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NIST Traceable Thermometer Uncertainty

B

b58chtec

#1
Hello,
I work as a QC Chemist for a chemical manufacturing company. We're trying to obtain an ISO 9001:2008 certification.

NIST Traceable thermometers, purchased from vendors, have a stated uncertainty of +/- some value at various temps. Older lab thermometers that have been in use for many years are calibrated, by me, against high precision NIST Traceable Thermometers. These are the really expensive ones that are designed for calibration only purposes.

Question. If I have a thermometer that is minus -0.1C versus the NIST thermometer, do I report it as +/- 0.1C of do I report it as -0.1C? What is the correct protocol for reporting uncertainty for thermometers calibrated in the lab?
Thanks.

Bart
 
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BradM

Staff member
Admin
#2
Hello Bart! :bigwave:

Just to clarify, are we speaking of uncertainty or accuracy? They are two different things.

Uncertainty is typically an estimation; a calculation of sorts.

Here is a link giving a brief definition of uncertainty:

http://www.l-a-b.com/content/measurement-uncertainty

So you have thermometers (called lab thermometers) and then the high precision thermometer. So you use the high precision ones to verify the accuracy of the lab thermometers. Correct?

So you have the high precision thermometers verified by a competent lab. This thermometer should be substantially higher accuracy than the lab thermometers.

You verify the lab thermometers and find a deviation. So if your high precision thermometer displays 0.1°C and the lab thermometers displays 0.3°C, then you can either generate a report with the actual values observed. You could also display the test value of 0.1°C and report the lab thermometer as +.2°C.

Not sure if you are estimating a true uncertainty. If so, that's another matter. If not, then you will want to assure the high precision thermometer has sufficiently better accuracy than the lab thermometers. A 4 to 1 ratio is typical.:)
 
B

b58chtec

#3
Brad,
Thanks for your reply.

Okay. Didn't realize there was a difference between uncertainty & accuracy. I'll check out the link you sent. I assumed they were interchangeable.

Anyway, the high precision thermometer(s) have values like +/- 0.06C at 20C or +/-0.03 at 30C & the regular thermometers came from the manufacturers with stated accuracies of +/- 1.5C or +/-1.0C. I think I'm ok there.

So, I don't have to report the regular thermometers as +/- value. Instead, I report the correction only of either + value or -value. Correct?

Thanks.

Bart
 

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#4
Brad,

Every lab is different. It's not quite that easy to take the difference in your temperature and just transfer it. I'm not a testing expert, but if you would like to give me a call next week when one of our testing program managers are in the office, we can tackle that question with the expert. <phone number removed> I'm the only Gary in the office.

Many people have questions about uncertainty budgets. We also have calculators you may find uselful in the resources tab on our website at <removed>
Last warning - This is not a place for you to come and drum up business. Your post has been reported and it's apparent you haven't read the ToS, or you have and are intentional violating it. You are making yourself look pretty bad - Who wants to do business with someone who either doesn't read the rules, or just decides not to follow the rules?
 

BradM

Staff member
Admin
#5
Anyway, the high precision thermometer(s) have values like +/- 0.06C at 20C or +/-0.03 at 30C & the regular thermometers came from the manufacturers with stated accuracies of +/- 1.5C or +/-1.0C. I think I'm ok there.

So, I don't have to report the regular thermometers as +/- value. Instead, I report the correction only of either + value or -value. Correct?

Thanks.

Bart
Well... I think I might have confused you and mixed two different things, here. If you are discussing the accuracy of the thermometer then it is usually expressed as a +/- value. The actual error that you observe while verifying the device is + value or -value.

You have two thermometers- S (standard) and B (bench thermometer).

S should have a stated accuracy that the thermometer should be able to meet. If it does not meet that accuracy, then it would be considered out of tolerance. It should be sufficiently more accurate than B. S should be calibrated by an outside lab, and be assessed against an identified specification (either your own, or mfg.).

Then you test B using S. B should have a stated specification also. This specification is what you expect the thermometer to meet while testing.

Let's say it is +/-5°C. When the S displays 50.0°C and B states 50.4°C, then you should list a +.4°C for the B thermometer @50°C.

However, that is still within your +/-5°C tolerance for the thermometer.

Now... as mentioned in this thread, the more correct method is to estimate the uncertainty of your measurement system. However, that tends to be a little more involved for the average individual. It's the better way; but not the simplest. :) So that is why I keep mentioning accuracy here.

I hope that explained things a bit better. :agree1:
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#6
BradM has been giving some great advice. Only one thing I would add at this point.

For the high accuracy thermometers, when there is +/- 0.06 C then that suggests that the value presented is the uncertainty. If so, the calibration cert should state if it is reported at k=2 to approximate 95% confidence, of if from a NMI then maybe k=3. If k=2, then 0.06 divides by 2 (assumes normal distribution) to return the value to standard uncertainty, necessary for calculation of your own uncertainty.
 
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