Trace Oxygen Analyzer
Operational Theory
Teledyne
Analytical
Instruments
17
In the presence of oxygen the cell generates a current. A current to
voltage amplifier converts this current to a voltage, which is amplified in
the second stage amplifier.
The second stage amplifier also supplies temperature compensation
for the oxygen sensor output. This amplifier circuit incorporates a
thermistor, which is physically located in the cell block. The thermistor
is a temperature dependent resistance that changes the gain of the
amplifier in proportion to the temperature changes in the block. This
change is inversely proportional to the change in the cell output due to
the same temperature changes. The result is a signal that is temperature
independent. The output from the second stage amplifier is sent to an 18
bit analog to digital converter controlled by the microprocessor.
The digital concentration signal along with input from the control
panel is processed by the microprocessor, and appropriate control
signals are directed to the display, alarms and communications port. The
same digital information is also sent to a 12 bit digital to analog
converter that produces the 4-20 mA DC and the 0-1 VDC analog
concentration signal outputs, and the analog range ID outputs.
Signals from the power supply are also monitored, and through the
microprocessor, the system failure alarm is activated if a malfunction is
detected.
2.5 Temperature Control
For accurate analysis this instrument is temperature controlled not
to fall beneath a certain temperature. This temperature is 22°F. This is
to prevent the sensor from freezing in cold environments.