Crowcon Tetra MO7237, Руководство пользователя, Страница: 34 / 36

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
particular situations and suggest alternative sensors if the instrument is likely to
experience extreme conditions.
Tetra uses a catalytic flammable 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 pel-
listor 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 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 breath-
ing limits it should be assumed that the flammable 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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Appendix: Limitations of sensors Tetra