Poisons
Some compounds will decompose on the pellistor and form a solid barrier over its surface.
This action is cumulative and prolonged exposure will result in an irreversible decrease in
sensitivity. The most common of these substances are:
lead or sulphur containing compounds,
silicones, phosphates.
It may be possible to recalibrate a sensor that is known to have been exposed to a poison.
However Status Scientific Controls strongly advise sensor replacement in these
circumstances.
Inhibitors
Certain compounds (especially H
2
S and halogenated hydrocarbons) are absorbed or form
compounds that are absorbed by the pellistor. This absorption is so strong that reaction
sites within the pellistor become blocked and normal reactions are inhibited. The resultant
loss of sensitivity is temporary and in most cases a sensor will recover after a period of
operation in clean air.
Pellistors will detect numerous flammable gases. The level at which they respond varies
depending upon the flammable gas. Contact Status Scientific Controls for details regarding the
cross sensitivity of flammable gases.
5.5.2 Oxygen Gas detectors
At very high levels (i.e. % levels), highly oxidising gases (e.g. ozone and chlorine) will interfere to
the extent of their oxygen equivalent
*
, but most other commonly occurring gases will have no
effect. For example:
Acid gases such as CO
2
and SO
2
will be slightly absorbed by the electrolyte (within the oxygen
sensor) and tend to increase the flux of oxygen to the electrode. This gives an enhanced oxygen
signal of about 0.3% of signal per 1% CO
2
. The oxygen gas detectors are not suitable for
continuous operation in concentrations of CO
2
above 25%.
*
Gas detectors should not be subjected to prolonged exposure to highly corrosive atmospheres
as this will cause premature failure.
5.5.3 Toxic Gas Detectors
Toxic gas sensors are known to be cross sensitive to gases other than those for which they have
been designed to detect. The amount of response observed by a sensor will vary depending upon
the sensor type and the interfering gas.
For example:
This table shows the cross-sensitivity of the H
2
S sensor fitted to the FGD2 and 3 Detector Heads.
The table shows the typical response to be expected from a sensor when exposed to a given test
gas concentration.
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