Land 4500 MKII Premier, Operation And Maintenance Manual

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Model 4500 Premier Dust and Opacity Monitor
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light originally reflected by the beamsplitter (3) falls on the concave mirror (8) which focuses it onto
reference detector (7). The opacity value can be calculated from the ratio of the signals on the two
detectors.
This is true only as long as long as the responsivity of the detectors (6 and 7) and the gain of their associated
electronics remains constant. In practice, temperature variations and ageing of the components means that
this cannot be guaranteed. The Model 4500 Premier uses a novel method to compensate for such
changes by using Flood LED (2) to flood both detectors (6 and 7) with light. Because it uses no focusing
optics, there is no possibility of misalignment and so any relative change in sensitivity between the
detectors
(6) and (7) will lead to a change in the ratio of signals from the flood LED (2). By alternating
light sources (1) and (2) once every second, the normalisation procedure is then continuous.
The microprocessor performs the calculations required to perform the normalisation and to convert
the opacity measurement into optical density or dust density as required.
Calibration Check
The transceiver periodically performs a two step calibration check. In the first step, a zero-point reflector
(10), mounted on the outside of the Transceiver is placed into the optical beam. In the second step, the
span filter (9) is also placed in the light path. The monitor reads the difference in signal level between
the current value and that established during the last clear stack calibration and makes a correction The
opacity value of the span filter (9) is shown on the calibration certificate which accompanies the instrument.