Gasman Appendix: Limitations of sensors
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Appendix: Limitations of sensors
Sensor limitations
The sensors used in Gasman have limitations common to all such gas
sensors, and users should be aware of the points listed below. Crowcon
can advise on particular situations and suggest alternative sensors if the
instrument is likely to experience extreme conditions.
Gasman Flammable uses a catalytic gas sensor, which measures the
flammability of the gas. For this reason, readings displayed on the unit will
be unreliable over concentrations of approximately 120% LEL. Oxygen
is necessary for catalytic sensors to operate. A 'pellistor saver' is used to
disconnect power to the pellistor sensor in the event of over-range to
prevent burn out. This locks out the unit for 200 seconds after which a
button press will reconnect power to the pellistor. If the sensor power is
reconnected when the unit is exposed to an over-range gas concentration
there is a risk of damage to the pellistor sensor. Restart should be carried
out in a known fresh air environment. Depleted oxygen levels can reduce
the flammable gas reading, and if oxygen levels are below safe breathing
limits it should be assumed that the flammable reading is low.
Electrochemical gas sensors, toxic gases or oxygen, contain chemicals.
Extreme levels of humidity can also cause the sensor to be unstable. The
sensors are rated for an (average) ambient of 20-90% R.H. However they
are used from the tropics to deserts to tundra.
Water should not be allowed to collect on the sensor as this may impede
gas diffusion.
Persistent exposure to high levels of toxic gas will shorten the life of a toxic
sensor. If the high level gas is corrosive (e.g. hydrogen sulphide) damage
may occur over time to metal components.
Sensors may be cross sensitive to other gases. If unsure, contact Crowcon
or your local agent.
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