TanGO TX1 - Appendix A; Supplemental Information about Sensors and Gases; Toxic Gases

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Appendix A
SUPPLEMENTAL INFORMATION ABOUT SENSORS AND GASES
Toxic Gases
A sensor is designed to detect for and measure the presence of a particular gas, the "target gas"; however,
it may also respond to other gases. When this is the case, the sensor is said to have "cross-sensitivity" to
another gas, which will interfere with the target-gas readings. Table A.1 provides insight to the levels of
cross sensitivity that can exist and whether a nontarget gas will have the effect of adding to or subtracting
from the target-gas readings.
For example, a site is being monitored for H
2
S; the air also contains NO
2
. According to table A.1, the H
2
S
sensor will respond to NO
2
, so the H
2
S readings will account for both gases. Because the NO
2
cross-
sensitivity value is negative (-25%), its presence will subtract from the H
2
S readings, which will generate an
H
2
S reading that is lower than the actual concentration of H
2
S contained in the air sample.
When a cross-sensitivity value is positive, the opposite will happen. When a gas has a positive cross-
sensitivity value, it will add to a sensor's target gas reading, which will generate a reading that is higher
than the actual concentration of the target gas contained in the air sample.
Table A.1 Sensor cross-sensitivity guidelines (percent response)
Target gas
Sensor type
Carbon
Monoxide (CO)
%
CO/H2 Low
%
Hydrogen Sulfide
(H
2
S)
%
Nitrogen Dioxide
(NO
2
)
%
Sulfur Dioxide
(SO
2
)
%
Carbon Monoxide (CO) 100 100 1 0 1
Hydrogen Sulfide (H
2
S) 5.0 5 100 -40 1
Sulfur Dioxide (SO
2
) 0 5 5 0 100
Nitrogen Dioxide (NO
2
) -5 5 -25 100 -165
Chlorine (Cl
2
) -10 0 -20 10 -25
Chlorine Dioxide (ClO
2
) — — — — —
Hydrogen Cyanide 15 — — 1 50
Hydrogen Chloride 3 — — 0 5
Phosphine — — — — —
Nitric Oxide 25 40 -0.2 5 1
Hydrogen 22 3 0.08 0 0.5
Ammonia 0 0 0 0 0