[volt-nuts] traceable calibration

[Andreas Jahn]

>>...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?

Hello Greg,

after thinking a while I have a guess what Keithley is actually doing.
They seem to add a constant 6uV tolerance geometrically to
the 5V and 10V tolerances.
so   sqrt(15 ^ 2 + 6 ^ 2)    = 16.1
and sqrt(27.5 ^ 2 + 6 ^ 2) = 28.1

so for a 7V I would correct my uncertainity to 20.9 uV.

This leads to another interesting question:
Which errors are statistical and thus can be added geometrically
and which errors have to be added linear?


> 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.

So I would have to use the +/-260 uV for the 7V value as MU.
I still hope that I can narrow down the MU for my LM399 #2 measurement.

Is there really no chance of narrowing this MU to a lower value
with the knowing that: over 2 years (3 calibrations) both Keithleys
have shown always a constant difference of 70-80uV for the 7V reference
and: during last year the difference of both instruments against LM399 #2
has been 10uV?

> My best,
> Greg

With best regards

Andreas