Teledyne Insta Trans - trace and percent oxygen transmitter - Manual (Page 19)

Trace Oxygen Transmitter Operational Theory
Teledyne Analytical Instruments 11
2.4 Electronics and Signal Processing
The Insta-Trans Trace Oxygen transmitter uses standard electronic
circuitry. Power is supplied via the 4-20 mA current loop.
The processing electronics are located inside the transmitter case.
Three boards are accessible after removing the top housing. Figure 2-5 is
a block diagram of the transmitter electronics.
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 the
3 1/2 digit DVM/Display and voltage to current converter/driver.
The 3 1/2 digit DVM/Display provides a visual indication of the
oxygen concentration in ppm or % depending on the range selected.
Range selection and calibration information are entered from the key
pad and processed through the key pad control, the input range selection
and the calibration control circuits. All power needed to run the Insta-
Trans transmitter is derived from the signal current loop. Calibration
settings are maintained indefinitely when the signal current loop is
disconnected. Range settings are maintained for a minimum of 72 hours
through the use of a 0.1 Farad storage capacitor when the signal current
loop is disconnected. The transmitter always defaults to Run mode when
the signal current loop is connected.