Table of Contents
Advertisement
22.
APPENDIX E:
22.1. Cross Sensitivity - Toxic Sensors
By varying the material construction of the electro-chemical cells, sensors can be optimised towards different gas
types along with differing sensitivities. An important point to note however, is that , for any given gas sensing cell
type, even though optimised for a particular target gas, there may be sensitivity to other gases. This is known as
'cross sensitivity' and is inherent in the mechanism by which the cells operate. The following table gives an indication
of cross sensitivities for each of the D-Guard
of false readings or possible alarm conditions due to the effects of cross sensitivity. Locating a detector in an
environment that is likely to contain gases other than the one specifically being measured can cause operational
problems.
Table of percentage cross sensitivity for typical D-Guard
2
Interfering gas
ppm
D-Guard
CO
CO
400
n/a
H
S
20
0
2
H
400
45%
2
SO
20
0
2
NO
10
0
2
NO
50
8%
C
H
400
52%
2
4
Cl
10
1%
2
NH
20
0
3
CO
5%
0
2
As an example:-
400ppm of hydrogen sulphide would produce a response equivalent to 180ppm in a carbon monoxide sensor per the
table value of 45% cross sensitivity (0.45 x 400ppm = 180ppm equivalent response).
The actual displayed value and the value transmitted over the current loop will ultimately depend on the actual
detector full scale range. Cross sensitivity data relates to the raw signal from the gas sensor before any scaling is
applied.
* O2 sensors do not respond to CO
22.2. Cross Sensitivity - Infra-Red Sensors
The cross sensitivity for infra-red sensors includes all hydrocarbon gases that share the same absorbance wavelengths.
For a detector specified to measure methane (CH
employed. The sensor will respond to other hydrocarbons but will not necessarily produce an accurate or linear
response. For applications designed to monitor methane where multiple hydrocarbons may be present, allowance
must be made for the possibility that the detector will read higher than expected due to the increase in sensor
response to the multiple gases.
2
detectors. This information is very useful in determining the likelihood
2
2
D-Guard
NH
D-Guard
H
S
D-Guard
3
2
0
3%
0.5%
2%
n/a
0
0
0.6%
0
-
15%
n/a
-
-25%
-100%
-
0
-110%
-
0.5%
30%
-
-20%
-38%
n/a
0
0
0
0
0
, but exhibit a change in sensitivity to Oxygen when exposed to CO
2
), a calibration curve that provides a linear response to methane is
4
2
gas detector types.
2
2
2
SO
D-Guard
ETO
D-Guard
NO
2
40%
0.1%
-
60%
-
0.1%
-
4%
-
5%
-
n/a
-
-
-
5%
-
0.1%
-
0.1%
61
D-Guard
2
2
D-Guard
NO
D-Guard
Cl
D-Guard
2
2
0
0
-
-88%
-88%
-
0
0
-
-1%
-1%
-
n/a
100%
-
0.4%
0.4%
-
0
0
-
100%
n/a
-
0.2%
0.2%
-
0
0
-0.03/% *
2
2
2
O
2
Advertisement
Table of Contents
