Electronics > Metrology

Developing a CMC and calculations

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moosebiscuit:
Good morning everyone. I am developing a CMC template for my calibration lab. We plan to adhere to 17025 requirements, but I am having some difficulties. I figured I would start with a Calibration and Measurement capability sheet. I am using a template that I obtained online for uncertainty calculations. I am starting with DCV - Source with a 5560A Multiproduct Calibrator.

This is my current understanding: The CMC format should disregard uncertainty values from the UUT. Aim to have the lowest uncertainties as possible, so when performing repeatability or reproducibility tests I am using an 8588A DMM. Possible sources of uncertainty for the CMC are Repeatability, Reproducibility, Ref standard stability, ref standard uncertainty, resolution, and bias. These are what I am using, anyway. I took a look at the specifications of the 5560A:

"The specifications include stability, temperature, and humidity; within specified limits,
linearity, line and load regulation, and the reference standard measurement uncertainty. The
product specifications are provided at a level of confidence of 99 %, k=2.58, normally
distributed. In some cases, additional specifications with a level of confidence of 95 %, k=2,
normally distributed are also listed. Fluke Calibration guarantees product performance to the
99 % level of confidence."

We send this unit in for calibration annually, I am using the 1 year spec @ 95% confidence. I would like to format the CMC with ranges for example 120mV, 1.2V, 12V, 120V and 1020V.  I am using a 100mV test point for reference for this first range. The uncertainty value published and calculated is 0.0000938000V. I took repeatability measurements with the DMM and calculated a STDEV (another uncertainty value) of 0.0000000316V. This is rather insignificant in comparison to the published uncertainty I mentioned. For my Bias check all I did was transfer over the error obtained from the 5560's latest certificate which was 0.0000001900V. Again, extremely insignificant compared to that first value.

Is it safe to say that I can just use these published specifications in my CMC for sourcing DCV? I am very new to this, any help would be greatly appreciated  ;D


mendip_discovery:
Until you have some history you are limited to just published specifications.

Did you take into account the thermal EMF caused by the material contacts?

I would avoid having your UoM written as 0.0000938000V, first, there needs to be a space between the number and the V. But I would refer it as 93.8 µV but you need round to two digits so 94 µV.

I just looked at my UoM for my Transmille and I have 7.5 µV at 100 mV. I will double-check this once I have cooked dinner.

EDIT:
Now that dinner has been dealt with.

Taking assumption,
9.3 µV/V + 0.8 µV for 1yr 95% spec.
0.01 µV Resolution
0.0316 µV Repeatabilty
1.2 µV to cover 0.3 µV/°C over a ± 2 °C swing of temp

I don't know your Imported UoM so I can take a stab at 4.9 µV/V from Fluke's Schedule in the UK

I get an Uncertainty of 2 µV at 100 mV. But it would be best to quote yours at FS, so 2.2 µV.

Remember it is fine having a low number but you have to prove you can achieve this and maintain it. So it is advisable to add a bit extra at first just so you have room to move if it starts to drift out.

Start doing a monthly cross-check between the multimeter and calibrator. Having some data will help you.

moosebiscuit:
Thank you for your input!


--- Quote ---Did you take into account the thermal EMF caused by the material contacts?
--- End quote ---

What would your recommendation be? For most general applications we use the Pomona banana leads, is this characteristic published anywhere?

Thank you for your input regarding the two digits. I wanted to play with the full numbers straight before making that conversion if that makes sense.


--- Quote ---1.2 µV to cover 0.3 µV/°C over a ± 2 °C swing of temp
--- End quote ---

I am curious as to how you came up with these numbers. Referring to my quote in the original post, isn't temperature already accounted for here: 9.3 µV/V + 0.8 µV? Or am I misinterpreting it? Also, when you mention importing UoM, are you referring to taking the expanded uncertainty from my certificate's 100mV point? If so, should I divide it by their coverage factor of 2 and then simply add it to my values? 

mendip_discovery:

--- Quote from: moosebiscuit on April 16, 2024, 06:36:17 pm ---Thank you for your input!

--- Quote ---Did you take into account the thermal EMF caused by the material contacts?
--- End quote ---

--- Quote ---1.2 µV to cover 0.3 µV/°C over a ± 2 °C swing of temp
--- End quote ---

I am curious as to how you came up with these numbers.

--- End quote ---

"Watch Out for Those Thermoelectric Voltages!" by Martin L. Kidd of Fluke using the Copper > Gold value of 0.3 µV per °C


--- Quote ---Referring to my quote in the original post, isn't temperature already accounted for here: 9.3 µV/V + 0.8 µV? Or am I misinterpreting it? Also, when you mention importing UoM, are you referring to taking the expanded uncertainty from my certificate's 100mV point? If so, should I divide it by their coverage factor of 2 and then simply add it to my values? 

--- End quote ---

Yes temp is, but the emf of the leads isn't included. It's worth noting in any procedures to watch out for issues if using the current features and then trying to do 120 mV measurements.

If the budget is for the range then pick the worst values from the range. With imported it gets divided by two in the maths anyway.

Do a screenshot of your maths and I will take a squint at it.

moosebiscuit:
I attached the document I am using for this. Here is what I know -

-Sensitivity coefficient stays as 1 because units are all the same here

-I believe my Type's are correct

-Distribution.. I'm about to make everything Normal because this topic seems to always go over my head  |O

-Divisor kind of makes sense, it is dependent on the distribution.

-Degrees of Freedom, I believe this value is just a representation of how many measurements were taken?

-(c^4*u^4)/v <---- I am not sure what this column is, does this look familiar?

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