11
carbon dioxide. The oxygen sensors used in
Sperian 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
Sperian Instrumentation’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:
1. From the current gas reading screen,
press the MODE button three times within
two seconds to begin the fresh air/zero
calibration sequence. The ToxiPro 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 ToxiPro alternates
between the following two screens:
↔
3. For instruments equipped with a toxic gas
sensor, 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 button. For
further instructions concerning the span
calibration of toxic sensor-equipped
models, proceed to section 3.5.
For ToxiPro O
2
models, calibration is
complete when the instrument returns to
the current gas readings screen. If the
fresh air/zero calibration attempt fails,
proceed to section 3.6.1.
Fresh air/zero
calibrations may only be performed in an
atmosphere that is known to contain
20.9% oxygen and 0 PPM toxic gas.
Performing the fresh air/zero calibration in
an atmosphere that is not comprised of
20.9% oxygen and 0 PPM toxic gas may
lead to inaccurate and potentially
dangerous readings.
3.4
Functional (bump) testing (toxic
sensor versions)
The accuracy of ToxiPro instruments may be
verified at any time by a simple functional
(bump) test. To perform a functional (bump)
test, do the following:
1. Turn the ToxiPro on and wait at least
three minutes to allow the readings to fully
stabilize. If the sensor has just been
replaced, it must be allowed to stabilize
prior to performing a functional (bump)
test. See section 4.2 for further details.
2. Make sure the instrument is located in
fresh air.
3. Verify that the current gas readings match
the concentrations present in fresh air.
The reading for toxic gases should be 0
parts-per-million (PPM) in fresh air. If the
reading is anything other than 0 PPM
proceed to section 3.3 and perform a
fresh air/zero calibration before
continuing.
4. Apply the calibration
gas as shown in
figure 3.4.
Figure 3.4 Proper
bump-test/span
calibration set-up
for toxic sensor-
equipped
instruments.
Note: ToxiPro instruments equipped with
a chlorine dioxide (ClO
2
) sensor require a
chlorine dioxide generator as a calibration
gas source.
