Flammable gas detectors calibration and repair
Principle of Operation
The most widely used technique for the detection of flammable gases in air is that based upon the principle of catalytic combustion which takes place on a specially prepared and heated detector element.
The detector head comprises two elements, one is called active and the other one is called reference or Inactive or compensator.
The active detector element known as a bead or pellistor consists of a fine platinum wire coil on which a bead of refractory oxide coated with a precious metal catalyst has been formed.
The reference (compensator) element is identical but it is not coated with the metal catalyst to prevent the gas oxidation on its surface.
The detector elements, active and reference are electrically matched to form two arms of a Wheatstone bridge and are mounted in the sensor behind the stainless steel sintered flame arrestor.
The other two arms of the bridge are precise resistors within the encapsulated electronics module which include also the high efficiency differential amplifier, supplies a constant current to heat the two elements.
In pure air the two beads are heated equally as the result of a controlled electrical current applied to the platinum coils from the bridge supply.
On exposure to a flammable gas in air, oxidation occurs on the catalyst surface causing a rise in the active bead temperature and thus increasing the resistance of its platinum coil. This change in resistance unbalances the wheatstone bridge and results in a change in the output signal 4-20 mA proportional to the gas/air concentration as well as local readout in LEL %.
Any change in output signal due to variation in ambient temperature and pressure is compensated by the inert reference (compensator) element.
In DAS plants, the hydrocarbon gas detectors are mounted level with or slightly above potential leak outlets. The HC smart detector has a blue front panel with the relevant information on it and blue is the accepted color for hydrocarbon detectors. As a rule HC detectors are usually mounted at head height.