Principle Of Operation; System Limitations - Vega SOLITRAC 31 Quick Setup Manual

2 Product description
Application area
Functional principle
Activity of the source
Non-linearity of the pro-
cess value
External radiation
Span
6
2.2

Principle of operation

The instrument is suitable for applications in liquids and bulk solids in
vessels under difficult process conditions. There are application pos-
sibilities in nearly all areas of industry.
The level is detected contactlessly right through the vessel wall.
Neither a process fitting nor a vessel opening are required. The instru-
ment is thus ideal for retro installation.
In radiometric measurement, a Caesium-137 or Cobalt-60 isotope
emits focussed gamma rays that are attenuated when penetrating
the vessel wall and the medium. The PVT detector on the opposite
side of the tank receives the radiation, whose strength is dependent
on the level. The measuring principle has proven to be very reliable
in conjunction with extreme process conditions because it measures
contactlessly from outside through the vessel wall. The measuring
system ensures maximum safety, reliability and plant availability,
independently of the medium and its properties.
2.3

System limitations

There are several measuring principle-specific factors which can influ-
ence the measuring result. Keep these factors in mind in order to fully
utilize the capabilities of the instrument with respect to measurement
reliability and non-repeadability.
The implemented radioactive isotope and its activity must be selected
according to the properties of the vessel and the medium. The neces-
sary radioactive activity must be calculated on the basis of the plant
data.
To this end, make use of our planning service for an optimum layout of
the measurement and selection of the isotope. This applies particu-
larly to SIL applications.
Due to the physical properties of the radioactive radiation, the pulse
rate is subject to slight fluctuations. Set a suitable damping level to get
a stable measured value.
The relation between level and the pulse rate measured by the sensor
is not linear.
Set up a linearization table to get a linear level signal. For precise
mesurement results, make sure when setting up the linearization
table that the actual filling height of the measuring points is entered as
exactly as possible.
External radiation sources can influence the measured value (e.g.
welding joint tests). In safety-relevant applications, the safety function
must be treated as unreliable as long as the external radiation occurs.
If necessary, you must take measures to maintain the safety function.
Make sure during the planning that for the planned application, a
possibly large different of the pulse rate with empty and full vessel is
SOLITRAC 31 • Four-wire 4 ... 20 mA/HART