Natural Science Forum / Physics / Acoustics / October 2005

Calibration is a funny thing.   The most useful calibration is the one that
is done after the measurement. If it is OK it is very unlikely that the
meter was not working properly when the measurement was done (count the
negatives!)  But that's not the calibration certificate that you put a copy
of in your report.   Of course the calibration before the measurement is
also useful: that makes it less likely that you are wasting effort by making
a measurement that will later have doubt cast on it.
As you say "barring any malfunction".  Some malfunctions can be subtle, for
example non-linearity in the A to D converter.  One has to take a view as to
the likely stability of the equipment in question, and the likelihood that
you would notice anything important being wrong.  I do think it is important
to do what checks are possible between calibrations.
Satisfying the lawyers is another matter.  Most organisations suggest 2
years but some manufacturers say 1 year  intervals, in which case the only
way to be really safe from criticism is to follow the shorter time.

We recently went through a discussion of this in a standards committee.  A
proposal was made to put a requirement in a measurement standard that all
instruments had to have been certified by a calibration lab within a year
before the measurements.  However, many people (including one who operates a
calibration lab) argued that this was not necesarily reasonable.  It was
noted that in the US now the laboratories that meet one of the accreditation
standards are not allowed to put a sticker specifying a due date on the
instrument unless the customer requests it, and then they must put the
interval requested by the customer.  Basically it is up to the user to
specify what he believes to be the necessary interval based on usage of the
instrument, experience with it, and expectations of his customers.  That is
where you have to look at any requirements in regulations that you must meet
when doing measurements.  The guidance given by one good US accredited
calibration lab can be found here

http://www.scantekinc.com/calibration.htm#5

In our case for the measurement standard, we wound up requiring that the
calibration date be reported in the test report but not specifying an
interval.

Tony Woolf makes a good point that was brought up in our discussions, that
the most significant calibration would be one done after the measurement.
After all, something could happen to the instrument during shipment from the
lab rendering it out of calibration once your receive it.  This is a reason
I would never be without at least two calibrators and some means of checking
some meter properties other than one level at one frequency.

The old coal mine noise standard (Pre 2000) required that a noise dosimeter
and an acoustical field calibrator be laboratory calibrated annually.

When a noise dosimeter (or sould level meter) is used for a field
measurement, a field calibration is required both before and after the
survey on which it is used.

Although not in the current regulations, we follow the same policy.

The ANSI standard for acoustical calibrators recommends an annual laboratory
calibration.

The ANSI standard for the Measurement of Occupational Noise Exposure
recommends an annual laboratory calibration for noise dosimeters, SLMs, and
acoustical calibrators, plus before and after checks on all equipment used
in a survey.

You may wish to take a look at an article entitled "Calibration of
Acoustical Instruments for Noise Exposure Measurements" by Seiler and
Giardino, Sound and Vibration Magazine, March 1996,