2.0 Sensor Assembly
2.1
Sensor Signal Processing
General Monitors uses a low temperature
catalytic bead to detect the presence of com-
bustible gases. Combustible gases are found
in many gases and vapors. The catalytic bead
converts the combustible gas to heat.
A
change in heat is converted to a change in re-
sistance.
By taking a matched pair of catalytic beads
and coating one so that it does not respond to
the presence of combustible gases, we 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 (figure 1). Environmental factors
can also influence the temperature of the cata-
lytic beads. Because the beads are a matched
pair, they will respond to changes in ambient
temperature, humidity, and pressure equally.
figure 1
By connecting one end of each catalytic bead
together, a series circuit is formed. This cir-
cuit is supplied with a constant voltage. 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,
causing the 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
(figure 2).
figure 2
The baseline and the gain for the amplifier are
determined by digital potentiometers.
They
are adjusted by the microprocessor during
calibration.
3
GENERAL MONITORS
Model S104
