ABB LWT300 series User Manual page 12

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Figure 1
The measurement cycle
The head sends a very short pulse of microwave energy through the coupler and down the probe
That pulse travels along the length of the probe
of the energy is reflected and travels back towards the coupler
the coupler, it is sensed by the electronics
and the reflected one, the electronics can calculate the product level
Since microwave energy travels at the speed of light, one complete measurement cycle is made up of
several thousand pulses. The electronics use the time domain reflectometry (TDR) sampling technique
to reconstruct, from these numerous pulses, a waveform that can be processed by the microprocessor
(see "Diagnosing from a waveform" on page 71). Measurement cycles are made up to five time every
second (depending on configuration and probe length) and results are processed before generating a
current output proportional to the level of the product.
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User Guide
5
4
1
3 2
4
. By measuring the time elapsed between the initial pulse
2 and, when it encounters the product surface, some
3 . When the reflected energy reaches
5 .
1 .