11
calibration as discussed below in section
3.3. The fresh air/zero calibration takes
less than ten seconds to perform and
does not require an external gas source
when it is performed in a fresh air
environment.
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 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.
Accuracy of
ToxiLtd instruments 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.
3.2 Effect of contaminants on
ToxiLtd sensors
The atmosphere in which the ToxiLtd is
used can have lasting effects on the
sensors. Sensors may suffer losses in
sensitivity leading to degraded
performance if exposed to certain
substances.
3.2.1 Effects of contaminants on toxic
gas sensors
Sperian’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.
3.3 Fresh air/zero calibration
To initiate the fresh air/zero calibration
procedure:
1. From the regular operating screen,
press the MODE button three times
within two seconds. The ToxiLtd will
briefly display “CAL” and then begin a
5-second countdown with the 0-CAL
icon lit.
→
2. Press the MODE button before the
end of the 5-second countdown to
begin the fresh air/zero calibration.
The fresh air/zero calibration has been
successfully initiated when the ToxiLtd
alternates between the following two
screens:
3. The fresh air/zero calibration is
complete when the instrument begins
a second 5-second countdown for the
span calibration. If span calibration is
not required, allow the countdown to
reach 0 without pressing the MODE
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