K-TEK MT5000 Installation & Operation Manua page 5

A measurement cycle consists of the following:
1. A very short ―pulse‖ of microwave energy is applied at the cou-
pler, to the Probe.
2. The pulse travels along the length of the probe and when it en-
counters a discontinuity that is a dielectric constant change,
such as 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. By measuring the time elapsed between the
initial pulse and the reflected one, the
late the product level.
4. Since the microwave energy travels at the speed of light, one
complete measurement cycle is made up of several thousands
of Pulses. The electronics uses Time Domain Reflectometry
(TDR), a sampling technique to reconstruct a waveform dupli-
cating the actual real time signal, but at a much lower speed, so
that it can be processed by the microprocessor. This process
can be compared to using the stroboscope effect as when ob-
serving a piece of
strobe light.
5. The measurement cycles are made 2 times per second and
processed by special filtering techniques, before generating a
current output proportional to the level of the product.
A simplified signal trace as seen on the graphic display (Figure 1) can be divided into three identifiable sections:
Coupler Reflection, Signal Reflection and End of Probe Reflection.
The measurement principle using TDR is based on the fact that a dielectric constant discontinuity or geometric
change will yield a positive pulse having certain amplitude above the baseline. The greater the dielectric constant
difference, the greater the positive amplitude of the return signal. This means that a signal will show up on the base-
line if there is a substantial change from a nozzle diameter to an open tank, for example, as signal plot at the proc-
ess connection. This fact will be taken into account in the configuration of the MT5000 (Consult Basic Setup
(Section 4.3) on Commissioning).
Blanking
Coupler Reflection
Note:
Operation is subject to the following two conditions:
1. This device may not cause harmful interference.
2. This device must accept any interference received, including interference that may cause undesired operation.
MT5000-0200-1 Rev f (10-2010) DCN0528
electronics can calcu-
machinery turning at high speed with a
Signal Reflection
Figure 1
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1
4
3 2
Threshold
End of Probe Reflection
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