Hello, I’m Jim Salsbury with Mitutoyo America Corporation and welcome to the
Metrology Training Lab. In this episode, we’re going to discuss Calibration, which
is critical to maintaining the quality and accuracy of your measuring equipment.
Unfortunately, there is a lot of confusion in practice associated with
“what is” calibration. It’s been our experience here at the Metrology
Training Lab that this confusion in calibration is very costly to
organizations – sometimes directly in poor use of your calibration dollars but more
importantly, by increasing quality risks that could explode into costly problems
with your products. Let’s start with an example. Say you have hired somebody to
calibrate this Mitutoyo linear height gage. What do you expect they will do?
And are you sure they know your expectations? Is it enough for them to
say take a few readings on some gage blocks, put a new calibration label
on the unit, and give you a Calibration Certificate with those readings? Did you
expect them to compare the readings to a tolerance? And to what tolerance? Did
you expect them to make adjustments to the accuracy if the unit is out of
tolerance? And are length measurements enough, or are there other
important items to check on this linear height gage? There really is no single
or correct answer to these questions – it depends on what you need. In our
experience, there are five distinctly different activities that are often
called calibration. So let’s start with a review of the basic concept of
calibration. In its simplest sense, calibration is about transferring some
measurement unit, like length in inches or mm, from some traceable
measurement standard to another piece of measuring equipment. For example, this
Mitutoyo Checkmaster, a type of step gage or length standard. To calibrate this, I could use a comparison
technique, for example with this gage block, which was itself previously calibrated, and transfer traceable length
from the gage block to the Checkmaster. Now let’s get a little more
official, as there are national and international
standards that define the term calibration. Let me get my glasses
I’ll read you the most widely accepted definition of calibration. This is from a
document called ISO/IEC Guide 99:2007, otherwise known as the (VIM)
International vocabulary of metrology so according to this definition and by the
way you can download a free copy of this document the vehm from our on-demand
educational resources at mitutoyo.com alright so here we go here's the
definition of calibration according to the top experts it's an operation that
under specified conditions, in a first step, establishes a relation between the
quantity values with measurement uncertainties provided by measurement
standards and corresponding indications with associated measurement
uncertainties and, in a second step, uses this information to establish a relation
for obtaining a measurement result from an indication” I’m
sorry. I had to be complete and give you the official
definition, but it is quite a mouthful. So much for the theory – let’s get more
practical. What the second part of that definition is saying is that calibration
really begins with an understanding of how you are using the measuring
equipment. What that means, is as the owner of the measuring equipment, you
need to realize that how you are using that equipment impacts how it needs to be
calibrated. And since not everybody uses measuring equipment in the same way, that
leads to different organizations having different understandings of calibration practices.
Alright, as I said earlier, what we’ve learned over the years here in the
Metrology Training Lab, is that there are five very different activities, in
practice, that are commonly called calibration. So let’s use this linear
height to discuss these various “types” of calibrations.
One of the most common goals of calibration is to see if the accuracy of measuring equipment is
within some specified tolerance limits. This type of calibration is often called
a test, or conformity acceptance test, or
a verification. For this linear height gage, this type of test is often done using
a length gage like this Checkmaster that you see here it would involve
taking a series of linear height points at various locations
I can zero on the surface plate, measure a point on the Checkmaster,
and compare the measured value with the calibrated value of the
Checkmaster. The resulting error is then compared to the tolerance.
This conformity acceptance type of calibration does not apply to all
measuring equipment. For example, for this Checkmaster, the purpose of the calibration
or what I need as the owner of this gage in order to use it the way I want – are
calibrated values, or reference values for each point of the Checkmaster.
