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

ScottBP

Involved In Discussions
#1
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. Is the customer fooling themselves by wanting the gauge to be tighter than manufacturer's specs? Is it up to the calibrated eyeball of the technician to guesstimate what the resolution will be?

Even if we can adjust it to be dead on, there's no way we can guarantee it'll meet the customer's required specification after the calibration interval if the manufacturer's spec for that same interval is larger! If we say we calibrate something in accordance with the manufacturer's procedure but then we try to go tighter (as in the example above), then that means we're NOT following the manufacturer's procedure and can get whammied for it on an audit. So how would I word that on a calibration certificate, or do we just tell the customer if they want a tighter specification then go buy a more accurate gauge?
 
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Jerry Eldred

Forum Moderator
Super Moderator
#2
Re: When is it ever OK to calibrate something tighter than manufacturer specs?

It's always okay to calibrate looser than tolerance. But to certify something as tighter than tolerance, you, as a lab (in my opinion) are attesting that the meter is capable of performing better than it's design, and that it will do so for a given interval.

I would think of a polite way to tell them that you can't certify better than it's specifications. It has to have adequate resolution, stability, repeatability, etc. to meet the spec.

This is not to mention that if you attempt to calibrate it and check at various points along the scale, you can't guarantee that it will even test to those specs (even if you optimize it).

My gut feeling is that if you certify in such a manner, you may even potentially incur some liability for it meeting those specs.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#3
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.
You have to help me understand how this works. If you apply a pressure, measured on a high much higher resolution than the gage, and when gage to get calibrated reads 100PSIG, if your gage reads +/-30PSIG of that value its good? If that is the scam, then I guess you could trim the pass/fail tighter to +/- 15 PSIG.... Not sure how you deal with fat needles in your calibration procedure...

Is the customer fooling themselves by wanting the gauge to be tighter than manufacturer's specs?
Maybe...it might not pass. That is usually the manufacturer's point. And if it doesn't, they don't want to hear about it.

Is it up to the calibrated eyeball of the technician to guesstimate what the resolution will be?
The resolution is still 50 PSIG. The markings don't change.

Even if we can adjust it to be dead on, there's no way we can guarantee it'll meet the customer's required specification after the calibration interval if the manufacturer's spec for that same interval is larger!
How could you guarantee it'll meet the customer's required specification after the calibration interval ever? It should be up to their usage to determine the interval. They might play racquetball with the thing...who knows?

If we say we calibrate something in accordance with the manufacturer's procedure but then we try to go tighter (as in the example above), then that means we're NOT following the manufacturer's procedure and can get whammied for it on an audit. So how would I word that on a calibration certificate, or do we just tell the customer if they want a tighter specification then go buy a more accurate gauge?
If you say you calibrate something in accordance with the manufacturer's procedure but then you try to go tighter, and identify the exception on the report, you are not lying. Now if you didn't mention the exception on the report, then you would be hurting. Does your procedure say you only calibrate something in accordance with the manufacturer's procedure? If so, then simply send it back to the customer uncalibrated. Done.

So how would I word that on a calibration certificate...
"Calibrated per manufacturer's procedure except verified to ±15 PSIG (±30 PSIG manufacturer specification) at the following PSIG: 100, 500, 1000, 1500, 2000, 2500, 2900 PSIG" (or whatever you checked it at...)

...or do we just tell the customer if they want a tighter specification then go buy a more accurate gauge?
Your call...

There a starting point for discussion. :tg:

My gut feeling is that if you certify in such a manner, you may even potentially incur some liability for it meeting those specs.
I don't know about liability. I think either it does or it doesn't pass calibration. If it doesn't, it fails, then they know they have the wrong gage.

It has to have adequate resolution, stability, repeatability, etc. to meet the spec.
I still think it is kooky to have a +/-30 PSIG manufacturer's spec if it reads in 50 PSIG increments...:confused:
 

Stijloor

Staff member
Super Moderator
#4
Re: When is it ever OK to calibrate something tighter than manufacturer specs?

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. Is the customer fooling themselves by wanting the gauge to be tighter than manufacturer's specs? Is it up to the calibrated eyeball of the technician to guesstimate what the resolution will be?

Even if we can adjust it to be dead on, there's no way we can guarantee it'll meet the customer's required specification after the calibration interval if the manufacturer's spec for that same interval is larger! If we say we calibrate something in accordance with the manufacturer's procedure but then we try to go tighter (as in the example above), then that means we're NOT following the manufacturer's procedure and can get whammied for it on an audit. So how would I word that on a calibration certificate, or do we just tell the customer if they want a tighter specification then go buy a more accurate gauge?
Scott,

Do what they want, invoice accordingly ;), and add a disclaimer.

Stijloor.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#5
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

Short answer, yes you can, and document it under Clause 4.4 Contract Review.

Now the question becomes is there any value in it? That is best left to the customer.
 

BradM

Staff member
Admin
#6
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

My answer is no. Every general calibration procedure I have ever seen specifies no tolerance tighter than manufacturer; and no resolution finer than 1/2 the increments. There would have to be interpolation/ interpretation of the actual values due to limited resolution; adding to the uncertainty of the instrument.

Yes, you can hit the cardinal points on the gauge during calibration. But what about reality when the gauge reads processes that are not on cardinal points? There is no way the instrument can display to comply with the accuracy (Is that 17? It looks like 22 to me; it's closer to 100 than 150...:tg:).

It is not advisable to "force" an instrument to comply with process requirements. Instruments must be purchased for their intended use; and calibrated to their requirements/ stated specifications.

I would suggest offering to calibrate to mfg. specifications, and help them spec out a different instrument suitable for that process tolerance.:)
 
S

SpongeMouse

#7
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

i agrre with the cover peeps above.

talk to your client for the reason that it cannot adjust to the specs needed. maybe another gauge in the market can be applicable.

usually, the guage limits from the maufacturer are standardized to all user, so its up to the user to provide means to adjust to its needed limits.

in your case, maybe another type of gauge that can provides more specific limits is applicable.

:notme:
 

ScottBP

Involved In Discussions
#8
Re: Is it OK to calibrate a measurement device tighter than manufacturer specificatio

Thanks for the input... In one case, the instrument would have enjoyed about a 6:1 T.U.R. under mfg. specs, but when the customer wanted it tighter, it cut the T.U.R. to 3:1, so I ended up providing an uncertainty statement for all to see... But in another instance where we were barely getting by with a 1:1 calibration by mfg. specs, I had to tell the customer no. :nope:
 

Jen Kirley

Quality and Auditing Expert
Staff member
Admin
#9
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:
 
D

Daniel Walker - 2011

#10
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.
 
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