Scope:

ASTM D257 test method covers direct-current (DC) procedures for measuring DC insulation resistance, volume resistance, and surface resistance. Volume and surface resistivity of electrical insulating materials are calculated from volume and surface resistance values and specimen and electrode dimensions. Corresponding conductances and conductivities are also calculated consequently. Volume resistivity or conductivity determinations are often used to check the uniformity of an insulating material during its production and processing. Resistivity values provide an indirect measure of moisture content, degree of cure, mechanical continuity, or deterioration of materials. ASTM D257 test results also help in detecting conductive impurities that affect the quality of the material and that are not readily detectable by other methods.

Test Procedure:

For the ASTM D257 test, a standard size sample is placed between two electrodes. Then, a potential of specified voltage is applied for 60 seconds and the resistance is measured. Finally, the volume or surface resistivity is determined and the apparent value is given for 60 seconds of electrification time.

Specimen size:

A 4 inch disk is desirable, or a 4 inch square can also be used for the ASTM D527 test. The least possible size of a specimen is a 3.5 inch disk.

Data:

Surface and Volume resistivity is calculated.
Volume Resistivity is given in ohms-cm
Surface Resistivity is given in ohms (per square)

Conclusion:

ASTM D257 is a standard test method for DC Resistance or Conductance of Insulating Materials.