Missing value of Uncertainty B

fahimk

Involved In Discussions
#1
Dear Fellows,

In my lab, while estimating one of the Type B uncertainties, i have to consult the uncertainty value provided in the calibration certificate. However, in some cases the relevant uncertainty values are not given in the certificate.

Example:
For instance, for AC Current @ 50 Hz the certificate provides me uncertainty for the following values:

1 mA
10 mA
100 mA
5 A
10 A

What should i do if i intend to insert Type B uncertainty for the following values?
5 mA
55 mA
200 mA
500 mA
7 A

Thanks and looking forward....
 
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dgriffith

Quite Involved in Discussions
#2
Your manual should explain the MU for your DMM (or similar). Usually that's a % of Range +a % of reading, added. The range for 5 mA is 10 mA, and the % of reading is % of 5 mA. The two are added.
Sorry, posted this quick. It may not be exact, but it should be similar.

Added: What instrument do you have?; what were the values tested at 1,10,100 mA and 5,10 A?
The vendor should have made sure the instrument was in tolerance for each range and condition. The uncertainty stated on the cert is their uncertainty for the measurement, not necessarily the instruments..
 
Last edited:

fahimk

Involved In Discussions
#3
Your manual should explain the MU for your DMM (or similar). Usually that's a % of Range +a % of reading, added. The range for 5 mA is 10 mA, and the % of reading is % of 5 mA. The two are added.
Sorry, posted this quick. It may not be exact, but it should be similar.

Added: What instrument do you have?; what were the values tested at 1,10,100 mA and 5,10 A?
The vendor should have made sure the instrument was in tolerance for each range and condition. The uncertainty stated on the cert is their uncertainty for the measurement, not necessarily the instruments..
Thanks mate for the detailed response.
Well, from your reply, i think you are talking about the uncertainty as stated in the Calibrator's manual. In my case that's UB1 uncertainty. For UB2, i am supposed to consult the Uncertainty values as stated in the certificate, issued to my lab from the upper body in the trace-ability chain. Unfortunately they haven't listed the results against a range of input values but rather for fixed values. I hope you understand what i mean.
 

dwperron

Trusted Information Resource
#4
Typically you would use the calibration uncertainty from the report for the closest value larger than the value you are calculating uncertainty for.
So for your 55 mA value you would use the 100 mA calibration uncertainty value.
 
Last edited:

BradM

Staff member
Admin
#5
Typically you would use the calibration uncertainty from the report for the closets value larger than the value you are calculating uncertainty for.
So for your 55 mA value you would use the 100 mA calibration uncertainty value.
Agreed. That was my suggestion. I would also consult the calibration provider and make a suggestion that they make it more clear on their calibration certificates what the measurement uncertainty estimation is for all the applied measurements.
 

dgriffith

Quite Involved in Discussions
#6
I have another point of view on this. In my post above I responded before reading fahimk's other post on a similar topic. I believe he is referencing a Fluke calibrator.
The calibrator has a (usually time-related) specification that already includes the uncertainty of measurement using the laboratory's primary or corporate standards. The certification uncertainty is Fluke's uncertainty in the measurement (which is also the uncertainty in the error); they are telling the customer the range of values within which their reported measurement lies with stated probability. It isn't the uncertainty in the calibrator's output, it's the combined uncertainty in their instrument(s).

Ex: My Fluke 8508 Ref DMM, 10Vdc on 20V range, the uncertainty spec is given as 4ppm rdg+0.25ppm rng @99%CL, reducing to [email protected]%. But, the Fluke cal cert says at 10V the MU was [email protected]%. Would I dare say every time I measure ten volts for the next year the uncertainty was six micro-Volt?

There are of course exceptions-- gage blocks, DC Volt standards, etc. where a single value is assigned or predicted with stated uncertainty.

Sorry for rambling, and my boss says I must now do real work for a while if I want to get paid....
 

dwperron

Trusted Information Resource
#7
Ex: My Fluke 8508 Ref DMM, 10Vdc on 20V range, the uncertainty spec is given as 4ppm rdg+0.25ppm rng @99%CL, reducing to [email protected]%. But, the Fluke cal cert says at 10V the MU was [email protected]%. Would I dare say every time I measure ten volts for the next year the uncertainty was six micro-Volt?

Fluke says this in their Extended Specifications document:
"The Absolute specifications include the uncertainty of the standards used to perform calibration of the 8508A at manufacture and may be used to determine the uncertainty of measurements made with the 8508A for periods up to 1 year and over a temperature range of ±5 °C from calibration. If the user has their 8508A calibrated with different uncertainties, the Relative specifications can be combined with the uncertainties applicable to that calibration to determine the effective absolute uncertainty following that calibration. "

So that means that the Fluke Calibration Uncertainty is included in the Absolute specifications, so you don't need to add the 6 µV to it.
If you have it calibrated somewhere else you need to use the Relative specifications and add the Calibration uncertainty that your lab provides.

This is kinda like what HP/Agilent/Keysight did with their 3458A specifications. They had a column on the outside of the specifications page that showed the Calibration uncertainties that needed to be added to their specifications if you had it calibrated by Keysight, otherwise you added your own numbers.
 

fahimk

Involved In Discussions
#8
Typically you would use the calibration uncertainty from the report for the closest value larger than the value you are calculating uncertainty for.
So for your 55 mA value you would use the 100 mA calibration uncertainty value.
Thanks for the response. I got it now.
So, what should i do when i am supposed to write uncertainty for the reading 200 mA when the maximum range, in the calibration certificate, is 100 mA?
 

dwperron

Trusted Information Resource
#9
Thanks for the response. I got it now.
So, what should i do when i am supposed to write uncertainty for the reading 200 mA when the maximum range, in the calibration certificate, is 100 mA?
You said:
For instance, for AC Current @ 50 Hz the certificate provides me uncertainty for the following values:

1 mA
10 mA
100 mA
5 A
10 A

So the next range would be the 5A range. I'm really surprised that there were no results provided between 100 mA and 5A (I would check on that), so you can only use what was provided to you.
 

fahimk

Involved In Discussions
#10
Thanks a lot for your elaborate responses.

Taking the discussion further, what am i supposed to do if the Calibration Certificate, from the NMI, provides me Uc for values at a single frequency, i.e. 50 Hz while a client desires Uc for values at 5k Hz and 40 Hz for instance?


FYI
In order to calculate the combined uncertainty for electrical measurements, i have used following B type uncertainties:
UB1: Calibrator's Inaccuracy
UB2: Calibration Certificate (based on the Calibration Certificate from the upper body in traceability chain)
UB3: Hysterisis
UB4: UUT Measurement Limitation (Resolution)

This time i am having issues with finding the value of UB2 at Current and Voltage frequencies other than thoses stated in UB2.

Hope you understand what i mean...
 
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