VEGA VEGAPULS 81 - Manual (Page 41)

VEGAPULS 64 and 81 41
Mounting and installation
4.4 Measurement through the
vessel wall
Radar signals can penetrate very non-conduc-
tive products such as glass or plastic. This
feature is very important for some applications.
In processes where high-purity is required
such as for semi-conductor production or
substances which are very aggressive it is
favourable that the system remains closed and
the measurement can be carried out through
the walls of plastic vessels.
A completely non-contact level measurement
is possible for products with good reflecting
features. Products with good electrical con-
ductivity or with an
ε
r
of more than 10 can be
measured through closed plastic vessels.
Reflection laws
Physically the fundamental laws of reflection
should be noted. The radar signal is partly
reflected when penetrating plastic. Another
part of the radar signal reaches the product in
the vessel, to reach from the radar sensor as
the real useful reflection.
Gating out of vessel interfering reflections
Emitting power, vessel reflection and radar
signal power reaching the product
The part of the radar signals reflected on the
vessel ceiling is detected by the sensor as
false echo.
When the vessel ceiling or the penetrated
plastic window is inclined in an angle of 35 ° to
45 ° to the sensor axis and the sensor with the
antenna keeps a distance to the vessel or
window of more than 400 mm the false echo is
reflected laterally and can no more be re-
ceived.
Gating out of window interfering reflections
emitted power
power reaching the
product
Plastic tank
~ 400 mm
35°…45°
~ 400 mm
35°…45°
Plastic tank
Glass/plastic
window