Introduction
Principle of Operation
1-2 48i Instruction Manual Thermo Electron Corporation
• “Principle of Operation” on page 1-2 describes the principles by which
your analyzer operates.
• “Specifications” on page 1-3 is a list of the analyzer’s performance
specifications.
Thermo Electron is pleased to supply this CO analyzer. We are committed
to the manufacture of instruments exhibiting high standards of quality,
performance, and workmanship. Service personnel are available for
assistance with any questions or problems that may arise in the use of this
analyzer. For more information on servicing, see Chapter 7 “Servicing”.
Principle of Operation The Model 48i operates on the principle that carbon monoxide (CO)
absorbs infrared radiation at a wavelength of 4.6 microns. Because infrared
absorption is a non-linear measurement technique, it is necessary to
transform the basic analyzer signal into a linear output. The Model 48i uses
an internally stored calibration curve to accurately linearize the instrument
output over any range up to a concentration of 10,000 ppm.
The sample is drawn into the Model 48i through the sample bulkhead, as
shown in Figure 1-1. The sample flows through the optical bench.
Radiation from an infrared source is chopped and then passed through a gas
filter alternating between CO and N
2
. The radiation then passes through a
narrow bandpass interference filter and enters the optical bench where
absorption by the sample gas occurs. The infrared radiation then exits the
optical bench and falls on an infrared detector.
The CO gas filter acts to produce a reference beam which cannot be further
attenuated by CO in the sample cell. The N
2
side of the filter wheel is
transparent to the infrared radiation and therefore produces a measurement
beam which can be absorbed by CO in the cell. The chopped detector signal
is modulated by the alternation between the two gas filters with an
amplitude related to the concentration of CO in the sample cell. Other
gases do not cause modulation of the detector signal since they absorb the
reference and measure beams equally. Thus, the GFC system responds
specifically to CO.
The Model 48i outputs the CO concentration to the front panel display, the
analog outputs, and also makes the data available over the serial or ethernet
connection.
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