8/12/2019 Why We Need Calibrate
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Why calibrate
test equipment?
What is Calibration?
Many people do a field comparison check of twometers, and call
them calibrated if they give thesame reading. This isnt
calibration. Its simply afield check. It can show you if theres a
problem,but it cant show you which meter is right. If bothmeters
are out of calibration by the same amount
and in the same direction, it wont show you any-thing. Nor will
it show you any trending youwont know your instrument is headed for
an outof cal condition.
For an effective calibration, the calibration stan-dard must be
more accurate than the instrumentunder test. Most of us have a
microwave oven orother appliance that displays the time in hours
andminutes. Most of us live in places where wechange the clocks at
least twice a year, plus againafter a power outage. When you set
the time onthat appliance, what do you use as your
referencetimepiece? Do you use a clock that displays sec-onds? You
probably set the time on the digits-challenged appliance when the
reference clock is
at the top of a minute (e.g., zero seconds). Ametrology lab
follows the same philosophy. Theysee how closely your whole minutes
track thecorrect number of seconds. And they do this atmultiple
points on the measurement scales.
Calibration typically requires a standard thathas at least 10
times the accuracy of the instru-ment under test. Otherwise, you
are calibratingwithin overlapping tolerances and thetolerances of
your standard render an in calinstrument out of cal or vice-versa.
Lets look athow that works.
Two instruments, A and B, measure100 V within 1 %. At 480 V,
both arewithin tolerance. At 100 V input, A reads99.1 V and B reads
100.9 V. But if youuse B as your standard, A will appear tobe out
of tolerance. However, if B isaccurate to 0.1 %, then the most B
willread at 100 V is 100.1 V. Now if youcompare A to B, A is in
tolerance. Youcan also see that A is at the low end ofthe tolerance
range. Modifying A to bringthat reading up will presumably keep
Afrom giving a false reading as it experi-ences normal drift
between calibrations.
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f
l u k e . c o m / l i b r a r y
Application Note
Youre serious about your electri-cal test instruments. You buy
topbrands, and you expect them to beaccurate. You know some
peoplesend their digital instruments to ametrology lab for
calibration, andyou wonder why. After all, theseare all electronic
theres nometer movement to go out of bal-ance. What do those
calibrationfolks do, anyhow just changethe battery?
These are valid concerns,especially since you cant use your
instrument while its out for cali-bration. But, lets consider
someother valid concerns. For example,what if an event rendered
yourinstrument less accurate, ormaybe even unsafe? What if youare
working with tight tolerancesand accurate measurement is keyto
proper operation of expensiveprocesses or safety systems? Whatif
you are trending data for main-tenance purposes, and two metersused
for the same measurementsignificantly disagree?
