Rae FMC-40, Operating Handbook

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7
Ventilation of the room will of course alter the situation significantly but it should be
remembered that if the ventilator is not at ceiling level, a dangerous concentration can still
occur between the top of the ventilator and the ceiling.
For heavier than air gases such as propane or butane, the formation of dangerous layers
occurs at ground level. These gases tend to behave like water and will run down gradients
and pool at the lowest point.
The number of heads required in individual rooms is determined by the number of possible
hazards in the vicinity.
Gas leakage may occur around valves, flanges and anywhere where gas pipes are jointed.
It may be possible to cover several probable gas leaks in one room by the careful siting of
a single head. Cable ducts, trenches and manholes are also likely places where a build up
of heavy gases may collect.
When siting a head in such places it is most important to ensure that there is no likelihood
of flooding by water, or excessive dust which may block the sintered disc and prevent gas
reaching the sensor.
When monitoring gases outside, those lighter than air will be quickly dispersed, but gases
heavier than air will tend to form in layers and again cause a dangerous hazard. When
siting heads outside prevailing winds must be taken into consideration and adequate
protection given against wind and rain.
POISONING OF CATALYTIC SENSORS
Catalytic elements used in flammable gas sensors are liable to be rendered inactive due to
ʻpoisoningʼ by certain groups of compounds.
In general contact with any gaseous compound capable of producing an involatile residue
upon heating is to be avoided.
Examples of such substances are:
a. Silicon containing vapours, as emitted by silicone polishes, greases and oils.
b. Petroleum vapours containing tetra-ethyl lead or other organo-metallic compounds.
c. Phosphorus in the form of phosphate esters.
These compounds will permanently affect the detector and if their presence is suspected
the response of the detector should be determined by the calibration procedure.
It is also possible that the reaction of the detector to a flammable gas could be inhibited by
halogen containing gases such as chloroform, carbon tetra chloride and trichloro-ethylene.
this effect is not permanent.