Determining which gas sensor you need is one of the first challenges of choosing a gas detection system for your facility.
Your first step to making that choice is understanding the various types of sensor technologies that are used in gas detection.
What are the various types of gas sensor technologies?
Catalytic Sensors
Catalytic sensors can sense all combustible gases, but respond at
different rates, so need to be calibrated for each particular gas. They
are mostly used for combustible gases like ammonia and hydrocarbon
refrigerants at high concentrations.
The sensor operates by burning the gas at the surface of the bead and
measuring the resultant resistance change, proportional to
concentration. Signal response time is about 20-30 seconds and they are
most effective at as levels of 2,000PPM up to 100% LEL.
Catalytic sensors are low-cost and reliable with an expected life-span of up to 5 years.
Electrochemical Sensors
Electrochemical cells are mostly for toxic gases which cannot otherwise
be detected, or where high accuracy levels are a must. They are suitable
for ammonia but not refrigerants. They typically consist of two/three
electrodes within in an electrolyte medium.
Electrochemical sensors work by providing an oxidation / reduction
reaction which generates an electric current proportional to gas
concentration. They are very accurate (0.02PPM) and tend to be used
principally for toxic gases which cannot be otherwise detected or where
high levels of accuracy are needed. These sensors are relatively
expensive with a short life span, but provide very high levels of
accuracy.
However, new developments have meant some sensors are now able to cover
the key range of 0-1000 PPM with a longer lifetime of about 3 years.
Infrared Sensors
Infrared sensors utilise the principle that most gases have a
characteristic absorption band in the infrared region of the spectrum
and this can be used to detect them. They were typically used where a
high level of accuracy and specificity was required, but this very high
level of precision performance meant they were somewhat expensive.
Although incredibly accurate (reading down to one part per million) when
they were first introduced infrared sensors were not suitable for
monitoring more than one gas.
However, as mixed gas installations developed so did the need for a
different model of detection. New models were developed based on broad
infrared wavelength monitoring that could detect a mixture of gases,
although their high levels of specificity and accuracy were reduced as a
result.
Semi-Conductor Sensors
Semi-conductor sensors operate by measuring resistance change
(proportional to concentration) as gas is absorbed onto the surface of a
semi-conductor material, (normally made from metal oxides).
Semi-conductor sensors can be used for a broad spectrum of gases
including combustible, toxic and refrigerant gases. They have been known
to perform better than catalytic sensors in the detection of
combustible gases at low concentrations, up to 1,000PPM.
Semi-conductors have many advantages. They are low-cost and long life,
sensitive yet stable, resistant to poisoning and can be used to detect a
large range of gases including all the CFC, HCFC, HFC refrigerants,
ammonia and hydrocarbons.
What they are not suited to is detecting a single gas within a mixture
or for use where high concentrations of interfering gases may be
present.
Although cross interference can be minimised by using filtered gas
sensors and by calibrating for the specific gas and incorporating a
delayed response.
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