Catalytic detectors are based upon the principle that when gas oxidizes it produces heat, and the sensor converts the temperature change via a standard Wheatstone Bridge-type circuit to a sensor signal that is proportional to the gas concentration. The sensor components consist of a pair of heating coils (reference and active). The active element is embedded in a catalyst. The reaction takes place on the surface of the catalyst, with combustible gases reacting exothermically with oxygen in the air to raise its temperature. This results in a change of resistance.

There is also a reference element providing an inert reference  signal by remaining non-responsive to gas, thereby acting as a stable baseline  signal to compensate for environmental changes which would otherwise affect the sensor s temperature.

Advantages

The major advantages of catalytic detectors:

• Robust.
• Simple to operate.
• Easy to install, calibrate and use.
• Long life with a low replacement cost.
• Proven technology with exceptional reliability and predictability.
• Easily calibrated individually to gases such as hydrogen which cannot be detected using infrared absorption.
• Can perform more reliably in dusty & dirty atmospheres as they are not as sensitive as optics to the build up of industrial contaminants.
• Can perform more reliably in high temperature applications.
• Are less sensitive to humidity and condensation.
• Not as significantly affected by changes in pressure.
• Can detect most combustible hydrocarbons.

Disadvantages

The limiting factors in catalytic detector technology:

• Catalysts can become poisoned or inactive due to contamination (chlorinated & silicone compounds, prolonged exposure to H2S and other sulfur &/or corrosive compounds).
• The only means of identifying detector sensitivity loss is by checking with the appropriate gas on a routine basis and recalibrating as required.
• Requires oxygen for detection.
• Prolonged exposure to high concentrations of combustible gas may degrade sensor performance.
• If flooded with a very high gas concentration, may show erroneously low or no response, and sensor may be damaged or rendered inoperable.