Is it OK to calibrate a measurement device tighter than manufacturer specifications?

Q

Qualqueen

#11
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

I would never try to calibrate to a tighter tolerance than the gage will read. Half a mark or 1/4 a mark? Phooey. If one wants a tight tolerance, one gets a gage designed and built for accurately measuring to said tolerance. Period. :2cents:
:applause::applause::applause: Couldn't say it better
 
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bobdoering

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#12
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

I would never try to calibrate to a tighter tolerance than the gage will read. Half a mark or 1/4 a mark? Phooey. If one wants a tight tolerance, one gets a gage designed and built for accurately measuring to said tolerance. Period. :2cents:
I guess it depends on the calibration method, huh? If you adjust the pressure until you are on the gage to be calibrated mark (right on it, ok?), then read your 10:1 standard gage and have it determine the error, then I don't see the problem. E.g., set the pressure to exactly 50 PSIG on the gage being calibrated. The standard gage reads 50.3 PSIG. You're in.

Besides, the manufacturer's specification does not meet your mark to mark tolerance requirement. +/-30 PSIG with 50 PSIG increments. For mark to mark calibration you would need +/-50PSIG just to get adjacent marks.

Some folks might be mixing up resolution with bias error. The OP did not have clear info on the customer use to clarify if resolution is even an issue...although it is suspicious...
 

Wesley Richardson

Wes R
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#13
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

So get this... A customer sends us a dial pressure gauge, 0-3000 PSIG, rated at 1% of full scale by the manufacturer (±30 PSIG.) The customer wanted it calibrated to 0.5% of FS, which is ±15 PSIG. The gage is marked in 50 PSIG increments, with a needle that is maybe 1/4 as wide as the spacing between the markings and no parallax mirror behind the needle.
It sounds like the customer is wanting a more accurate gauge, without paying the additional cost to purchase what is required for the application.

ASME B40.1 provides pressure grades and accuracies. Note that these do not include the effects due to measurement uncertainties. From the description, the customer has a Grade 1A (1% full scale) gage, and wants it calibrated to then become a Grade 2A (0.5% full scale) gage.

I recommend against providing a calibration certificate that the gage meets the tighter specification. This is based on the fact that the inherent characteristics of the gage are not changed by calibration. If it met the tigher requirements, then the manufacturer would have specified it as being the tighter tolerance gauge.

Wes R.
 

bobdoering

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#14
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

If it met the tigher requirements, then the manufacturer would have specified it as being the tighter tolerance gauge.
Or, if it doesn't it will fail calibration and the problem is solved.
 

Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#15
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

I guess it depends on the calibration method, huh? If you adjust the pressure until you are on the gage to be calibrated mark (right on it, ok?), then read your 10:1 standard gage and have it determine the error, then I don't see the problem. E.g., set the pressure to exactly 50 PSIG on the gage being calibrated. The standard gage reads 50.3 PSIG. You're in.

Besides, the manufacturer's specification does not meet your mark to mark tolerance requirement. +/-30 PSIG with 50 PSIG increments. For mark to mark calibration you would need +/-50PSIG just to get adjacent marks.

Some folks might be mixing up resolution with bias error. The OP did not have clear info on the customer use to clarify if resolution is even an issue...although it is suspicious...
I was responding to a post describing how the tolerance was being attempted to less than one measurement increment on the dial. The gage has a dial with those increments for a reason - the gage is designed and constructed to function in a range. A large range gage will generally have larger accuracy tolerances. One selects the gage one needs based on range or tolerance.

In a quality system, as a good practice I wouldn't dream of asking for a gage with, for example .001" increments to produce accuracy of .0005" and the gage user to read .0005" with the needle floating somewhere between two lines.
 

bobdoering

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#16
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

In a quality system, as a good practice I wouldn't dream of asking for a gage with, for example .001" increments to produce accuracy of .0005" and the gage user to read .0005" with the needle floating somewhere between two lines.
Yes, I agree, resolution 10:1 to the tolerance or 10:1 to the control limits - no need to interpolate.

However, calibration is looking to how close the gage reads to the standard - is 50PSIG really 50PSIG? For that the tolerance of the standard gage must be 10:1 (or some say 4:1) to the gage being calibrated.


I was responding to a post describing how the tolerance was being attempted to less than one measurement increment on the dial. The gage has a dial with those increments for a reason - the gage is designed and constructed to function in a range. A large range gage will generally have larger accuracy tolerances. One selects the gage one needs based on range or tolerance.
Fact is, they may have done exactly that (we don't know from the info we received what the actual measurement tolerance is), but they just want to be sure that if the gage they bought reads 50PSIG, it is really between 35 and 65PSIG, not 20 and 80 PSIG. That is bias, not resolution.
 
Last edited:

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#17
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

If the customer wants it, and it's properly documented in contract review, I don't see how you could be liable for anything. I would also put a note on the cert stating the customer's requirement.

Now common sense tells you that you should really explain the reasoning to the customer as to why this shouldn't be done.

But, bottom line, if the customer wants it and it's documented, you're operating within the requirements of 17025 as a cal provider.
Daniel is correct.

The technical issues aside, it is allowed under 17025 though not good practice, as your uncertainty for the cal will likely approach 20% or even more because now resolution and parallax will drive the Type B's, and subsequently affect the Type A's as well.

But if the customer understandards that any reading he makes using that item will be indication +/- a 25% error (uncertainty) and is comfortable with that, then document it really well, and make sure the customer signs off on it.

But do suggest acquisition and use of a different instrument that will actually provide what the customer truly needs.
 

ScottBP

Involved In Discussions
#19
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

Ok, well I just had another situation where a customer wanted a thermocouple measuring device to read within ±1.0°F at certain test points. All the readings were well within that limit (the largest deviation was 0.2°F), but when we put down the manufacturer's spec of ±1.4°F like we always do for this particular model, the customer got bent out of shape, so I ended up amending their certificate to show the deviation was within the ±1.0°F spec that the regulation they were using called for (AMS-2750D raises its ugly head again). I left the manufacturer's spec in place for all to see so the customer can decide for themselves if it is acceptable to rate the meter tighter than what the manufacturer's specs call for, or go out and get a more accurate meter.
 
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