Model TA102A
36
6.0 Appendix
6.1 Principle
of
Operation
General Monitors uses a low temperature catalytic bead to detect the presence of
combustible gases and vapors. These gases and vapors are found in many
applications. The catalytic bead converts the combustible gases and vapors to heat.
This change in heat results in a change in the electrical resistance of the bead.
By taking a matched pair of catalytic beads and coating one, so that it does not respond
to the presence of combustible gases and vapors, we can compare the change in
resistance between the two beads. The bead that is coated is called the reference bead
and the other bead is the active bead (Figure 62). Environmental factors can also
influence the temperature of the catalytic beads. Because the beads are matched pairs,
they will respond to changes in ambient temperature, humidity and pressure equally.
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 bead will be
identical in the absence of combustible gases and vapors. As combustible material is
converted to heat, the resistance across the active bead increases, causing the voltage
drop across each bead to be different. This difference is proportional to the amount of
combustible gas or vapor that is present at the sensing elements (catalytic beads).
Figure 62 – Catalytic Sensor Diagram
