Carbon Monoxide Alarm User Manual

Page 45
Appendix G 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
W ith 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.
Lengthening the Intervals between
Verifications of Accuracy
W e 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. W hen 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.