Calibrate - Metrology Training Lab (What is Calibration?)

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 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