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Teledyne API T803 CO2/O2 Analyzer Operation Manual Principles of Operation
207
This current is passed to a feedback loop, which generates a second current to a
wire winding (in effect, a small DC electric motor) mounted on the suspended
mirror. The more O
2
present, the more the dumbbell and its attached mirror
moves and the more current is needed to move the dumbbell back to it’s zero
position. Finally, sensor measures the amount of current generated by the
feedback control loop which is directly proportional to the concentration of
oxygen within the sample gas mixture.
12.2. CO
2
SENSOR
12.2.1. NDIR MEASUREMENT OF CO
2
The CO
2
sensor is a silicon based Non-Dispersive Infrared (NDIR) sensor. It uses
a single-beam, dual wavelength measurement method.
An infrared source at one end of the measurement chamber emits IR radiation
into the sensor’s measurement chamber where light at the 4.3 μm wavelength is
partially absorbed by any CO
2
present. A special light filter called a Fabry-Perot
Interferometer (FPI) is electronically tuned so that only light at the absorption
wavelength of CO2 is allowed to pass and be detected by the sensor’s IR detector.
A reference measurement is made by electronically shifting the filter band pass
wavelength so that no IR at the CO
2
absorption wavelength is let through.
Figure 12-3. CO
2
Sensor Theory of Operation
The sensor computes the ratio between the reference signal and the measurement
signal to determine the degree of light absorbed by CO
2
present in the sensor
07276B DCN6418