Siemens Pointek CLS 200 Instruction Manual page 91

Appendix A: Technical References
Operating Principles
1
Capacitance measurement operates by forming a variable capacitor resulting from the
installation of a measurement electrode in a vessel or silo. The environment at the time of
setup acts as a reference.
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.
If everything in the probe's environment stays consistent, with the exception of the vessel
contents, the change in capacitance caused by a change in vessel contents can be used
to provide level detection.
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.
The Pointek CLS 200 high frequency oscillator
The Pointek CLS 200 probe is equipped with a high frequency oscillator which responds
to the capacitance. The inverse of frequency is proportional with the capacitance. A small
change in frequency is easy to detect, resulting in high resolution and accuracy.
Counts
Frequency is always represented in 'counts' , a dimensionless value generated from the
inverse of frequency. The measured value is displayed in counts. (If you are using
PROFIBUS PA, you can set the measuring limits, in counts.)
The Pointek CLS 200 electrode
The Pointek CLS 200 electrode
electrical capacitance value. The reference is the environment at the time of setup.
The design of the Pointek CLS 200 probe makes it very sensitive to changes in
capacitance in the immediate vicinity of the antenna tip. This makes it possible to operate
Pointek CLS 200 in a plastic tank, though a properly grounded metal tank will shield the
sensor from external influences.
1.
For definitions relating to capacitance, see
2.
Usually insulated.
Page 94
2
is the primary sensor of the system. It supplies the
Pointek CLS 200 – INSTRUCTION MANUAL
Glossary
, page 127.
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