Berthold LB 444 - Principle of Measurement

Density Meter LB 444 SECTION 2. SYSTEM DESCRIPTION
4
2.2 The Principle of Measurement
The density measurement is based on the irradiation method. It utilizes the
physical law of the attenuation of gamma radiation passing through the product
being measured. The resulting measurement effect is the ratio I/I
0
between the
un-attenuated radiation I
0
and the radiation I which is attenuated by the product
being measured. The remaining radiation picked up by the detector (scintillation
counter) represents the density of the product being measured. Figure 1 illus-
trates the principle of measurement. The radiation is attenuated according to the
following formula:
I = I
0
* e
- µ' * ρ * d
I = Radiation picked up by the detector
I
0
= Un-attenuated radiation
µ = Mass attenuation coefficient (absorption coefficient) in cm
2
/g
ρ = Density of the absorbing material in g/cm
3
d = Thickness of the absorbing material in cm
Source Detector
ρ
ρρ
ρ
I
d
I
0
Figure 1: Principle of measurement
The intensity of the radiation picked up by the detector is also dependent on the
distance between source and detector. As in the case of light, the function in-
volved is a square function, i.e. doubling the distance reduces the radiation in-
tensity to ¼ if all other conditions remain the same.
Assuming a constant distance between source and detector and a fixed meas-
uring path, the radiation picked up by the detector is only dependent on the den-
sity of the material to be measured.
NO contamination of the product being measured or of the pipeline wall by
gamma radiation is possible. (Physics).