Transmitter
The transmitter uses a high intensity light emitting diode (LED) as the light
source. An internal detector monitors the transmitted light intensity. A lens in the
front of the enclosure focuses the light from the LED across the duct.
Current to the LED is modulated at a frequency of 512Hz. The wavelength of
the light emitted is 637 nanometres which is in the red part of the visible
spectrum and the source intensity is sufficient for duct widths up to six metres to
be accommodated.
A fully-sealed (to IP65) epoxy-coated aluminium enclosure houses the
transmitter.
Receiver
The receiver consists of a silicon cell detector and its associated electronics
housed in an identical enclosure to the transmitter. A lens focuses the light
received onto the detector.
Because the light from the transmitter is modulated, the receiver is able to
measure ambient light levels, and the levels compensated for within the signal
processor.
Signal Processor Unit
The signal processor unit is a remotely mounted and provides power for the
transmitter and receiver, and produces an output of opacity from the receiver
output signal. The following functions are provided by the processor:
It accepts power, 48V DC from the power supply unit, and
provides the DC voltage rails required by the transmitter, receiver
and micro-processor circuitry.
A measurement of Opacity is obtained from the receiver signal to
provide a 0 or 4 to 20mA current output, proportional to either
dust, Ringelmann, opacity or extinction.
Change-over contact relays are provided to warn of a high opacity
level, and of a system complication.
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