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
‘poisoning’ 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.
FIRE SENSORS
See separate supplier manual
COMMISSIONING
Gas Sensors
Before applying power to the instrument ensure that all detector heads are connected to the sensor
terminals on the printed circuit board (fig3 & 4) and that each detector head is connected to its
appropriate channel, identified by a small circular, coloured label:
Red = flammable
Yellow = toxic
Blue = oxygen
Green = others
Switch on power to the instrument.
Allow ten minutes for the detector head to stabilize.
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