Fake Calibration Companies on the rise...

[OlDave]

Don't misunderstand me. I didn't say it can't be done. Just that it's not as simple as just reading the uncertainty off the cal certificate and running with that number. I've seen too many people think like that. The point I was trying to make was that the uncertainty of measurement is not an absolute value and only reflects the measurement at the time of calibration under conditions at the time of calibration. If other factors that can and do affect the accuracy and stability of the instrument over time aren't considered then people are fooling themselves.

It sounds like you have taken the necessary steps to maintain that level of uncertainty so it is usable and transferable. I never meant to imply otherwise, just pointing out that there is more to it than just getting an accredited calibration on an instrument and reading the numbers.

[flew-da-coup]

I just wanted to make sure that you knew that we calculate our expanded  uncert. correctly. We do RSS (2 sigma).  Some people who have worked only in government labs have no understanding of expaned uncertinty and they get confused about how things are done this way.

[MIRCS]

I just wanted to make sure that you knew that we calculate our expanded  uncert. correctly. We do RSS (2 sigma).  Some people who have worked only in government labs have no understanding of expaned uncertinty and they get confused about how things are done this way.

Now could you and Old Dave please come and explain this to a fellow technician here that believe our S-1 weights can be used as Ultra class, E1, 0 regardless of where he is just due to the uncert. given by Troemner.

For the love of god, I have tried enough times already.

[OlDave]

MIRCS, Lets run with this example and see how many people want to take a shot at it....

Actually I will challenge anyone to tell by any method other than weighing, whether a Troemner weight is an UltraClass, Class 1, Class 2, Class 3 or even a Class 4 weight. I can't, and I'll bet you can't either. They all are made of the same material, are two-piece construction, and even have the same surface finish. The only difference is their closeness to the nominal value.

Lets say you are calibrating a Mettler XP205 balance. That's a 220 gram 0.01mg resolution balance. You would typically check span at 200 grams. If I remember the Mettler spec correctly the span tolerance is ±0.40 mg. Now lets say you have set of UltraClass weights with you that range from 1 gram to 500 gram. In it there are 2 200 gram weights. You put one of the weights on and the balance reads 199.99989 grams. Outstanding! We're good to go. But out of curiosity you decide to drop the other 200 gram weight on the pan and oops....it now reads 199.99951 grams. Hmmm....we gotta problem. Is the balance good? Is the balance bad? Are one of the weights bad? We don't know.

Lets think about this, the tolerance for a 200 gram UltraClass weight is ±0.30 mg. Lets say the first weight is heavy by 0.21mg (200.00021 g), still within tolerance, not a problem. And the second weight is light by 0.17 mg (199.99983 g), again within tolerance. But since we don't have our weights charted we don't know this.

This balance is actually reading 0.32 mg light isn't it. So the balance is within its tolerance of ±0.40 mg, but we can't prove that with the standards we have.

But suppose I had a set of Class 3 weights that have the actual mass value and uncertainty of measurement reported. Troemner's typical uncertainty at 200 grams is around 0.027 mg. However the Class tolerance for a 200 gram Class 3 weight is a whopping 2.0 mg. I place this 200 gram weight on the balance and it reads 199.99867 grams. Ouch! But before I panic, I look at the report of measurement and see that the actual mass value of that weight is 199.998986 grams with an uncertainty of 0.027 mg (well within the class tolerance of ±2.0 mg). Since I'm out in the field I'll most likely just blow off the uncertainty of 0.027 mg since the balance has an accuracy of ±0.40 mg and just subtract the balance indication from the mass value of the standard (199.99867 – 199.998986) and see that the balance has an error of –0.000316 grams. Within my calibration limit of ±0.40 mg.

Now which weight set allowed you to perform the better calibration? The UltraClass without a report of measurement or the Class 3 set that did?

[flew-da-coup]

Well said Ol' Dave. However, remember that you need to do your expanded uncertinty of the calibration uncert. of 0.027mg. Even still your explaination was perfect. We use Rice Lake standards, but it's all the same. We calibrate mass up to 25kg class 1 weights and we can do 1:1 cal's with class 0 weights. Doing mass with the lowest uncert. is obtained using 3 in1 weighing design. However, useing double substitution weighing design is good to do up to class 1 and class S weights.

[OlDave]

I know, you need to use the 0.027 mg as the uncertainty of the mass standard but my example was getting long enough to put most people to sleep as it was. And since we weren't really trying to discuss a full blown uncertainty analysis I took a couple of short cuts.

I only use a 3 in 1 weighing design for all of our check weights in the Institute. Since we don't perform outside calibrations I have a good deal of control over how stuff gets done here....

[flew-da-coup]

I gottcha Ol'Dave.

[Newbie]

Just interviewed with a company in Atlanta area, the manager stated in the interview that a person with my experience should have no trouble calibrating a scope in 15 minutes A signal generator should take no more than 30 minutes. I guess he was interviewing for a lick it stick it position...funny it was posted as a calibration tech. Thats why Im going to try to get into nuclear work.

[scottbp]

Holy crap...  Cal a scope in 15 minutes? I can't cal a scope that fast using MET/CAL... I'll bet they have a pay rate that keeps their turnstile spinning, too.