Endress+Hauser silometer FMC 671 Z, Руководство по эксплуатации, Страница: 10 / 68

1.3 Measuring principle
The Silometer FMC 671 Z/676 Z measures level on the basis of the capacitance and
hydrostatic measurement principles. In both cases the measured value is processed by
the electronic insert and passed on as a frequency signal.
Capacitance
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 properties
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.
Hydrostatic
measurement In an open vessel, the level is derived from
the hydrostatic pressure exerted by a
column of liquid on a probe placed at its
foot. The pressure exerted is:
p
1
=
ρ
x g x h (2)
whereby
p
1
= hydrostatic pressure
ρ
= density of the liquid
g = acceleration due to gravity
h = height of the liquid column.
At a constant density, the level is
proportional to the hydrostatic pressure.
C
1
BA063E05
d
L
D
Fig. 1.4
Capacitance measurement principle
BA063E06
d
Conducting
product
L
D
Fig. 1.5
Measurement in conducting media
BA063E07
p
1
=
ρ
x g x h h
atmospheric
pressure
p
1
Fig. 1.6
Hydrostatic measurement principle
Silometer FMC 671 Z/676 Z Chapter 1: Introduction
8