General Monitors S216A, Instruction Manual

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This chapter discusses signal processing, the
on board power supply and the control
electronics for the Model S216A.
2.1 Sen sor Sig nal Proc ess ing
General Monitors uses a proprietary Metal
Oxide Semiconductor (MOS) film on the
sensor for detecting hydrogen sulfide (H
2
S)
gas. The MOS film is deposited onto a
substrate between two electrodes (figure 1).
With no gas present, the measured resistance
between these two electrodes is very high
(in the mega-ohms). As H
2
S adsorbs onto
the film, the resistance between the two
electrodes decreases (to kilo-ohms). This
decrease in resistance is logarithmically
proportional to the concentration of H
2
S that
is present.
fig ure 1
The change in resistance is converted to a
change in voltage and amplified by the input
circuit (figure 2). This amplified signal is
fed to an Analog to Digital (A/D) Converter,
converted to a digital signal and sent to the
microprocessor to be processed. The
process of H
2
S adsorbing onto the MOS film
is most effective at an elevated temperature.
On the outer edge of the substrate is a heater
ring. The temperature of this heater ring is
measured with a thermistor and kept
constant by a circuit located inside the body
of the sensor.
fig ure 2
As H
2
S adsorbs onto the film, electrons
move more freely from one electrode to the
other (figure 3). This is represented as a
decrease in resistance. The process of H
2
S
adsorbing onto the MOS film is completely
reversible. As the concentration of H
2
S
decreases (H
2
S desorbs), the resistance
between the electrodes will increase.
fig ure 3
2.2 Power Sup ply
The Model S216A operates from a +24VDC
(nominal) input. This unregulated source is
fed to a converter board that produces
regulated voltages (see figure 4). These
voltages are the source for operating all of
the circuitry on the control and the display
boards, and supplying the sensor with
power.
3
Model S216A Sensor Assembly