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Gases Characteristics
Gas Glossary

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C Catalytic Bead
E Electrochemical
I Infrared - Carbon Dioxide
S Solid State sensor

Catalytic Bead

Principle of operation

Combustible gas mixtures will not burn until they reach an ignition temperature. However, in the presence of certain chemical media, the gas will start to burn or ignite at lower temperature. This is known as catalytic combustion.

The electrical circuit used to measure the output of catalytic sensor is called a Wheatstone bridge. When the gas burns on the active sensor, the heat of combustion causes the temperature to rise, which in turn changes the resistance of the sensor.

As the bridge is unbalanced, the offset voltage is measured as the signal. For the proper detection of hydrocarbon gases, the sensor requires a heated surface temperature between 900 C and 1000 C so the sensor can properly react with gases at a sufficiently high and stable rate. The sensor’s output is directly in proportion to the rate of oxidation.


  • Linear output
  • Accurate
  • Fast response time
  • Good repeatability
  • Up to 3 year life
  • Sensitive to all combustible gases
  • Expensive
  • Subject to poisoning by vapours of silicones, chlorine and heavy metals



Electro Chemical

Principle of operation

A typical electrochemical sensor consists of a sensing electrode (or working electrode), and a counter electrode separated by a thin layer of electrolyte. Gas that comes in contact with the sensor passes through a small capillary-type opening and then diffuses through h a hydrophobic barrier.

The gas that diffuses through the barrier reacts at the surface of the sensing electrode involving either an oxidation or reduction mechanism. These reactions are catalyzed by the electrode materials specifically developed for the gas of interest. With a resister connected across the electrodes, a current proportional to the gas concentration flows between the anode and the cathode. The current can be measured to determine the gas concentration.


  • Linear output
  • Minimal cross-sensitivity to other gases
  • Accurate
  • Good repeatability
  • Low power consumption
  • Life - 5 years (long-life cell)  2-3 years (standard cell)

  • Inexpensive (long-life cell)  Moderately expensive (standard cell)
  • Long-life cell only available in Carbon Monoxide
  • Standard cell available in several toxic gases


Specifications - Carbon Monoxide (long-life)

Specifications - Carbon Monoxide (standard)

Carbon Monoxide  

Specifications - Nitrogen Dioxide

Nitrogen Dioxide  

Specifications - Hydrogen Sulphide

Hydrogen Sulphide  

Infrared - Carbon Dioxide

Principle of operation

In infrared (IR) spectroscopy, IR radiation is passed through a sample of gaseous molecules.  Some of this radiation is transmitted through while the rest is absorbed by the sample, producing an infrared spectrum, or “molecular fingerprint”.  Because each molecular structure has a unique combination of atoms, each produces a unique infrared spectrum.  From this, identification and quantitative analysis of the gas is possible, the two major applications of infrared spectrometry.


  • Linear output
  • Gas specific
  • Accurate
  • Can be continuously exposed to gas
  • Minimum maintenance
  • 10 year life
  • 5 year calibration
  • Inexpensive

Specifications - Carbon Dioxide

Solid State sensor

(CMOS – ceramic metal oxide semi-conductor)

Principle of operation

A solid state sensor consists of one or more metal oxides such as tin oxide or aluminum oxide. These metals are prepared and processed into a paste which is used to form a bead type sensor. A heating element is used to regulate the sensor temperature, since the finished sensors exhibit different gas response characteristics at different temperature ranges.

The sensor is then processed at a specific high temperature which determines the specific characteristics of the finished sensor.  In the presence of gas, the metal oxide causes the gas to dissociate into charged ions  which result in the transfer of electrons.

The built-in heater, which heats the metal oxide material to an operational temperature range that is optimal for the gas to be detected, is regulated and controlled by a specific circuit. A pair of biased electrodes are imbedded into the metal oxide to measure its conductivity change. The changes in the conductivity of the sensor resulting from the interaction with the gas molecules is measured as a signal.


  • 10 year life
  • Inexpensive
  • Reacts to a number of gases
  • Non-linear output
  • +/- 15 % accuracy
Gases Range


Published: February 16, 2004 Last Updated: March 25, 2004