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Pointek CLS500 – INSTRUCTION MANUAL
7ML19985GG03
mmm
mm
Oper
at
ion & Appl
icat
ion
Capacitance measurement in a cylindrical metal tank
In a cylindrical tank, the initial capacitance in air is calculated by factoring in the length of
the probe, diameter of the probe, diameter of the tank, and the relative dielectric constant
of air.
The formula
1
is:
where: C = capacitance
K = constant
= dielectric constant
L = active measurement length
D = diameter of tank
d = probe diameter
When the material inside the vessel changes, the relative dielectric constant changes,
which results in a capacitance change.
The transmitter measures the capacitance of the measurement electrode relative to the
environment (reference electrode). Any material that covers the probe will cause an
increase in capacitance relative to an uncovered probe surrounded by air. As the product
level rises the capacitance will increase. The minimum change in capacitance required to
detect a change
2
in the medium is 1pF (within a range of 330 pF).
The Pointek CLS500 electrode
The Pointek CLS500 electrode, comprising a measurement section and an active shield
section, is the primary sensor of the system. It supplies the electrical capacitance value
of the measurement section relative to the environment.
The Pointek CLS500 patented Active-Shield Technology electrically isolates the
measurement section and prevents any non-measurement capacitance from interfering
with the measurement. (Capacitance changes could result from uncontrolled variations
occurring in the connection cable, process connection, and non-active parts of the
probe.) This gives a better ratio of initial capacitance to total capacitance, resulting in
higher accuracy
1.
This formula applies to a centrally mounted probe: for a probe mounted off-cen-
tre, the formula must be adjusted.
2.
From absence to presence, or vice versa.
C
K
L
Log D d
--------------------------
=