Emerson Rosemount CCO 5500, Reference Manual

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Source and Receiver Diagram Figure A-1:
A. Chopper blade detector
B. Chopper blade & motor
C. Calibrating cell
D. Detector D2
E. Gas cell
F. Detector D1
G. Beam splitter
H. Interference filter
I. Stepper motor and gearbox
J. Reference detector
K. Heated infrared source
L. Control detector
A.2 Infrared receiver unit
The precision and reliability with which the CO concentration levels are measured governs
the performance of the instrument. For this reason, Rosemount
™
has concentrated design
efforts to produce an extremely simple and robust receiver unit. It contains no moving
parts, is fully sealed, and is designed to give many years of trouble-free and maintenance-
free operation.
The unit includes a lens to focus radiation received from across the duct, followed by a
precision interference filter to limit the band wave of energy used. This filter's 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 beam-splitter
transmits the other half of the radiation 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 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 the background radiation from the flue
or flue gas. The detector signals are amplified and fed to the control unit.
Theory of operation
104 Rosemount CCO 5500