Section 1 System Description - EMS EMS520 Manual

SECTION 1 SYSTEM DESCRIPTION

TRANSMISSOMETER /RETRO REFLECTOR
The EMS520 is a precision, double-pass, dual beam Transmissometer that consists of a transceiver (transmitter/receiver) mounted on
one side of a stack or duct and a passive reflector mounted on the opposite side. The LED source, photo detectors, and all
measurement/reference optics used in opacity measurement are housed in the transceiver.
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 narrow beam LED (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 LED
source or contamination of the optics associated with the collection and focusing of the energy from the LED. 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 upscale 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.
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