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7 Functional Safety Volume 1
7.5 Project Planning
7.5.1 Safety Function
The measuring system SENSseries LB 480 is used for non-contact
measurement of level, density or concentration. A gamma radiation
source (Co-60 and Cs-137) generates a radiation field which is
attenuated or absorbed by the product to be measured and is
detected by the scintillation detector LB 480.
The following descriptions are exemplary for the respective applica-
tion. The safety integrity, that is the non-detected measured value
deviation is <2%. If TowerSENS detectors are used the deviation is
<5%. Information on the accuracy of the measurement can be
found in the technical data section of the operating manual.
7.5.1.1 Level detection
The measuring system consists of a detector and a radiation source.
These are mechanically positioned on the filling level and trigger an
alarm once this level is reached. The safety function is to monitor
the fill level of the product to be monitored at a defined fill level.
This can be both an overfill and underfill protection.
Principle of measurement As long as the level is below the beam path, the detector receives
radiation from the radiation source and the signals "Empty". If the
level rises and reaches the height of the beam path, then the radi-
ation is absorbed by the product. The significantly reduced radiation
intensity evaluates the detector signal as a "Full" signal.
The "Empty" signal is displayed on the analog current output with
4 mA, the "Full" signal with 20mA.
Signal
"empty"
Signal
"full"
Shielding with
radiation source
Shielding with
radiation source
Radiation path
Radiation
absorbed by
product
Product
Product
Container
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