "one level within the range of operation" mean? We typically weigh samples between 5-10 grams. If we were to perform a Drift Check as described here, what weight(s) would we be using?

4) We recently moved some of our labs to new buildings. Do we need to have our balances re-calibrated by 3rd party? We haven't noticed any strange readings. Can we just wait until their annual due date?
 

Stijloor

Leader
Super Moderator
Re: Questions about Balance Calibration Verification

A Quick Bump!

Can someone help Jodi?

Thank you very much!!
 
G

Gmzita

Re: Questions about Balance Calibration Verification

Hi Jodi -

You may be able to find some answers on NIST's website - NIST.gov and do a search for the 2012 Edition of NIST Handbook 44; Section 2 of the Handbook covers the calibration of Scales/Balances. As an accredited calibration vendor, this is what our Procedure is based on.

Hope this helps.

Geralyn
 
G

Gmzita

Re: Questions about Balance Calibration Verification

Thanks Chance - Because I am new - the forum doesn't let me post links.
 
J

JodiB

Re: Questions about Balance Calibration Verification

Thanks y'all, I'll take a look!
 
J

jimatsntc

Re: Questions about Balance Calibration Verification

Hi jodi-

if you planning to have ISO/IEC 17025 in the futre, i suggest to practice what is acceptable.. try to find UKAS LAB 14 calibration of weighing machine.... you need to consider not only the resolution of your balance but also the capacity..
Hope this helps.
 
J

JodiB

Re: Questions about Balance Calibration Verification

Thank you Jimatsntc! I wasn't aware of the UKAS site, and their guidance docs look very helpful. I've downloaded two of them so far and will probably download more.
 
P

Pezikon

Re: Questions about Balance Calibration Verification

I imagine at this point you have already found your answer but I'll give my input anyways.

1) To calibrate your 0.001g scale using class 4 weights would result in a "limited" calibration status. This happens any time something is calibrated to a spec worse than the manufacturer's warranted spec. Such items must be labeled conspicuously indicating what about the scale makes it a less than normal calibration. Example: "calibrated to +/-0.01g." This is legitimate even by 17025 standards. When the uncertainty is calculated, the reported uncertainty will likely exceed the tolerance of the device.

2) If the scale already has been calibrated 17025, then the calibration cert should already have uncertainties on it. But when the scale becomes due for calibration and assuming your lab and weights are 17025 accredited, you will indeed need to calculate your own measurement uncertainty if you plan to use your own weights.

3) I would interpret "operating range" as the range in which you typically use the scale. Since you said between 5 and 10 grams, I would use a 5 and/or 10 gram weight for your "drift check."

4) It is said that analytical scales should be calibrated in place where they are to be used and then ideally never moved. But that's more of a recommendation than a requirement. I doubt you will find anything in writing that says more than that. I would only recommend re-calibration in the event that you were suspicious of the measurements. If you are performing the drift check every day and maybe even performing the auto self-cal once in a while (if installed) and you still remain confident in the performance of your scale, I wouldn't worry about it. Don't forget to re-level your scale when you move it.
 
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