Biosystems PhD Ultra Reference Manual - page 59
59
Appendix F 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