[volt-nuts] High precision calibration for the poor man

[Dr. Frank Stellmach]

  Hi Samuel,

your question has to be split into two problems.

First one is the transfer of the unit of the Volt from the 3458A to the 
DUTs.

For sure it is possible to calibrate your DUTs relative to the 3458A as 
you described, provided your DUTs require about 10 times less accuracy, 
as your 3458A.

Some manuals of DVMs, as the HP34401A, recommend the usage of an 
appropriate calibrator as the Fluke 5700/5720A, or alternatively, the 
3458A plus appropriate stable sources.

In the latter case, the 3458A acts twofold, as a volt reference analog 
to the 732A, and as a transfer standard for scaling the 5500As output, 
analog to the 720A, due to its ultra high linearity. So the accuracy of 
a calibrated 3458A can be easily transferred to the DUT, plus taking 
into account diverse additional errors, like EMF, but this problem would 
also exist with other standards.

Due to its limited DAC resolution, the 5500A will never provide an exact 
10.000000V reading on the 3458A.

Trimming the output voltage to an exact value by a resistor divider is 
also critical, due to the necessary stability of the divider, or you 
have available a Fluke 720A.


Your proposed method, yet being an "accepted metrological practise", it 
is nether the less quite cumbersome, and in most cases of DUTs it is not 
necessary at all.

Most modern DUTs are easily calibrated electronically, ie they also 
accept odd values for the reference voltages.

So you simply measure the 10.000073V output by the HP3458A and type this 
reference value into your DUT.

Just read in the DUTs manual, how far off the reference may be from the 
standard value, to be accepted by the DUT and to bring it within its 
specification limits.

For example, the HP34401A accepts everything between 0.9 to 1.1 of Full 
Scale (eg 9.000 - 11.0000V) for most ranges and functions.

In the same manner, if your DUT is analogously calibrated (by a 
trimmer), simply trim its reading as near as possible to your reference 
value, ie 10.000073V, and you are done.



So, now for the 2nd problem, calibration "for the poor man".

Well, your are not a poor man, because you don't have a 5720A or a 
5440A, but only a 5500A
Instead you have have the 3458A. In fact, owning this device, you must 
be really rich.

But you are in a poor mid- and long term situation, as your 3458A is not 
a very good volt standard (8ppm/yr.), and therefore needs very frequent 
calibration (which will make you really poor), if you want to maintain a 
defined level of uncertainty.

So your real problem is the transfer of the official volt from NIST or 
an accredited cal lab to your lab.

To have less frequent recalibrations, a better volt standard, like a 
732A/B or similar would mitigate this 2nd problem.

Frank