Ametek Land 4200, Руководство пользователя

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User Guide Model 4200 Dust & Opacity Monitor
4.1.1 Transmissometer
The Land Model 4200 has been developed to be one of the most accurate and
economical opacity/ dust monitors available. It uses a high-intensity LED source
very low noise electronics and a sophisticated normalisation system to minimise
calibration drift. The LED is electronically modulated at a frequency of 3kHz in
order to reduce noise and eliminate errors due to background light. Electronic
modulation eliminates the need for a mechanical chopper thus eliminating the
need for moving parts.
A 4 digit 7 segment LED display combined with a simple 4 button keypad are
all protected by a removable front detail plate allowing direct user interface for
simplicity in alignment and setup.
The instrument is shown schematically with optional failsafe shutters in Fig.
4-1. Light from source LED1 passes through the diffuser and on to the 50/50
beamsplitter. The transmitted portion passes on to the lens which projects a
well-defined collimated beam across the measuring path to the distant retro-
reflector. Light returned from the retro is focused by the lens onto measurement
detector D
m
. The portion of light originally reflected by the beamsplitter falls on
the concave mirror which focuses it onto reference detector D
r
. The opacity value
can be calculated from the ratio of the signals on the two detectors.
This is true only if the responsiveness of the detectors and the gain of their
associated electronics remains constant. In practice temperature variations and
ageing of the components means that this cannot be achieved. The Model 4200
uses a novel method (Fig. 4-1) to compensate for such changes by using LED2
to flood both D
m
and D
r
with light. Because it uses no focusing optics there is
no possibility of misalignment and so any relative change in sensitivity between
D
m
and D
r
will lead to a change in the ratio of signals from LED2. By alternating
LED1 and LED2 between the measure and normalisation mode approximately
once every second the normalization procedure is then effectively continuous.
The instrument is controlled by a sophisticated electronic system with a
microprocessor which performs the calculations required to perform the
normalisation.