Electronics > Metrology

Shoud we ask the Cal Lab to adjust the multimeter to 24h specs?

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rodpp:
Hi,

When sending a multimeter to calibration we have the option to only check if its measurements are according with its specs, OR we can ask the Cal Lab to make adjustments too, reporting the measurements before and after adjustments.

I read some posts here saying that when the equipment to be calibrated has a history of various calibrations, adjusting it will ruin that history.

For equipments that have to adjust physical trimpots or similar, I understand it. 

But for multimeters calibrated without opening the case, only adjusting software parameters, this statment against adjustments to 24h specs remains valid? If yes, could you explain the reasons considering that every calibration + adjustments will provide a report with the measurements before and after ajustments?

Regards,
Rodrigo. 

aronake:
If you have old calibration history and want to be able to track drift, the possibility to some extent predict drift and adjust for drift out of later measurements you do not want to have it adjusted.

If you have no calibration history, or do not care about being able to track drift, you want also to have it adjusted.

Overspeed:

--- Quote from: rodpp on January 19, 2025, 12:12:42 pm ---Hi,

When sending a multimeter to calibration we have the option to only check if its measurements are according with its specs, OR we can ask the Cal Lab to make adjustments too, reporting the measurements before and after adjustments.

I read some posts here saying that when the equipment to be calibrated has a history of various calibrations, adjusting it will ruin that history.

For equipments that have to adjust physical trimpots or similar, I understand it. 

But for multimeters calibrated without opening the case, only adjusting software parameters, this statment against adjustments to 24h specs remains valid? If yes, could you explain the reasons considering that every calibration + adjustments will provide a report with the measurements before and after ajustments?

Regards,
Rodrigo.

--- End quote ---

Hello

Not all calibration lab run adjustments , depend of their calibration procedures / equipment.
In some cases ( most) that request hardware tests / checks even visual checks as torque / corrosion / ... if there is drift and why .
Making adjustment on a single value is gambling , as a 10 V range tested at 10V don t make any proof  for linearity .

Regards
OS

bdunham7:

--- Quote from: rodpp on January 19, 2025, 12:12:42 pm ---But for multimeters calibrated without opening the case, only adjusting software parameters, this statment against adjustments to 24h specs remains valid? If yes, could you explain the reasons considering that every calibration + adjustments will provide a report with the measurements before and after ajustments?

--- End quote ---

It depends on which calibration service you purchase and the policies of the calibration lab.  You have to understand that those policies are generallly based on the needs of their customers, not a few random voltnuts.  Many basic services don't provide data unless there is an OOT (Out Of Tolerance) measurement.  When you get into services that do offer data, then adjustment policies are often based at least in part on the capabilities of the lab.  A lab might have the abilility (accuracy) to determine that your meter is within its one-year spec but not to adjust it down to the last digit.  In this case, they'll only perform an adjustment when the meter is off from nominal by more than their uncertainty, IOW they'll only make adjustments where they are sure they are improving things.  It is not uncommon for calibration labs to have uncertainties of 5-8ppm @ 10V and still take in 6.5-digit meters and provide calibration certificates for them.

As far as 24h specs, if you look at the entire set of ranges on many meters some will have 24h specs that aren't much better than the 1-year.  A better way of thinking about what the cal lab will do is that they will adjust all the ranges to nominal values and then use the 24h specs as their go/no-go during the performance check after the calibration adjustment.  Adjustment this way, along with as low as possible uncertainties, is what most people would hope for as a best-case when they send their DMM in.  Just getting data and avoiding adjustment is a thing for references and single-range instruments--DMMs have too many ranges to keep track of.  As you said, if you did want to track its drift or other long-term performance characteristic, you have the data and closed-case calibration doesn't affect drift like a potentiometer tweak might.

rodpp:

--- Quote from: aronake on January 19, 2025, 01:10:15 pm ---If you have old calibration history and want to be able to track drift, the possibility to some extent predict drift and adjust for drift out of later measurements you do not want to have it adjusted.

If you have no calibration history, or do not care about being able to track drift, you want also to have it adjusted.

--- End quote ---

But if no physical adjustments will be done, one can track drift with or without adjustments because only the equipment software will be adjusted.

For example, we are tracking the 10V readings considering no adjustments:

1st cal: reference
2nd cal: reference + 5ppm
3th cal: reference + 7ppm
4th cal: reference + 8ppm
5th cal: reference + 8ppm

Now if that same multimeter had got adjustments:

1st cal: reference
2nd cal: ajusted -5ppm => now reading same as reference again
3th cal: adjusted -2ppm => now reading same as reference again
4th cal: adjusted -1ppm => now reading same as reference again
5th cal: no adjustments needed - already reading same as reference

In both cases it is trivial to track the drift.

What am I not getting?

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