On 06/02/2013 12:19 AM, Charles P. Steinmetz wrote:
Bob wrote:
At least the way I read the pdf's NIST seems to believe that GPS is
legally traceable to NIST. It is the same "measure and then look up
the data" sort of thing that LORAN used to be. Took a while to read
through them all…
Yes, that is correct.
Magnus wrote:
However, just taking time from GPS does not achieve NIST traceability.
* * *
You can achieve NIST traceability (or to any other NIH) if you do a
whole bunch of things _right_ and in accordance with relevant
standards. Few do.
That is also correct. Instruments are *not* "NIST traceable." However, a
measurement made with the equipment can be "NIST traceable" under
certain conditions. It is the PROCESS that produces traceability. (I use
"NIST" here for convenience -- any National Metrology Institute can be
substituted.)
Sadly, you are also correct that few do the process right. In my
experience, about 80% of labs that claim they are calibrating
frequency-measuring equipment to "NIST-traceable" standards really
aren't. (I've had this rather adversarial discussion with several dozen
lab managers over the years.) Of course, this does not mean their
calibrations are not accurate, just that, from the standpoint of legal
metrology, the instruments they calibrate cannot be used to make
NIST-traceable measurements or perform NIST-traceable calibrations.
(When it comes to all of the "NIST calibrated" equipment you see on
ebay, the figure approaches 100% very, very closely.)
The same is true the other way around. You can have all of your
equipment calibrated by a lab that works to NIST-traceable standards,
but if you do not follow through with the traceability process the
measurements you make with those instruments (including calibrations
done using them) will not be NIST-traceable, from a legal metrology
perspective.
Think of the traceability process as a chain. One broken link and
traceability vanishes.
One of the required criteria for traceability is *demonstrated
competence*. Generally, this is done by becoming accredited to the
relevant ISO/IEC standard by an accreditation body. No matter how
sophisticated and meticulous someone is, if their lab is not accredited,
they cannot make NIST traceable measurements or perform NIST-traceable
calibrations. Period. This is where home labs and other "little guys"
inevitably fail to preserve traceability. Again, this does not mean that
their measurements or calibrations are junk -- just that they are not
NIST-traceable, as far as legal metrology is concerned. The A2LA shows
you what you need to do (see link in my last e-mail).
The OP may have thought he was making NIST-traceable measurements using
WWVB and/or LORAN standards. But if his lab was not accredited, that was
not so. Nothing has changed, in that regard, with the advent of GPSDOs
(except that the uncertainty levels are better now).
I agree fully with the above. I was just too lazy to write down the
nitty gritty.
A word of advice for everyone involved. Do not say "NIST traceable"
unless it by particular rules (in US) explicitly says NIST. It's
traceable measurements and calibrations in general, it may be to NIST,
SP, METAS, NPL or whatever, and through the system of international
agreements of measurements they are the same. Only on very rare occasion
you need to go to a particular lab.
"NIST traceable" is what people that doesn't know what they are saying
slap on the front to create some form of confidence. They can have all
honest reasons in the world for it, but very few swinging the term has
any credibility in that claim in my book.
I enjoy whenever I hear "Well, it will not be NIST traceable but...
eh... kinda", which only shows that for once it is someone who has an
idea about the traceability stuff.
Reading up should include the BIPM VIM and GUM, and ISO 17025. Realizing
how legal traceability put requirements on things, it should be apparent
that just adjusting up to GPS time like a GPSDO do, has nothing what so
ever to do with "NIST traceability".
Another aspect with calibration and traceability is that even if you
achieve it, you can then blew it all up by not doing your measurements
correctly, and hence break the chain. This is why ISO 17025 requires
training and continuous training of staff to make sure that the chain
remains unbroken.
Cheers,
Magnus
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