1.2
Measuring principle
The Silometer FMX 570 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.
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
ε
r
=
rel. dielectric constant of product
D=
diameter of vessel
d=
diameter of probe
L=
length of probe immersed in
product in meters
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.
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.
Assuming a constant density, the level of
the liquid can be calculated from the
pressure measured by the Deltapilot.
BA119Y05
d
L
D
C
1
Fig. 1.3
Capacitance measurement principle
BA119E06
d
Conducting
product
L
D
Fig. 1.4
Measurement in conducting media
BA119Y07
p
1
=
ρ
x g x h
h
atmospheric
pressure
p
1
Fig. 1.5
Hydrostatic measurement principle
Capacitance
measurement
Hydrostatic
measurement
Measurement in
conducting media
Silometer FMX 570
Chapter 1: Introduction
7
