Volume and Surface Resistivity

Volume and Surface Resistivity

ASTM D257, IEC 60093

Volume resistivity represents an insulating material’s resistance to leakage current through its body. It 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).

Surface resistivity is the resistance to leakage current along an insulating material’s surface. Two parallel electrodes within a distance of each other equal to their contact length come 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 lengths in the surface resistivity and quotient cancel are generally measured in Ohms because the electrodes’ four ends form a square. However, some test results use Ohms per square due to its more descriptive nature.

Volume Resistivity and Surface Resistivity Testing Techniques

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.

Volume Resistivity Testing Measurements and Applications

Volume resistivity represents the electrical resistance through a cube of insulating material. As measured in Ohm-centimeters, it demonstrates the electrical resistance through a one-centimeter cube of the sample. Similarly, when using Ohm-inches, it indicates the electrical resistance through a one-inch cube of the material.

Electronic devices contain a variety of chemicals meant to insulate or conduct. Volume resistivity testing for these chemicals ensures electricity travels through these components as intended. Determining the volume resistivity of electrical consumer products is an important part of safety standard testing. Volume resistivity in conductive pastes and other electronic components can indicate contamination if the desired level of resistivity or conductivity isn’t achieved.

Surface Resistivity Measurements and Applications

Surface resistivity defines a fixed surface length’s electrical resistance on an insulating material. 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. Accordingly, surface resistivity is measured between electrodes along the insulator material’s surface.

In materials testing, this measurement can determine the surface resistivity of plastics. In situations involving static electricity dissipation such as electronics manufacturing, low surface resistivity is ideal. On their own, engineering plastics have high levels of surface resistance. To increase conductivity, manufacturers often add carbon or a surface treatment. In general, surface resistivity testing is rarely applied to metals because they already have high conductivity.

Volume and Surface Resistivity Testing From NTS

NTS uses only the highest testing standards to determine the effectiveness and compliance of products and materials across multiple industries. As one of the largest networks of commercial laboratory testing, we can help you achieve your manufacturing goals with our cutting-edge equipment and training. Our vast selection of testing standards allows us to evaluate multiple products and follow industry criteria. To learn more about resistivity testing and our other services, get in contact with our team online.

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