In this case, there are no tolerances to be
checked in the calibration. We see that these two pieces of equipment the
calibration of the check master and the calibration a linear height we need
something different from calibration the check master needs the reference values
and the linear height needs to be tested to see if it conforms to tolerance these
are the first two types of calibration now before we sketched the third type of
calibration what we need to talk about is what needs to get calibrated on your
measuring equipment for this linear height if you're only measuring Heights
then this test with the check master for measuring height measuring accuracy is
the only thing that you need calibrated however this linear height has
additional functions it can measure things like straightness and
perpendicularity and if you're using these additional functions then you need
additional stuff to be calibrated for example we have a ceramic square over
here and if I'm not using the straightness and perpendicularity
functions then I don't want to waste my money getting that calibrated so as the
manufacturer of the height gage we can give you some advice in the calibration
but the ownership of what needs to be calibrated belongs to you the user of
the measuring equipment and with today's sophisticated measuring equipment this
is a real problem organizations are often not getting the
right things calibrated which can waste money or even worse expose your
organization's to risks associated with quality all right now let's move on and
discuss the most controversial use of the term calibration there are times
when measuring equipment needs to be adjusted or corrected or tweaked or
compensated to make it more accurate possibly to bring it back to within the
specified tolerances these adjustments as important as they are do not meet the
official definition of calibration now please don't get mad at me for this or
post nasty comics I'm just the messenger on this one
honestly in our experience here the metrology train lab a majority of
organizations that we work with will claim that in their opinion calibration
includes proper adjustments and that measuring equipments isn't calibrated
unless it has been demonstrated to be working within tolerance so this is a
big problem in the calibration business you could hire somebody to calibrate
this linear height they could do some tests find the height to be
significantly higher tolerance and they could still issue you an accredited
calibration certificate with those results they can still stick a new
calibration label on the instruments hand you an invoice and say all done
possibly without ever discussing the auto tolerance condition with you
so this bothers me and hope it bothers you as well this is a problem born from
the different historical usages of the term calibration and also price pressure
if you are hiring somebody to do a calibration protect yourself by being
clear in your purchase orders don't just say calibrate this gauge as we often see
in this business instead be more specific say something like calibrate
midotaur linear Heights check against the
accuracy tolerance for Heights and straightness and perpendicularity if you
need that perform facture recommended preventive
maintenance make necessary adjustments to be within original manufacturer
specified tolerance limits report the as found readings prior to any service and
adjustment and then report the as left readings after all the final adjustments
and finally issue an iso/iec 17025 accredited calibration certificate okay
we've discussed three different activities often called calibration the
determination of reference values as we would want for this check master a
conformity test against tolerance as we would want for this linear height and
the question of adjustments there are two other calibration activities that we
need to discuss that are a little less formal and usually done by the user in
the routine daily use of the measuring equipment on this linear height for
example there are interchangeable probes when each of the different probes is
used at some point the diameter of the tip of the probe needs to be calibrated
on this linear height this is usually done with this probe diameter
calibration block these type of user calibrations are quite common from tip
sizes on things like coordinate measuring machines the magnification for
optical systems the detector gain adjustments even the zero setting on
large outside micrometers so these calibrations need to be done correctly
as they are often tweaking some temporary adjustment to the measuring
equipment if I'm off five microns when calibrating the probe diameter all
my measurements will be off five microns it's also important to recognize that
doing this probe calibration is not calibrating the entire instruments
measuring equipment that has these user calibrations still need to be calibrated
as in the first three types of calibrations we discussed earlier okay
the fifth and final type of calibration we want to discuss in this episode is
sometimes called verification of calibration or interim test or check
standard so to reduce risk between full calibrations which may be a year or
longer many organizations like to implement
some type of quick interim test to monitor the status of the measuring
equipment these are usually shortened versions of
a full calibration for example as linear height the user may have say one single
gauge block that's measured every month or every week or possibly every day
depending on the acceptable level of risk and the use of the linear height so
there you have it five very different activities that are sometimes called
calibration well post a summary of those at the end of this episode I hope you've
learned a few things that will help you reduce risks in your organization and
the next time that somebody asks you to calibrate something like this linear
height you should ask do you need this calibrated or probably just calibrate it
and I assume you want it calibrated if needed and don't forget to check the
calibration and always do a calibration before use with this calibration gauge
which by the way also needs to be calibrated thank you I'm Jim Salisbury
and we'll see you next time from the metrology training lab