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1.3 Principle of operation
The detection and measurement of carbon monoxide in the Model 300 is based on the
absorption of Infra Red (IR) radiation by CO molecules at wave lengths near 4.7 microns.
In practice, the Model 300 uses a high energy heated element to generate broad-band IR
light. This light is passed through a rotating Gas Filter Wheel which causes the beam to
alternately pass through a gas cell filled with Nitrogen, (the Measure Cell) and a cell
filled with CO/Nitrogen Mixture (the Reference Cell). This alternation occurs at a rate of
30 cycles/second and causes the beam to be modulated into Reference and Measure
pulses. During a Reference pulse, the CO in the gas filter wheel effectively strips the
beam of all IR energy at wave lengths where CO can absorb. This results in a beam
which is unaffected by any CO in the Sample Cell. During the Measure pulse, the
Nitrogen in the filter wheel does not effect the beam which can subsequently be
alternated by any CO in the sample cell. The Gas Filter wheel also incorporates an
optical chopping mark which superimposes a 360 Cycles/Second Light/Dark modulation
on the IR Beam. This high frequency modulation is included to maximize detector
signal-to-noise performance.
After the gas filter wheel the IR beam enters the multi-pass sample cell. This sample cell
uses folding optics to generate a 16 meter absorption path length in order to achieve
maximum sensitivity.
Upon exiting the sample cell, the beam passes through a band-pass interference filter to
limit the light to wave length of interest.
Finally, the beam strikes the detector which is a thermoelectricly cooled solid-state
photo-conductor.
This detector, along with its pre-amplifier and bias voltage supply convert the light signal
into a modulated voltage signal.
The detector output is electronically demodulated to generate two DC voltages, CO
MEAS and CO REF. These voltages are proportional to the light intensity striking the
detector during the Measure Pulse and Reference Pulse, respectively.
P/N 02163G1 Teledyne API Model 300 CO Analyzer Instruction Manual - Page 3