Hi welcome back to our series on calibration. I'm Bob Stern your host I'm a senior metrologist
at Agilent Technologies. In this module we are going to examine the
concepts related to traceability. So the actual definition of traceability is
that it is the property of the result of a measurement or the value of a standard whereby
it can be related to international system of units via national metrology institutes
through an unbroken chain of comparisons with stated uncertainties.
OK that's a lot of words but what it says in plain English is that you have a measurement
and you want to be able to relate that back to the most accurate standard by a an organization
like NIST or NRL. And the part I've that high-lighted in yellow,
the unbroken chain of comparisons all with stated uncertainties. When they say stated
uncertainties the definition means ISO GUM. So this is the official definition from international
terms for calibration. Let's look at an example in DC Volts. In
the early 1980s, NIST invented a technique for very very accurate generation of DC Voltage
using a Josephson junction which is a voltage across a semiconductor junction when its cooled
by liquid nitrogen. So just to show you the path of traceability
you have going down this little list here the calibration system and then you have the
measurement uncertainty on the right. And the main thing I want to show you is that
the first line, the Josephson junction, that is a primary standard.
So the one that Agilent has is the exact same accuracy as what NIST officially maintains
for the rest of the United States. Or you can go to any number of other national laboratories
around the world and they also use the exact same Josephson Junction
When you use that to calibrate the service test executive volt meter cal system which
we use in Agilent and when you use that to calibrate a Fluke 5720 calibrator then your
accuracy that you can measure other volt meters for DC you can see that it degrades a little
bit. It's not quite as good obviously. It moves down to 4.2 x 10 -6. And if you then
if you use that system to calibrate for lower performance volt meters you can see it grows
from 4.2 x 10 -6 to 7 x 10 -6.
So what are you are seeing there is the traceability path if you were to measure DC volts using
of any one of those three systems Now let me move and show you how it works
for microwave measurement and in this case power traceability. So the international standard
is the Joule or the Watt. And NIST in Colorado and other NMIs around the world have national
standards. In the case of NIST they have a microcalorimeter. That is it is something
they invented. It actually converts RF energy to heat and it's a very very sensitive thermometer.
And what they are able to do is have a transfer standard with that, which then laboratories
such as Agilent can send in our reference power sensors and that becomes the reference
sensor which then can be used as a production working standard. And finally that can be
used to calibrate your power sensor that you send in. So this is the traceability path
for power. So here's the photo of the calorimeter and
thanks very much to Ron Ginley and his section in NIST for providing these figures. So you
see here is the calorimeter and then watch carefully as these go into this assembly in
the gentleman's right hand here. And then this is the water bath and that is
the national standard for power. They then use that to transfer to a standard over here
on the right in the picture. And then over on the right is where you can send in your
standard, your reference standard, to be calibrated by NIST.
So this gives you an idea as you look through each of those three states of the relative
uncertainty, you can see the uncertainty gets larger as you get more steps away from the
national standard. Perhaps most importantly in our traceability
discussion, is this chart here. The x axis is of course frequency, the y axis is the
actual measurement uncertainty of the efficiency factor of the power sensor. And so as you
look at each of these, the lower the line the better. The yellow line is what you would
get, in this case it is coming from the National Physics lab, that's the top national metrology
institute in Britain. That would be very similar to the same one you would get from NIST.
And then in our example, if you use a sensor in that bottom line there to calibrate a reference
sensor as we have done here, that is the special option 84 which we perform on our Roseville
site in California. So you see the uncertainty associated with
that. If you then use that special option 84 calibration to calibrate other sensors
you can see that the resulting uncertainty associated with those is the purple line.
So it all depends on how great your need for measurement uncertainty to keep that limited
determines really which one of these is most appropriate for use for you.
Now any time that you get measurement results from a outside calibration company as part
of that you get a trace report. And what you see here is a list of all the instruments
and then you are also able to see the ID numbers and certificates, so if you wanted to be able
to say OK and show me the exact path back to NIST or one of the other national metrology
institutes, that certificate number would allow you to begin drilling and you could
go back until you get to and in this case say to the Josephson Junction at some point
for DC Volts I want to make one other note here, and that
is in my experience, whether or not the calibration labs provide you a with an actual measurement
report I have yet to see anytime where they don't provide a trace report or said in
a positive way, every calibration lab that I've ever seen provides a cal certification
that has a list of instruments that were used. And that's really another way that can do
an audit because if you're thinking earlier on in our first module we said, think about
which specification are key that can tell you which performance tests should have been
tested. You can look at a trace report like this so
for instance, if you have a spectrum analyzer and you wanted to know was third order intermod,
TOI, tested, you could look at the trace report and would expect to see two microwave signal
generators and if you only saw one than you would know that important measurement could not
have been made. So this is another way where you might do
an audit. If you want to take a look at one of those and you are not sure how to do it,
please contact your Agilent support field engineer and they will help you, or put you
in contact with someone like me who can guide you through that process.
Thank you very much for tuning in and listening to this module on traceability. We hope you've
enjoyed it and I hope to see you again on the next module on standards compliance.