Mainstream CO
2
Measurement
60
Introduction to the Instrument
Theory of Operation for the M3016A Mainstream CO
2
The signals progress through the circuit as follows:
Microprocessor
This is in overall control of the mainstream CO
2
measurement functions. As well, it performs
the following functions:
• Serial communication, via optocouplers.
• Calculation of the calibration constants.
• Calculation of the raw data of the mainstream CO
2
waveform.
Temperature Sensor, Amplifier, AIO, Heater Control and Heater
The temperature of the transducer is maintained at approximately 43°C to prevent
condensation and to negate any discrepancies due to temperature changes. Signals from the
temperature sensor in the transducer are amplified and then passed through an Analog-to-
Digital converter. The microprocessor then adjusts the output of the heater control
accordingly.
Sense Coil, Motor Phase Comparator, Counters, Motor Drive, Drive Coil
The rotations of the chopper wheel in the transducer are regulated to 40 rotations per second.
The sense coil together with the motor phase comparator and the counters are used to
measure the actual number of rotations per second. The microprocessor reads the values out
of the counters and adjusts the motor drive output accordingly.
Infrared Detector, CO
2
Input Amplifier, Dual Slope A/D, AZ Pulse Monoflop
and Bias Regulation
Signals from the infrared detector are amplified and passed through a dual slope Analog-to-
Digital converter. This converter is controlled by the microprocessor via the counters. The
conversion is started at a predefined time and is performed synchronously with motor
rotation.
The following signals are produced and used as the input values for the mainstream CO
2
algorithm:
•Zero (GZ).
• Sample (GS).
• Reference (GR).
An auto zero signal (AZ), which is derived from the output of the motor phase comparator,
controls the bias regulation of the infrared detector. The AZ signal is enabled or disabled by
the microprocessor.
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