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Appendix B:
Calibration
Frequency
One of the most common
questions that we are asked at
Sperian Instrumentation is:
“How
often should I calibrate my gas
detector?”
Sensor Reliability and Accuracy
Today’s sensors are designed to
provide years of reliable service.
In fact, many sensors are designed
so that with normal use they will
only lose 5% of their sensitivity per
year or 10% over a two-year
period. Given this, it should be
possible to use a sensor for up to
two full years without significant
loss of sensitivity.
Verification of Accuracy
With so many reasons why a
sensor can lose sensitivity and
given the fact that dependable
sensors can be key to survival in a
hazardous environment, frequent
verification of sensor performance
is paramount.
There is only one sure way to
verify that a sensor can respond to
the gas for which it is designed.
That is to expose it to a known
concentration of target gas and
compare the reading with the
concentration of the gas. This is
referred to as a “bump” test. This
test is very simple and takes only a
few seconds to accomplish.
The
safest course of action is to do a
“bump” test prior to each day’s
use
. It is not necessary to make a
calibration adjustment if the
readings fall between 90%* and
120% of the expected value. As an
example, if a CO sensor is
checked using a gas concentration
of 50 PPM it is not necessary to
perform a calibration unless the
readings are either below 45 PPM
or above 60 PPM.
*The Canadian Standards
Association (CSA) requires
combustible gas sensors to
undergo calibration when the
displayed value during a bump
test fails to fall between 100%
and 120% of the expected value
for the gas.
Lengthening the Intervals
between Verification of
Accuracy
We are often asked whether there
are any circumstances in which the
period between accuracy checks
may be lengthened.
Sperian Instrumentation is not the
only manufacturer to be asked this
question! One of the professional
organizations to which Sperian
Instrumentation belongs is the
Industrial Safety Equipment
Association (ISEA). The
“Instrument Products” group of this
organization has been very active
in developing a protocol to clarify
the minimum conditions under
which the interval between
accuracy checks may be
lengthened.
A number of leading gas detection
equipment manufacturers have
participated in the development of
the ISEA guidelines concerning
calibration frequency. Sperian
Instrumentation’s procedures
closely follow these guidelines.
If your operating procedures do not
permit daily checking of the
sensors, Sperian Instrumentation
recommends the following
procedure to establish a safe and
prudent accuracy check schedule
for your Sperian instruments:
1. During a period of initial use of
at least 10 days in the
intended atmosphere, check
the sensor response daily to
be sure there is nothing in the
atmosphere that is poisoning
the sensor(s). The period of
initial use must be of sufficient
duration to ensure that the
sensors are exposed to all
conditions that might have an
adverse effect on the sensors.
2. If these tests demonstrate that
it is not necessary to make
adjustments, the time between
checks may be lengthened.
The interval between accuracy
checking should not exceed
30 days.
3.
When the interval has been
extended the toxic and
combustible gas sensors
should be replaced
immediately upon warranty
expiration. This will minimize
the risk of failure during the
interval between sensor
checks.
4. The history of the instrument
response between
verifications should be kept.
Any conditions, incidents,
experiences, or exposure to
contaminants that might have
an adverse effect on the
calibration state of the sensors
should trigger immediate re-
verification of accuracy before
further use.
5. Any changes in the
environment in which the
instrument is being used, or
changes in the work that is
being performed, should
trigger a resumption of daily
checking.
6. If there is any doubt at any
time as to the accuracy of the
sensors, verify the accuracy of
the sensors by exposing them
to known concentration test
gas before further use.
Gas detectors used for the
detection of oxygen deficiencies,
flammable gases and vapors, or
toxic contaminants must be
maintained and operated properly
to do the job they were designed to
do. Always follow the guidelines
provided by the manufacturer for
any gas detection equipment you
use!
If there is any doubt regarding your
gas detector's accuracy, do an
accuracy check! All it takes is a
few moments to verify whether or
not your instruments are safe to
use.
One Button Auto Calibration
While it is only necessary to do a
“bump” test to ensure that the
sensors are working properly, all
current Sperian gas detectors offer
a one-button auto calibration
feature. This feature allows you to
calibrate a Sperian gas detector in
about the same time as it takes to
complete a “bump” test. The use of
automatic bump test and
calibration stations can further
simplify the tasks, while
automatically maintaining records.
Don't take a chance
with your life.
Verify accuracy frequently!
Please read also Sperian
Instrumentation’s application note:
AN20010808 “Use of ‘equivalent’
calibration gas mixtures”
. This
application note provides
procedures to ensure safe
calibration of LEL sensors that are
subject to silicone poisoning.
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