VEGA VEGAPULS 45 Profibus PA - Manual (Page 16)

16 VEGAPULS 42, 44 and 45 – Profibus PA
24908-EN-041227
Emission cone and false echoes
The radar signals are focused by the an-
tenna system. The signals (type 42 and 44)
leave the antenna in a conical path similar to
the beam pattern of a spotlight. This emission
cone depends on the antenna used. Any
object in this beam cone will reflect the radar
signals. Within the first few meters of the
beam cone, tubes, struts or other installations
can interfere with the measurement. At a
distance of 6 m, the false echo of a strut has
an amplitude nine times greater than at a
distance of 18 m.
At greater distances, the energy of the radar
signal distributes itself over a larger area,
thus causing weaker echoes from obstruct-
ing surfaces. The interfering signals are
therefore less critical than those at close
range.
If possible, orient the sensor axis perpen-
dicularly to the product surface and avoid
vessel installations (e.g. pipes and struts)
within the 50% emission cone.
The illustrations of the emission cones are
simplified and represent only the main beam
- a number of weaker beams also exist. Un-
der difficult measuring conditions, the an-
tenna should be oriented so that the lowest
possible false echo values appear. Only
giving attention to the size of the useful echo
is not adequate when measuring conditions
are unfavourable.
In a difficult measuring environment, search-
ing for a mounting location with the lowest
possible false echo intensity will bring the
best results. In most cases, the useful echo
will then be present with sufficient strength.
With the adjustment software PACT
ware TM on
the PC, you can have a look at the echo im-
age and optimise the mounting location (see
chapter „5.2 Adjustment with the PC – Sensor
optimisation – Echo curve“).
Mounting and installation
If possible, provide a "clear view“ to the
product inside the emission cone and avoid
vessel installations in the first third of the
emission cone.
Optimum measuring conditions exist when
the emission cone reaches the measured
product perpendicularly and when the emis-
sion cone is free from obstructions.