also at 80.0% methane, well above the upper flammable
limit of methane. Although gas levels above the upper
flammable limit will not propagate a flame, it stands to
reason that somewhere between the leak and clear air
there will be a flammable mixture.
CAUTION
It is possible for the display to drop to a low % LFL
reading after going into high alarm and still have an
unsafe level of combustible gas present.
Therefore, precautions should be taken to ensure
that the combustible gas has been cleared before
classifying the area as safe.
V-5
95-8470
ACTIVE ELEMENT
REFERENCE ELEMENT
THERMAL BARRIER
FLAME ARRESTER
RED
WHT
BLK
B1124
Figure V–2—Catalytic Combustible Gas Sensor
0
90
5
10
15
20
30
40
50
60
70
80
100
100
90
80
70
60
50
40
30
20
10
0
CONTROLLER
DISPLAY
READING
(%LFL)
PERCENT OF OXYGEN IN MIXTURE (BY VOLUME)
B1126
50% LFL METHANE IN STANDARD AIR
4.5% METHANE
2.5% METHANE
90% LFL METHANE IN STANDARD AIR
Figure V–3—Effect of Oxygen Enriched and Deficient Atmosphere on the Response of a Typical Catalytic Sensor