91518 Multigas Analyzer Service Manual
3-9
Theory
The isothermal analyzer bench contains a temperature sensor that measures the temperature of the gas
sample, the optical filters, and the detectors. A pressure sensor measures the pressure of the gas sample.
The temperature sensor measures the gas measuring units’ temperature. It is used for temperature
compensation.
The gas measuring unit includes a TPX board located at the end of the unit. Its function is to connect the seven
thermopile signals and the temperature sensor signal to the CPU board.
Figure 3-8: Gas analyzing unit
Oxygen Measuring Unit
Oxygen concentration is measured with a paramagnetic oxygen sensor.
The sensor uses a differential pressure transducer to compare the pressure gradient produced when reference
and sample input gases are exposed to an oscillating magnetic field.
The oxygen measurement is based on paramagnetic susceptibility. The gas and the reference gas, which
usually is room air, are conducted into a gap with an electromagnet. A strong magnetic field switched on and
off at a frequency of approximately 165 Hz.
An alternating differential pressure is generated between the sample and reference inputs, due to magnetic
forces acting on the oxygen molecules in a magnetic field gradient.
The pressure is measured with a sensitive differential transducer. The transducer is rectified with a
synchronous detector and amplified to produce a DC voltage proportional to the oxygen partial pressure
difference of the two gases.
CPU board
The CPU board contains the processor and memories and analog-digital converters that are common to the
whole multigas analyzer. The CPU board also contains the preamplifiers for the TPX sensor and the drivers for
valves, fan, pump, and lamp. The multigas analyzer is connected to the analyzer bus through a RS-485 serial
channel.
