EMS 1304, Installation And Operation Manual

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10
PRINCIPLE OF OPERATION
Normal mode of operation
The Dual beam measurement system has a stack mounted Transmissometer sensor system consists of an optical
transceiver mounted on one side of the stack and a retro reflector mounted on the other. To avoid errors due to
ambient light, the
lamp (See Drawing) 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 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, 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 E.P.A. 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.