ASTM D150, IEC 60250 test method is used to measure Dielectric Constant (DC) and Dissipation Factor (DF) values for insulating materials. DC is the ratio between capacitance induced by two metallic plates when an insulator is kept between them, and the capacitance induced by the same plates with air or a vacuum filled in the space between the plates. Materials with low DC value make good insulators. A material with a higher DC value is used in high capacitance electric applications.
DF is the reciprocal of the ratio between the capacitive reactance and resistance of insulating material at a specified frequency. It is an indicator of the inefficiency of insulating material.
In the ASTM D150 test method, a specimen is kept between two metallic plates of the measuring device, and capacitance is measured. This activity is repeated without placing the specimen between the two electrodes. The ratio of these two values is calculated as the dielectric constant.
A flat specimen, larger than circular electrodes with a diameter 50 mm (2 inches), is used for the measurement for the ASTM D150 test method.
Dielectric constant (DC) = Observed Capacitance (Material as dielectric)/Observed Capacitance (Air or Vacuum) as dielectric.