Measuring the relative dielectric constant of the dielectrics

For the E-field thruster research project we need to know the relative dielectric constant of the used dielectrics with good accuracy. This is not always available for certain materials from general tables, and we have to measure it. A professional measurement method is described below, which can be simplified by substituting the expensive special equipment with some similar available solution.

The relative dielectric constant e_r or K of an insulator is determined so that first the capacitance of the test capacitor C₀ is measured with air as dielectric. Then using the same capacitor with the same distance between the plates we measure its capacitance C_x using the examined dielectric (sample) between the plates. The relative dielectric constant can be calculated as:

If higher accuracy is needed then the edge effect should be taken into account, and the following more accurate formula should be used for calculating the relative dielectric constant:

The C_e is calculated from the formulas in the third column of the following table, for different shapes of test capacitors.

Calculation of C₀ and the correction factor C_e for different test capacitors

The electrodes used for measuring the e_r of solid a) and liquid b), c) dielectrics are shown in the following figure:

Electrode shapes for measuring the e_r of solid a) and liquid b), c) dielectrics

The electrodes are usually made from stainless steel, and the insulating parts from melted quartz or borosilicate glass. The measured results are influenced not only by the frequency and temperature, but also by the humidity and the applied voltage, which should be taken into account.

Steven Dufresne has conducted some preliminary simplified dielectric constant measurements for some insulators, planned to be used in the E-field thruster project. His measurements and the results can be found at: http://rimstar.org/materials/dielectrics/dielectrics.htm .

In our simplified method, the capacitance of the test condenser is measured with digital multimeter. An important practical problem arising from this and falsifying the measurement results is demonstrated and explained with a practical measurement that can be found here.

Created by Zoltan Losonc (feprinciples@on.mailshell.com) on 21 May 2003 and it was available for the feprinciples-group members. Last updated and published on 6 September 2003.

Tutorial Measuring the relative dielectric constant