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Guru
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Calculating Uncertainty for Transmitters

02/23/2021 12:22 AM

I've been having discussions with other calibration professionals and can't get a concensus.

According to ISO/IEC 17025:2017, Clause 7.8.4.1a: Calibration certificates shall include...the measurement uncertainty of the measurement result presented in the same unit as that of the measurand or in a term relative to the measurand (e.g., percent).

Basically, what this particular clause is saying is that your uncertainty units should be the same as the measuring units. If you're measuring ºC then your uncertainty must also be in ºC; if you're measuring psi then your uncertainty must also be in psi.

This isn't a problem when you're calibrating, say, a pressure gauge. If your gauge is in psi, you apply a pressure in psi, read off the pressure in psi, and calculate the uncertainty in psi.

The problem crops up when you're calibrating pressure transmitters. You apply a pressure in psi and read off the output in milliamps. What do you do? Do you calculate the uncertainty in psi or in mA?

For me, the answer was obvious. 17025 says the uncertainty units must be the same as the measurand units, so the uncertainty must be in psi. I've found, however, that quite a number of calibration laboratories are giving uncertainties in milliamps! To me, this was a violation. So I was surprised to learn that they've been accredited! They passed!

So, my question is, how can this be? If you violate any clause in ISO, you're given a non-compliance notice. Why are these laboratories passing if they are not following this particular clause?

Note: same issue for temperature transmitters, flow transmitters, any type of transmitter that outputs a signal in different units from the measurand.

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Guru

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#1

Re: Calculating Uncertainty for Transmitters

02/23/2021 1:11 AM

It seems to me you would have to measure both in a defined range...but I would expect the accuracy to be in the gauge terms....

"For example, an electronic pressure transmitter may have an instrument range of 0–750 psig and output of 4-to-20 milliamps (mA). However, the engineer has determined the instrument will be calibrated for 0-to-300 psig = 4-to-20 mA. Therefore, the calibration range would be specified as 0-to-300 psig = 4-to-20 mA."

Example: you are assigned to perform the calibration of the previously mentioned 0-to-300 psig pressure transmitter with a specified calibration tolerance of ±2 psig. The output tolerance would be: 2 psig/300 psig*16 mA = 0.1067 mA

https://www.mgnewell.com/wp-content/uploads/2016/05/Calibration-Principles.pdf

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#2

Re: Calculating Uncertainty for Transmitters

02/23/2021 9:15 AM

One might keep in mind that for most instrument applications, 1 part in 2^15 is about the smoothest one can get; chips with everything these days...��

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#3

Re: Calculating Uncertainty for Transmitters

02/23/2021 12:06 PM

It seems to me that it depends on how you calibrate it. If you put in a calibrated input, the error is the actual output minus the correct output, in output units. If you adjust the input to get a fixed output, the error would be the correct input for that output minus the actual input, in input units.

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#4

Re: Calculating Uncertainty for Transmitters

02/24/2021 6:44 PM

What would be the value of a transmitter whose process value had an uncertainty of 0.01% FS when read on the local display but had a 4-20mA output whose uncertainty was ±0.2% ? The represented pressure values are then as good as thee 4-20mA output, good to only to ±0.2%, are they not?

A transmitter is a transmitter. Although it might have a local indicator, the purpose of a transmitter is to read the pressure and get the process variable value to some other destination via a representative analog signal, hence the signal representing the pressure is the object of interest in terms of the 'accuracy' of the transmitter. It's the representative 4-20mA signal that is used in the control room, not the local indication, so it's what is the determining factor in the 'accuracy' of the transmitter.

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#5
In reply to #4

Re: Calculating Uncertainty for Transmitters

02/24/2021 6:51 PM

Then why is there even a gauge?

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Guru
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#6

Re: Calculating Uncertainty for Transmitters

03/02/2021 8:39 AM

Actually, the issue has to do with putting the result in the calibration certificate/report. The ISO guide says the uncertainty stated in the report needs to be in the same units as the measurand but this is not followed when the instrument happens to be a transmitter. I don't see why it has to be different.

regards,

Vulcan

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