Endress+Hauser Silometer FMC 672 Z, Инструкция по установке и эксплуатации

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1.3 Measuring principle
The Silometer FMC 672 Z/677 Z measures level on the basis of the capacitance/
impedance measurement principle. The measured value processed by the electronic
insert and passed on as a frequency signal.
Capacitance/impedance
measurement The probe and vessel form the two plates
of a capacitor, the total capacitance of
which can then be calculated from the
formula:
C
tot
= C
1
+ 2
πε
0
ε
r
x L pF (1)
--------------------------------
ln (D/d)
whereby
C
tot
= total capacitance
C
1
= capacitance or feed through
ε
0 = dielectric constant of air (8.85)
ε
r
= rel. dielectric constant of product
D= diameter of vessel
d= diameter of probe
L= length of probe immersed in
product in meters
For vessels with a diameter greater than 1 m, the capacitance varies by approx. 12 pF
per meter of covered probe.
Measurement in
conducting media If the product conducts, the capacitance is
determined by the thickness and quality of
the insulating material surrounding the
probe. Equation (1) applies, whereby the
variable D is now the diameter of the probe
with insulation. In this case the
capacitance varies by approx. 300 pF/m.
Measurement is independent of dielectric
constant and not affected by changes in
this variable.
C
1
BA064E05
d
L
D
Fig. 1.4
Capacitance measurement principle
BA064E06
d
Conducting
product
L
D
Fig. 1.5
Measurement in conducting media
Silometer FMC 672 Z/677 Z Chapter 1: Introduction
8