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Operational Theory
In the presence of oxygen the cell generates a current. A current to
voltage amplifier (I–E AMPL) converts this current to a voltage.
The second stage amplifier (TEMP COMP) supplies temperature
compensation for the oxygen sensor output. The temperature
compensation amplifier incorporates a thermistor (THERM) that 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 temperature
changes. As a result there is negligible net change in the signal due to
temperature changes once the sensor comes to equilibrium. See
Specifications in the Appendix.
The output from the temperature compensation amplifier is sent to
an analog to digital converter (ADC), and the resulting digital
concentration signal is sent to the microcontroller.
The digital concentration signal along with input from the front
panel buttons (KEYBOARD) is processed by the microcontroller, and
appropriate output signals are directed to the display, alarm relays, and
RS-232 output. The same digital information is also sent to a 12-bit
digital to analog converter (DAC) that produces the 0-10 V dc analog
concentration signal and the 0-10 V dc analog range ID output. A
current to voltage converter (E–I CONV) produces the 4-20 mA dc
analog concentration signal.
Teledyne Analytical Instruments
Model 3190
14
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