[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






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