[volt-nuts] A Fluke 732A

Charles Steinmetz csteinmetz at yandex.com
Sun Mar 9 04:18:07 EDT 2014


Thomas wrote:

>You may find a local lab with less accreditation charging half then 
>price that is fully capable of of calibrating to the limit of the 
>732A but cannot document to the level of a primary standards lab.

Very, very doubtful.  Very few cal labs have a 732A or equivalent, 
much less anything better.  The only labs with *better* uncertainty 
than a properly working 732A are those with JJAs.  If you look at the 
NIST NVLAP accreditation list and run down it, looking at each lab's 
"Scope of Accreditation," you will only find 4 or 5 labs on the list 
with better uncertainty than a properly working 732A (I'm not sure 
you will find *any* on the A2LA accreditation list, but I haven't run 
down it lately).  The Fluke cal lab and the Los Alamos and Sandia 
standards labs are three of those four or five (plus, of course, NIST 
itself).  Boeing (Seattle) is another.  Interestingly, you will find 
many labs that are rigorously accredited to only .003% or so (30 
ppm), because the best voltage standard they own is an HP 34401A 
DMM.  Even the HP Houston cal lab is certified to only 0.0007%, or 7 
ppm (using a Fluke 5700A calibrator).

>Yes a 1-2PPM Cals is not as sexy as a .1PPM Cal but in the real 
>world the results when used in you home lab my be the same.

To get a calibration with an uncertainty of 1 or 2 ppm, the lab would 
need, at a minimum, a 732A or 732B to compare with (as well as a 720A 
Kelvin-Varley bridge, or equivalent, and a null meter that can 
reliably be read to 0.1uV, if you want the calibration certified to 1 
or 2 ppm at voltages other than 10v).  I don't think there are even 
ten labs on the NVLAP list that claim to have a 732A or B (the 
equipment used is often listed in the "remarks" column).

It does not take long to run down the whole list -- it's a short list 
and the "Scope of Accreditation" documents load fast.  I recommend 
the exercise, to get a feel for what's out there.  Same with the A2LA 
list, but it is longer and not as well organized and it usually takes 
2 or 3 steps (running off to the lab's site) to get to the "Scope of 
Accreditation."  (If you look at A2LA labs, pay attention to the lab 
class and only look at "open" commercial labs -- the non-commercial 
ones do not take in third-party calibration work.)

A list of NVLAP-accredited labs can be found here:

<http://ts.nist.gov/standards/scopes/dclow.htm>

There seems to be this myth of cal labs that can do just as good a 
job as the expensive, accredited labs, but don't bother with 
accreditation so they are much cheaper.  First, note that to do a job 
as good as an expensive, accredited lab, any lab would have to do the 
same documentation as the accredited lab.  If there is no 
documentation, there can be no claim as to the calibration's 
uncertainty.  Having done the documentation, which is the 
time-consuming (thus, expensive) part, no commercial cal lab is going 
to do without the accreditation (which is nothing but an audit of the 
lab's procedures and documentation).  I stress again -- if there is 
no documentation, there can be no claim as to the uncertainty of a 
lab's work.  And since the documentation is the part that contributes 
most to the cost, there simply are not any commercial labs that can 
claim to have uncertainties on par with accredited cal labs, but are 
not themselves accredited.

Best regards,

Charles





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