Appendix: Limitations of sensors
Sensor limitations
The sensors used in Tetra have limitations common to all such gas sensors, and
users should be aware of the points listed below. Crowcon can advise on par-
ticular situations and suggest alternative sensors if the instrument is likely to
experience extreme conditions.
Tetra uses a catalytic combustible gas sensor, which measures the combustibil-
ity of the gas. for this reason, readings displayed on the unit will be unreliable
over concentrations of approximately 120% LEL. Oxygen is necessary for cat-
alytic sensors to operate. A 'catalytic bead saver' is used to disconnect power
to the catalytic bead sensor in the event of over-range to prevent burn out. This
locks out for 200 seconds after which a button press will reconnect power to
the catalytic bead. 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 catalytic
bead sensor. Restart should be carried out in a known fresh air environment.
Depleted oxygen levels can reduce the combustible gas reading, and if oxygen
levels are below safe breathing limits it should be assumed that the combustible
reading is low.
Electrochemical gas sensors contain chemicals. Extreme levels of humidity can
also cause problems. The sensors are rated for an (average) ambient of 15-90%
R.H. However they are used from the tropics to deserts to tundra without this
normally being a problem.
Water should not be allowed to collect on the sensors as this may impede gas
diffusion.
Persistent exposure to high levels of toxic gas will shorten the life of toxic sen-
sors. 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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Charging batteries Tetra
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