[volt-nuts] traceable calibration

gbusg gbusg at comcast.net
Sat Sep 10 11:48:29 UTC 2011


Andreas wrote:

>...so I would use the calibrator tolerance as being 2.5ppm + 3.6uV as worst
>case
>and transfer this to the 7V of my reference voltage as 21.1uV uncertainity
>for the calibrator.
>But how do I have to regard the other tolerances up to the measurement of 
>my
>LM399 #2?

Hi Andreas,

Actually, assuming Keithley's Test Uncertainty Ratios (TUR) are reasonably 
high enough - say >4:1 -  you don't even need to carry-forward Keithley's 
calibration Measurement Uncertainties (associated with their process to 
calibrate your 2000) because Keithley's published specs (for their Model 
2000) include allowances for the Measurement Uncertainties (MU) of their 
processes to calibrate your 2000.

Therefore in metrology we assume the MU of *your* process mostly equals the 
rectangular distribution of Keithley's published specs for Model 2000.

If your specific Model 2000 has a "proven track record" (e.g., at least 3 
historical calibration test reports showing that your 2000 DC readings are 
very repeatable and stable between cal events), then at least on an informal 
basis you might use Keithley's test report data as *correction factors* in 
*your* process to enhance your accuracy (lower MU). However that strategy 
would involve a lot of additional statistical analysis and knowledge of your 
Model 2000.

My best,
Greg





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