The C's and D's of dielectric measurements

The dielectric properties of materials ranging from electrical insulation to copier toner can indicate a manufacturing process that needs adjustment. While accurate dielectric-constant and dissipation-factor measurements require adherence to standards, you can assess your manufacturing process with some simple measurements that are accurate to approximately ±10%.

Figure 1. Measure a material's capacitance and resistance (left) compared to that of air (right) to calculate dielectric constanct (k') and dissipation factor (Dx). Make all measurements at the same distance between electrodes.

Figure 1a shows a specimen between electrodes of an LCR meter or other capacitance-measuring instrument. Figure 1b shows the same setup for measuring air. To make the capacitance measurements, use the same distance, h, between the electrodes for both measurements. To get a repeatable distance between the probes with and without the specimen, you need to build or buy a test fixture that lets you move the electrodes while measuring the distance between them with a micrometer.

In addition to measuring dielectric constant, you can measure dissipation factor (D_x), a material's ability to store energy at a specific frequency. D_x is the ratio of an insulating material's resistance to its capacitive reactance at a specific frequency. You can use the dielectric-constant-measurements to calculate dissipation factor:

D_x = D_xm - D_air

To calculate D_x, measure the dissipation factor of the specimen at electrode distance h_m. D xm is the measured dissipation factor with the specimen in the fixture, and D airis the measured dissipation factor of air over distance h_m.

To learn more about how to measure dielectric constants, download a paper by Jim Richards of QuadTech.