VEGA VEGAFLEX 81 Foundation Fieldbus Coax probe - Manual (Page 11)

11
3 Product description
VEGAFLEX 81 • Foundation Fieldbus
44215-EN-130910
d
h
1
Fig. 4: Level measurement
1 Sensor reference plane (seal surface of the process fitting)
d Distance to the interface
h Height - Level
Probe end tracking
To increase sensitivity, the probe is equipped with probe end tracking.
In products with a low dielectric constant, this function is very helpful.
This is the case, for example, in plastic granules, packing chips or in
vessels with fluidized products.
Between a dielectric constant of 1.5 and 3, the function switches on, if
required. As soon as the level echo can no longer be detected, probe
end tracking is automatically activated. The measurement is contin-
ued with the last calculated dielectric constant.
The accuracy thus depends on the stability of the dielectric constant.
If you measure a medium with a dielectric constant below 1.5, probe
end tracking is always active. In this case, you have to enter the
dielectric constant of the medium. A stable dielectric constant is very
important here.
High frequency microwave impulses are guided along a steel cable or
rod. Upon reaching the product surface, a part of the microwave im-
pulses is reflected. The other part passes through the upper product
and is reflected by the interface. The running times to the two product
layers are processed by the instrument.
Functional principle - in-
terface measurement