Working Principle of Combustible or Flammable Gas Sensor

Many gases and vapors are combustible. The catalytic bead converts the combustible materials to heat. A change in heat is then converted to a change in resistance, which can be measured.

Taking a matched pair of catalytic beads and coating one so that it does not respond to the presence of combustible gases can compare the change in resistance between the two beads. The bead that is coated is called the reference bead and the other is called the active bead. Because the beads are a matched pair, they will respond equally to changes in ambient temperature, humidity, and pressure. This makes the sensor virtually immune to changing environmental conditions.

  By connecting one end of each catalytic bead together, a series circuit is formed. This circuit is supplied with a constant current. The voltage drop across each of the beads will be identical in the absence of combustible gases. As combustible material is converted to heat, the resistance of the active bead increases and causing a voltage drop across each bead to be different. This difference is proportional to the amount of combustible gas that is present.

 The voltage from the sensor is amplified and fed to an Analog to Digital (A/D) converter and then made available to the microprocessor. The baseline and the gain for the amplifier are set using digital potentiometers. They are adjusted by the microprocessor during calibration.