VEGA VEGAPULS 41 4 … 20 mA; HART compact sensor - Manual (Page 12)

12 VEGAPULS 41 – 4 … 20 mA
26620-EN-041227
Mounting and installation
Emission cone and false echoes
The radar signals are focused by the an-
tenna system. The signals 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 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.
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.
Examples of vessel echoes
The following vessel images show a typical
echo pattern in a vessel. The example shows
a process vessel with a slow double-bladed
stirrer. In the lower area, the vessel is
equipped with heating spirals. A thin, angled
inlet tube ends in the vessel centre between
the stirrer blades.