8 User Guide
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Figure 1 The measurement cycle
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4
5
The head sends a very short pulse of microwave energy through the coupler and down the probe 1.
That pulse travels along the length of the probe 2 and, when it encounters the product surface, some
of the energy is reflected and travels back towards the coupler 3. When the reflected energy reaches
the coupler, it is sensed by the electronics 4. By measuring the time elapsed between the initial pulse
and the reflected one, the electronics can calculate the product level 5.
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 78). 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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