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Instruction Manual
IM-106-5500, Original Issue
August 2005
1-3
CCO 5500
The radiation emitted by the heater is then 'chopped' by a motor-driven disc,
and focused across the duct by a lens. The chopper disc is driven by a small
DC motor. The phase and frequency of the chopper disc are monitored by a
radiation detector to provide a reference signal that is utilized by the signal
processor unit.
System calibration is achieved by a small calibration cell containing pure
reference gas (CO) that can be swung into the sight path by means of a
'stepper' motor and gearbox assembly to enable continuous calibration
updates to be maintained. The chopper motor and stepper motor represent
the only moving components in the entire system.
A printed circuit board mounted at the front of the unit provides control
circuitry for the heater, the motors, and the reference wave detector.
Figure 1-2 illustrates the transmitter and receiver units.
Figure 1-2. Transmitter and
Receiver Schematic
Infrared Receiver Unit The precision and reliability which CO concentration levels can be measured
governs the performance of the complete instrument. For this reason design
efforts have been concentrated in producing an extremely simple and robust
receiver unit. It contains no moving parts, is fully sealed and designed to give
many years of trouble-free and maintenance-free operation.
The unit comprises a lens to focus radiation received from across the duct,
followed by a precision interference filter to limit the wave band of energy
used. This filter tolerance is strictly controlled since it alone determines the
instrument scale shape and calibration. The radiation then passes to an
optical beam-splitter where approximately half the radiation is reflected at
right angles directly onto a radiation detector. The other half of the radiation is
transmitted, by the beam-splitter, through a gas cell containing pure reference
gas (CO) and onto a second radiation detector.
The detectors used are lithium tantalate pyro-electric detectors, renowned for
their sensitivity, stability and ability to operate at normal ambient
temperatures, without the need for cooling. They respond only to changing
levels of radiation and thus to the chopped radiation from the infrared source
unit and not to background radiation from the flue or flue gas. The detector
signals are amplified and fed to the signal processor unit.
Chopper Blade Detector
Chopper Blade & Motor
Calibrating Cell
Stepper Motor & Gearbox
Reference Detector
Heated Infrared Source
Control Detector
Detector D2
Gas Filter
Detector D1
Beam Splitter
Interference Filter