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Instruction Manual
IM-105-4000, Original Issue
August 2005
OPM 4000
1-2
Cabling
The standard cabling used between the stack-mounted units and the control
unit is at a minimum 6-pair, #20 AWG, twisted, shielded cable. More pairs or
larger than 20 AWG is acceptable.
PRINCIPLE OF
OPERATION
Normal Mode of
Operation
The dual beam measurement system has a stack mounted transmissometer
sensor system that consists of an optical transceiver mounted on one side of
the stack and a retro reflector mounted on the other (Figure 1-1). To avoid
errors due to ambient light, the lamp is electronically modulated and projects
a collimated beam of light, which is split into a reference beam, and a
measurement beam by an optical beam splitter. The reference beam is
directed to the reference detector (RD). The measurement beam is projected
across the stack to a retro reflector that returns the beam back across the
stack to a beam splitter and directs the measurement beam to the
measurement detector (MD). A portion of the returning light is also sent to the
TTL (Thru The Lens) bulls-eye target viewed through a window provided at
the rear of the transmissometer. The bulls-eye is used to correct changes in
alignment and is unique in that no moving parts are used.
The ratio of the measurement and reference detectors is used to provide a
transmittance
2
(T
2
) signal. Because the same light source is used for both
detectors, and a measurement / reference ratio is used throughout for the
calculations, the monitor is insensitive to variations in light intensity. Since all
measurements are made on a ratio basis, all resulting computations are
independent of the absolute intensity of the light source or contamination of
the optics associated with the collection and focusing of the energy from the
lamp. The (T
2
) signal is converted to a current format and sent to the control
unit for processing. At the control unit the signal is processed to read 0-100%
opacity, and provide alarms and outputs.
Internal Calibration
System, Zero Mode
Zero and span calibration checks can be initiated manually, automatically, or
by a PLC or computer. During the zero calibration mode a calibrated zero
reflector is placed in front of the transceiver optical package testing all optical
surfaces and electronic components to assure zero point has not changed.
Internal Calibration
System, Span Mode
In the span mode a Span filter of known neutral density is placed in the
measurement path and produces a specific up scale reading in accordance
with the latest EPA requirements. The zero and span cycle provides a
continuous check of all the optical components and surfaces, the main lamp,
the detector, interconnecting wiring, control unit, and computation analysis.