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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