Teledyne 1220 - Multipoint flammable gas and vapor detection system User Manual

Page 14

2-4

2 Operational Theory

Model 1220

Teledyne Analytical Instruments

When exposed to a mixture containing gases and oxygen, the measur-

ing bead coating allows the oxygen and combustibles to combine at its
surface. The energy produced by this reaction heats the measuring bead.
The rise in temperature changes the bead’s resistance and is related to the
concentration of the combustible gas. The reaction rate is dependent upon
the nature of the particular combustible gas. Raising the temperature of the
bead increases the reaction rate, which increases the sensitivity of the
sensor and reduces the observed differences of various gases. This rise in
temperature is generated by a constant-current supplied to the sensor.

The temperature of the measuring bead will be influenced by other

factors such as initial gas temperature, gas thermal conductivity, flow rates
and the temperature of its housing. The reference bead, having similar
electrical and thermal properties and being heated by the same current, but
lacking the catalytic material will be similarly affected by these extraneous
factors but not significantly affected by oxidation of the combustible gas.

These two beads are placed in close proximity to one another so that

they are affected by the same environmental factors. Thus the differences
between the changes in resistance of the two coils are directly related to the
concentration of combustible gases.

For area monitoring applications, these two beads are installed in a

metal housing as shown in Figure 2-2. Wires connected to the beads are
brought out the back of the detector through potting which provides me-
chanical retention of the wires and provides a gas-tight seal. A stainless-
steel flashback arrestor screen at the front of the detector covers the beads
and is held in place by a retaining cylinder pressed onto the housing. This
retaining cylinder is threaded internally to accept a flow-through adapter
for calibration.

123456789

RED

WHITE

BLACK

3/4 NPT

15/16” WRENCH FLAT

PROBE SENSING

SURFACE

INTERNAL THREAD FOR
CALIBRATION ADAPTER

Figure 2-2 Remote Combustible Probe