[volt-nuts] A Fluke 732A

Sun Mar 9 18:02:39 EDT 2014

I have to side with Tom on this one. Accreditation only provides independant verifcation of processess and proceedures. It provides increased confidence (in both the human and statistical senses) in the service provided. It does not prove that your standard is correct. The typical time nut does not need accredited calibration. True, if a commecial cal lab is any good it will have accreditation, but if they only do work in-house they may not need it. If I could get access to a JJ in a local non accredited research lab to compare my 10V  reference to, I'd have very high confidence that it is correct. If  I can make repeated visits over time and document those, even better. I personally think that this would be better than hot shipping it to a remote accredited lab. It might also release limited funds for a second or third reference for even greater confidence. I've had to recall aircraft equipment that was mis-adjusted due to a bench test set being
 calibrated incorrectly. All the links in the chain were accredited and documented. This was finally traced to the manufacturers reference standard being off following calibration (at a high level). The conclusion was a shift during transport. They had calibrated several bench test sets before we caught it. It was only because the unit failed badly on this minor parameter during an aircraft check at the same base as the workshop and then failed again after repair that it was spotted. I continually complain about over calibration at work. For example 12 monthly calibration of variable bench DC power supplies. just stick an "indication only" sticker on them and use an external meter if the voltage is critical. 

Robert G8RPI.

________________________________
 From: Tom Knox <actast at hotmail.com>
To: Discussion of precise voltage measurement <volt-nuts at febo.com> 
Sent: Sunday, 9 March 2014, 21:07
Subject: Re: [volt-nuts] A Fluke 732A

Charles is difficult to disagree you are simply explaining the system that has evolved over decades that insures the accuracy of our standards labs, and I have found I am barely smart enough to get coffee for some of the Senior Time and Volt Nuts members. You have made your point very eloquently and I don't necessarily disagree with anything you say. But I feel reading your response I have failed to make my point.
I will make one last effort using a example. If I send a 732A to Fluke and have it calibrated, and you stop by with yours shortly after mine returns, has a day or so to settle, and appear correct with my 3458A, you could then calibrate your 732A using of mine and feel fairly confident you standard is also a few PPM from 10 volts. Now lets stay you repeat this with another Volt-Nut when his standard returns. You do not have documents proving the accuracy of your standard because none of the standards used are parts of a accredited lab, but you can be confident yours is correct and You have save nearly $1000 dollars. There is what you can prove, but there is also what you know using common sense. Many Volt-Nuts also work in Metrology or know someone who does. In my work these days solutions at the cutting edge of Metrology are often as much art as science. That intuition often leads to advances that are both document-able and repeatable. So as I brought up
 earlier if Volt-Nuts compared
  standards and shared data locally they could build there own model and uncertainty algorithm that although not recognized by accrediting bodies could achieve similar results.  If you can afford to fully document the instruments in you lab I encourage you to do so and support the industry, but that can be beyond the budget of even some serious researchers and exploring alternatives seems to be what the Volt-Nuts are all about. Also I have been collecting data for a number of years hoping I can find time to do a serious study showing the effects of shipping on calibration. And I can tell you my early data points toward the fact that a substantial part of the yearly uncertainty happens during shipping. I am also convinvced that in that next decade instruments will include an environmental sensor package, and cal will be based more on environmental expose then simple time. As Physicists have pushed the limits of Metrology in recent years there has been
 little research in insuring these
  improved accuracies reach the end user. Expect changes in coming years.

Thomas Knox

> Date: Sun, 9 Mar 2014 04:18:07 -0400
> To: volt-nuts at febo.com
> From: csteinmetz at yandex.com
> Subject: Re: [volt-nuts] A Fluke 732A
> 
> 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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