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Appendix D Calibration Frequency
One of the most common questions
that we are asked at Biosystems is:
“How often should I calibrate my
gas detector?”
Sensor Reliability and Accuracy
Today’s sensors are designed to pro-
vide 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 any
significant loss of sensitivity.
A lot of sensors indeed do last that
long with only minimal loss of
sensitivity. However, there are a
number of reasons why a sensor may
unexpectedly lose additional
sensitivity or even fail to respond to
gas. Such reasons include
desiccation, poisoning, physical
restriction of airflow, overexposure,
leakage, and mechanical damage due
to dropping or immersion.
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
are 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 the
instrument 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
Verifications of Accuracy
We are often asked whether there are
any circumstances in which the period
between accuracy checks may be
lengthened.
Biosystems is not the only
manufacturer to be asked this
question! One of the professional
organizations to which Biosystems
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. Biosystems
procedures closely follow these
guidelines.
If your operating procedures do not
permit daily checking of the sensors,
Biosystems recommends the
following procedure to establish a
safe and prudent accuracy check
schedule for your Biosystems
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
Biosystems gas detectors offer a one
button auto calibration feature. This
feature allows you to calibrate a
Biosystems 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 Biosystems’
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.
All of Biosystems Applications Notes
are located on the Biosystems website
at
http://www.biosystems.com
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