Berthold LB 442, Operating Manual

Page: 10 / 158

2.2 The Principle of Measurement
8
2.2 The Principle of Measurement
The principle of determining mass with a radiometric Bulk Flow Me-
ter can be compared to that of X-raying material lying on the con-
veyor. To generate the radiation, which is physically identical to
that of X-ray radiation, radioactive material is used which is easy
to handle and whose energy is dependent on the load on the con-
veyor. The radiation is absorbed by the product on the belt and the
attenuated radiation is picked up by a detector and evaluated by
the microprocessor unit
The attenuation occurring during maximum conveyor throughput
determines the choice of source. Any material between source and
detector reduces the intensity to be measured at the detector.
The radiometric measurement utilizes the physical law of the at-
tenuation of gamma radiation passing through the product being
measured. The resulting measurement effect is the ratio I/I0 be-
tween the unattenuated radiation I0 and the radiation I which is
attenuated by the product being measured. The remaining radia-
tion picked up by the detector (scintillation counter) stands for the
mass of the product being measured. Figure 1 illustrates the prin-
ciple of measurement. The radiation is attenuated according to the
following formula:
I = Io * e - µ' * roh * d
I = Radiation picked up with load
I
0
= Unattenuated radiation without load
µ = Absorption coefficient) in cm
2
/g
roh = bulk density of the material on the conveyor in g/cm
3
d = height of absorbing material in cm
Detector
Figure 1: Principle of measurement
Source
ρ
d