Endress+Hauser gammapilot FTG 671, Руководство по эксплуатации

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1.3 Measuring principle
Gamma ray absorption When Gamma radiation penetrates a material, it is absorbed to a degree dependent
upon the density
ρ
, and thickness d of the material as well as the linear absorption factor
µ
, which is also dependent on the material and source. The attenuation F
s
is given by:
F
s
= e
−µ • ρ •
d
The radiation also decreases with the square of the distance between source and
detector. For limit switching
µ, ρ
, and d are constants and the detected radiation level
depends upon the presence or not of material in the beam. The countrate is at a maximum
when the beam is completely free and at a minimum when the all radiation has to travel
through the material. The maximum and minimum countrates N
max
and N
min
are obtained
when the transmitter is calibrated. Their dependency is given by:
N
min
= N
max
/F
s
= N
max
e
−µ • ρ •
d
The FTG 671 output is normalised to produce the end values 0 and 100, relay 1 trips at
38 in minimum and relay 2 at 72 in maximum fail-safe mode (10 and 90 for DG 17/27 (Z)
detectors). The detection band is dependent upon the orientation of the detector, which,
however, is usually mounted horizontally on the vessel.
Statistical assessment The precision of a gamma limit switch is dependent on the statistical variation in
countrate. Since the emission of gamma rays from a radioactive source is a random
phenomenom, several successive countrate measurements taken over a given period
of time will be seen to vary randomly about a mean value. The degree of variation between
measurements depends upon the time interval chosen. The measurement precision
depends on the confidence level - 1
σ
, 2
σ
or 3
σ
- which is to be attained. For a countrate
of N pulses per second:
±
1
σ
=
±
1
√
N 68.28% confidence level
±
2
σ
=
±
2
√
N 95.45% confidence level
±
3
σ
=
±
3
√
N 99.74% confidence level
Normally a confidence level of 2
σ
is used. The statistical variation is reduced by
increasing the sampling or integration time
τ
:
Statistical variation =
±
2
√
N
√τ
Barrier covered
Statistical fluctuation
in countrate Residual and
background radiation
BA082E05
Radiation
progressively
absorbed by product
Barrier free
Nmax
Nmin
Detection band
0
100
Display
Detector
Fig. 1.5:
Operating principle of FTG 671
radiometric level limit switch
Gammapilot FTG 671 Chapter 1: Introduction
8