7ML19985GG03
Pointek CLS500 – INSTRUCTION MANUAL
Page 9
mmm
mm
Oper
at
ion & Applicat
ion
Pointek CLS500 Transmitter
Operating Principles
Capacitance
1
measurement operates by forming a variable capacitor resulting from the
installation of a measurement electrode in a vessel or silo. The environment (for example,
the tank wall) acts as a reference electrode. Whatever material is sandwiched between
the two electrodes forms the dielectric. This will be composed of the vessel contents (air,
vapor, liquid, solid, or a combination) and, if the measurement electrode is insulated, the
insulating layer (PFA, for example). The dielectric gives a capacitance value.
Capacitance is affected by the surface area of the electrodes, the separation distance
between the electrodes, and the dielectric constant of the vessel contents. The relative
dielectric constant is the measure of a material’s ability to store energy. The relative
dielectric constant of air (vacuum) is 1: all other materials have a higher value.
The capacitance when the probe is uncovered (capacitance in air) will be different from
the capacitance when the probe is covered (for example, capacitance in water). If the
product is two immiscible liquids with different relative dielectric constants, (for example,
oil and water) the capacitance will change at the interface between the two liquids.
Pointek CLS500 variable frequency oscillator
The Pointek CLS500 probe is equipped with a variable frequency oscillator which
responds to the capacitance. A change in capacitance is registered as a change in
frequency. This technology provides high resolution and accuracy. The variable frequency
maintains a constant relationship to the reading.
1.
For definitions relating to capacitance, see the glossary, page 103.
dielectric = contents plus insulation (non-conductive contents)
dielectric = insulation (conductive contents)
(insulation)
probe
sleeve
internal
tank wall
probe
dia. (d)
internal tank diameter (D)