Hello, I’m Jim Salsbury with Mitutoyo America Corporation and welcome to the
metrology training lab in this episode we're going to talk about traceability
or sometimes called metrological traceability here's the question that
traceability answers say I make a measurements with this caliper how do I
know that the inches or millimeters on this caliper are the same as everybody
else's inches or millimeters how do I relate this measurement back to some
defined standard like the definition of the mirror or the inch clearly
traceability is critical at measurements so how do you insure it this is not a
new problem over 5,000 years ago in ancient Egyptian culture they understood
this their standard was the cubit it looks something like this and they're
traceable calibration system helped enable them to build some amazingly
accurate structures like the Great Pyramid
while the cubit in its history is pretty cool things got even more interesting in
the late 1700s with the birth of the meter which eventually led to the entire
metric system known as the SI system of units so back in the late 1700s a bunch
of moderately crazy scientists decided that a new standard of length the meter
needed to be based on something that we all own together and they chose planet
Earth they measured the circumference of the earth and a fraction of that was
used to establish the meter but what was most important was that countries all
around the world agreed to use this meter
so here's how traceability worked then in a high-level government lab near
Paris France they had what was called the official
prototype meter it was a bar made of a special platinum iridium alloy and then
other similar meter bars were made and compared to that one these bars were
sent all around the world in the United States we had meter bar number 27 which
was used to disseminate length traceability across our country our
meter bar number 27 is in a museum now we don't use it anymore as technology
advanced the world needed something better than passing around metal bars
today the meter is defined and how far light travels in a very short amount of
time in a vacuum so basically anybody can create their own meter all they need
is a stopwatch and a light well that would be a really really really accurate
light and a really really really accurate stopwatch so maybe most people
can't do that but here in the United States a group of smart scientists take
care of that for us they work for the US Department of Commerce at a place called
NIST the National Institute of Standards and Technology they do a great job on
this important task for our country so let me just say thanks to all those
smart men and women and neste who take care of this for us but how exactly are
the measurements made using this caliper traceable to the meter or inch that NIST
let's walk through an example say this caliper is used at some manufacturing
organization and they calibrate this caliper themselves in-house using a
caliper checker something like this to calibrate the
caliper checker the company then sends them to some local calibration lab and
that local calibration lab who has their own gauge blocks then sends these gauge
blocks to a higher level primary calibration lab like the one that we
have here at military America the company that owns the caliper
doesn't send their caliper checker to NIST the local calibration lab doesn't
send their gaze blocks to NIST the reason for that is simple
they don't need that level of accuracy and they sure don't want the cost in
lengthy turnaround time and NIST however here at mid Toya merica being a higher
level lab we send our master gaze blocks to NIST
and then internally here we calibrate what we call our working masters against
the NIST calibrated masters and then the working masters are used daily to
calibrate customers items in the end our master gaze blocks are actually used
very infrequently which allows us to absorb the NIST turnaround time and also
helps preserve our masters for decades of use at NIST our master blocks are
then compared to their masters which are in turn compared to laser systems which
are directly linked to the realization of the meter at NIST so it goes from
NIST masters to their gaze blocks to our masters to our working masters to the
local labs masters to the customers caliper checker to the customers caliper
to the parts being measured on the shop floor and that's traceability but how
does the owner of this caliper trust that system they are relying on a chain
of multiple calibration suppliers to ensure the quality of their measurements
made with this caliper the best way to ensure rock-solid traceability is to use
what we call accredited calibration laboratories the use of accredited
calibration labs is required in many industries but even if not it is highly
recommended the standard used in accreditation is called iso/iec
17 Oh 25 and it is an internationally recognized standard for competency and
calibration and testing now they become accredited a laboratory must demonstrate
traceability they are also audited regularly by measurement experts to see
if the work is being done correctly and they must produce what's called a scope
of accreditation which defines what they can do and how well they can do it
scopes are publicly available documents and are usually found on organizations
websites you can also easily find them on the website of the different
accreditation bodies so this is the scope for mitad TOI is accredited lab if
you're wondering if we can calibrate gates blocks then just look on our scope
the scope also lists what's called the CMC or the calibration measurement
capability which is a measure of how well our lab can calibrate something the
CMC is also sometimes called the best measurement uncertainty now we're not
getting into what uncertainty is in this episode but let's just say that a lower
uncertainty or a lower CMC means the lab can measure more accurately scopes are
pretty powerful in a very easy way for you to vet calibration suppliers or as I
like to do to spy on my competition so I hope you've learned a bit about
traceability and accreditation in this episode that will help you manage your
calibration systems better Thank you, I’m Jim Salsbury, and I’ll see you next
time from the Metrology Training Lab.