What is Volume and Surface Resistivity

05 Feb 2020 | Posted by Andrew Onions

Volume resistivity represents an insulating material’s resistance to leakage current through the body of the material – this is measure is used in testing electrical insulation matting and sheeting such as those covered by BS921, BS EN IEC 61111, BS EN IEC 61112, ASTM D178 and ASTM F2320. The test method calculates the ratio of the potential gradient in relation to the current in a material with the same density. A direct-current resistance between opposing faces of a one-meter cube of the material numerically equates to volume resistivity in SI (Ohm-m). Volume resistivity represents the electrical resistance through a cube of insulating material. If measured in Ohm centimetre’s, it demonstrates the electrical resistance through a one centimetre cube of the material. Equally Ohm-inches, indicates the electrical resistance through a one-inch cube of the material.

Volume resistivity can indicate contamination if the desired level of resistivity or conductivity isn’t achieved.

Surface resistivity is the resistance to leakage current along an insulating material’s surface – this measure is primarily used in testing static dissipative materials including those used in potentially explosive environments such as subsurface mining operations, and for flooring, worktop and workstation matting tested in accordance with standards such as IEC 61340-5-1, ASTM F150-06-2013, ANSI/ESD S20.20-2014, ANSI/ESD STM7.1-2013 and ANSI/ESD STM97.1-2015. Typically, two parallel sets of electrodes are used in contact with the material’s surface to measure surface resistivity. Therefore, the potential gradient’s quotient (V/m) and current per unit of electrode length (A/m) represent the resistivity.

The surface resistivity and quotient cancel are generally measured in Ohms, however because the electrodes’ four points form a square some test results use Ohms per square due to its more descriptive nature. This measurement does not take physical dimensions such as thickness and diameter into consideration. Since it only determines the surface’s resistivity, only one physical measurement is required.

In applications involving static electricity dissipation such as electronics manufacturing, low surface resistivity is ideal. In their base format rubbers and plastics have high levels of surface resistance. To increase conductivity, manufacturers will add carbon black or a surface treatment.

Volume Resistivity and Surface Resistivity Testing Techniques

Test methods used in Europe for measuring resistivity include ISO14309, ISO340, and IEC60093. Resistivity testing measures an insulator’s resistivity to leakage current by implementing the following steps:

• Applying a known voltage to the material

• Recording the current created by the voltage

• Using Ohm’s law to calculate the observed resistance

• Determining resistivity based on the specimen’s physical dimensions

The final resistivity measurement depends on many outside factors, including:

• Applied Voltage: The amount of voltage applied to the material greatly alters the test’s final results. To counter this factor, sometimes a test involves varied voltage to establish voltage dependence.

• Electrification Time: The tested material charges at an exponential rate when exposed to voltage for an extended period. Therefore, the resistivity of a sample increases over time during the test. This must be accounted for to get an accurate calculation.

• Environmental Factors: High humidity levels create lower resistivity compared to lower humidity levels. The testing environment’s conditions have a large impact on potential results.

Because of these variables, these conditions should stay constant between tests when comparing multiple tests. ASTM standards recommend the commonly used method of 500V applied for 60 seconds to make results easily comparable to one another. The results of this test can measure volume and/or surface resistivity, depending on their application.

MacLellan Rubber offer a wide range of Electrical Insulation and Static Dissipative sheeting and rubber matting products.