15
3.
Calibration
The ToxiPro features fully automated fresh air/zero and
span calibration functions. The MODE button is used to
initiate the automatic calibration sequence. Calibration
adjustments are made automatically by the instrument.
3.1
Verification of accuracy
3.1.1 Verifying accuracy and
response: ToxiPro O
2
To verify the accuracy of the ToxiPro O
2
, take the
ToxiPro O
2
to an area where the atmosphere is known to
be fresh and check the readings. If the readings differ
from those expected in fresh air (oxygen monitors should
read 20.9% in fresh air), then a fresh air/zero calibration
adjustment must be made as discussed below in
section 3.3. If fresh air is not available, see section 3.7
below for instructions for calibrating the ToxiPro O
2
in
contaminated air.
Honeywell Analytics also recommends that the response
of the oxygen sensor be regularly verified by any of
these methods:
Expose the O
2
sensor to a known concentration
of gas containing less than 19.0% oxygen. If the
descending oxygen alarm is set to 19.5% the
instrument should go into alarm a few seconds
after the gas reaches the sensor face.
Process the ToxiPro O
2
in an IQ Express Dock
that has a cylinder of calibration gas containing
18.0% oxygen (or less) connected to the gas port
and proceed with the oxygen bump test.
Breath test: Hold your breath for 10 seconds,
then
slowly
exhale directly onto the face of the
sensor (in the same way you would to fog up a
piece of glass). If the descending oxygen alarm is
set to 19.5%, the instrument should go into alarm
after a few seconds.
3.1.2 Verifying accuracy: ToxiPro with
toxic gas sensor
Verification of accuracy is a two-step procedure for
ToxiPro instruments equipped with a toxic gas sensor:
1.
Step one is to take the ToxiPro to an area where
the atmosphere is known to be fresh and check
the readings. If the readings differ from those
expected in fresh air (instruments equipped with a
toxic sensor should read 0 PPM in fresh air), then
a fresh air calibration adjustment must be made
as discussed below in section 3.3.
2.
Step two is to test sensor response by exposing
the sensor to a test gas of known concentration.
This is known as a functional (bump) test.
Readings are considered to be accurate when the
display is between 90% and 120% of the
expected values as given on the calibration gas
cylinder. If readings are accurate, there is no
need to adjust your gas detector. See section 3.4
for further details concerning the functional/bump
test.
If the readings are inaccurate, the instrument must be
span calibrated before further use as discussed in
section 3.5.
The accuracy of ToxiPro
instruments equipped with toxic gas sensors should
be checked periodically with known concentration
calibration gas. Failure to check accuracy can lead
to inaccurate and potentially dangerous readings.
See Appendix C for a discussion of calibration
frequency recommendations.
3.2
Effect of contaminants on
ToxiPro sensors
The atmosphere in which the ToxiPro is used can have
lasting effects on the sensors. Sensors may suffer
losses in sensitivity leading to degraded performance if
exposed to certain substances.
The ToxiPro O
2
uses a galvanic oxygen sensor, while
toxic sensor versions of the ToxiPro use an
electrochemical toxic gas sensor. Different types of
sensors use different detection principles, so the
conditions that affect the accuracy of the sensors vary
from one type of sensor to the next.
3.2.1 Effects of contaminants on O
2
sensors
Oxygen sensors may be affected by prolonged exposure
to "acid" gases such as carbon dioxide. The oxygen
sensors used in Honeywell Analytics instruments are not
recommended for continuous use in atmospheres
containing more than 25% CO
2
.
3.2.2 Effects of contaminants on toxic
gas sensors
Honeywell Analytics’s “substance-specific”
electrochemical sensors have been carefully designed to
minimize the effects of common interfering gases.
“Substance-specific” sensors are designed to respond
only to the gases that they are supposed to measure.
The higher the specificity of the sensor, the less likely
the sensor will react to other gases, which may be
incidentally present in the environment. For instance, a
“substance-specific” carbon monoxide sensor is
deliberately designed not to respond to other gases that
may be present at the same time, such as hydrogen
sulfide (H
2
S) and methane (CH
4
).
Although great care has been taken to reduce cross-
sensitivity, some interfering gases may still have an
effect on toxic sensor readings. In some cases the
interference may be positive and result in readings that
are higher than actual. In other cases the interference
may be negative and produce readings that are lower
than actual and may even cause the instrument to
display negative readings for the target gas.
See Appendix A for cross-sensitivity data.
