[time-nuts] Traceability after loss of LORAN and WWVB
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Sat Jun 1 18:19:09 EDT 2013
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).
Best regards,
Charles
More information about the time-nuts
mailing list