Pellistor is based on two thin platinum wires in aluminium beds connected to Wheatson bridge. One bed is covered with a special catalyst initiating oxidation of flammable gase (vapours), the other bed is modified to inhibit oxidation. The catalytic bed increases the temperature during the oxidation process in the presence of flammable gases (vapours). Oxidation process increases temperature of the bed with catalyst, which causes increase of resistance. The result of this status a bridge imbalance.
Electrochemical sensor consists of 2, 3 or 4 electrodes, which are located in a gel electrolyte. A system of electrodes and the electrolyte is separated from the atmosphere by a diffusion barrier. Gas molecules diffuse through this barrier and react with the electrolyte. There are oxidation and reduction reactions at the electrodes and they cause a change of cell potential. The higher the gas concentration is, the higher the potential will be.
A Photoionization detector (PID) uses an ultraviolet (UV) light source to ionize chemicals to positive and negative ions that can be easily counted with a detector. Ionization occurs when a molecule absorbs the light energy. The gas becomes electrically charged. These charged particles produce a current that is then amplified and displayed on the meter as "ppm" or even "ppb".
A Infrared detector (IR) uses an ability of gases with two ore more atoms to absorb infrared (IR) light, e.g. carbon dioxide, methane. Infrared detectors use wavelengths specific corresponding to vibration or rotation of a bond between two atoms in the gas molecule. The higher gas concentration the lower IR signal (approx. logarithmic function).
TC gas detectors, sometimes called Katharometers operate by comparing the thermal conductivity of the sample with that of a reference gas (usually air). A heated thermistor or platinum filament is mounted so that it is exposed to the sample and another, acting as a reference is enclosed in a sealed compartment.
If the sample has a higher thermal conductivity than the reference, heat is lost from the exposed element and its temperature decreases, whilst if the thermal conductivity is lower than that of the reference the temperature of the exposed element increases. These temperature changes cause electrical resistance changes, which are measured by means of a bridge circuit.
Because of the amount of power required to heat these sensors, they have to be mounted in a flameproof enclosure, in the same way as pellistors are. The precise mechanism, which occurs, is quite complex because the thermal conductivities of gases vary with temperature and convection as well as conductivity plays a role. Whilst most gases exhibit a linear output signal, this is not always the case.