98
Appendix A
Supplemental Information about Gases and Sensors
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 provide 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 Cross-sensitivity guidelines
(%)
Sensor
Target Gas
CO
CO/H
2
Low
H
2
S
SO
2
NO
2
HCN
NH
3
CO
100
100
1
1
0
0
0
H
2
S
5
5
100
1
-40
10
25
SO
2
0
5
5
100
0
—
-40
NO
2
-5
5
-25
-165
100
-70
-10
Cl2
-10
0
-20
-25
10
-20
-50
ClO
2
—
—
—
—
—
—
—
HCN
15
—
—
50
1
100
5
HCl
3
—
—
5
0
0
0
PH
3
—
—
—
—
—
425
—
NO
25
40
-0.2
1
5
-5
0
H2
22
3
0.08
0.5
0
0
0
NH
3
0
0
0
0
0
0
100
The values supplied above are estimates. They generally apply only to new sensors used for monitoring gases in these environmental
conditions: 20 °C (68 °F), 50% RH, and 1 atm. Values are subject to change.
“—” indicates no available data.