Determining relative permittivity
The relative permittivity of a test substance is the ratio of the capacitance of a capacitor – with the space between and around the electrodes being completely and exclusively filled with the tested substance – to the capacitance of the same electrode arrangement in a vacuum.
Capacitance of the electrode assembly in air can in general be selected instead of that in a vacuum to determine relative permittivity with sufficient accuracy.
Determining relative permittivity of liquids is based on DIN EN 60247 dated January 2005.
Permittivity is measured with the Epsilon+ measuring device from flucon fluid control GmbH. A schematic drawing of the electrode arrangement of the measuring cell is shown in Abbildung 1. The measuring cell is constructed like a tubular capacitor and consists of an inner and an outer electrode, and is filled with the test substance.
The measuring cell (see Figure 1) is placed in a thermostat from OMEGA. The relative permittivity can thus be measured in a temperature range from 20 °C to 180 °C. The temperature is set here with an accuracy of 0.03 K. Temperature curves can be measured directly using this method.
Figure 1: Schematic structure of the electrode arrangement of the measuring cell to determine relative permittivity (Epsilon+ from flucon fluid control GmbH)
Based on the relative permittivity Ɛr and the conductivity of the liquids, the relaxation time in a conductive or dissipative container or plant component can be determined as defined in TRGS 727[1] according to the following equation:
Ƭ = Ɛ0 · Ɛr / k
The relative permittivity is also required to adjust or correct functioning of capacitive level probes, among other things.
[1] TRGS 727 Avoidance of ignition hazards due to electrostatic charges, January 2016