Electrical surface resistivity is the electrical resistance of the surface of a material, in units of Ohms per Square (Ω/sq). The surface resistivity is independent of the material size or thickness. It is a calculated based on the surface resistance of the material and the known geometry of the electrodes. It signifies the material’s resistance to current flow when an electrical potential difference is applied to the surface. Surface resistivity differs from volume resistivity as it specifically addresses the electrical characteristics of a material’s surface layer, influencing its electrical behaviour in applications such as coatings, films, and insulating layers.
Factors that could influence a material’s surface resistivity the material composition, surface treatments, moisture, temperature, contamination, and mechanical stress can impact surface resistivity. These factors alter the surface’s conductivity or insulation properties.
Testing Principle and Methodology
Two electrodes with equal length and distance between them are arranged on the surface of the material and direct current voltage is supplied to one end of the electrode while measuring the current at the other end of the electrode. Alternatively, a concentric test cell with a known cell constant can be utilized. The electrical resistance can be measured directly by utilizing a megger-ohmmeter. The surface resistivity is then calculated from the formulae below.
Where:
P     is Resistivity in ohm meters (Ω/square)
RÂ Â Â Â Â Resistance
LÂ Â Â Â Â Length of electrode
DÂ Â Â Â Â Distance between electrodes
The test is typically performed under different humidity conditions applies to only solid material tested in its commercial form.
Factors that could affect the surface resistivity measurement include temperature, relative humidity and surface cleanliness. It is therefore imperative to ensure the surface is clean and free from contamination that could impact the results.
Surface Resistivity Test Electrode manufactured by ANKO.
Applicable Standard
The surface resistivity test is conducted in accordance with the American Standard Testing Method (ASTM) D257. ‘Standard Test Methods for DC Resistance or Conductance of Insulating Materials’, BS 5958: Part 1: 1991 and BS 7506: Part 2: 1996, and NFPA 77.
Data interpretation
The surface resistivity of the material is classified as follows:
| <105 | Conductive / Low Resistivity |
| 105 to 109 | Static Dissipative / Medium Resistivity |
| >109 | Non-Conductive / High Resistivity |
By knowing the surface resistivity of your material, measures can be put in place to safely handle the material. This can be achieved by handling these materials in a well-earthed environment. The build-up and retention of charge on the surface of a material or equipment possess the biggest threat if the charge is suddenly released in the form of a spark discharge, which can cause an ignition of a flammable atmosphere. Therefore, it is imperative to know and understand the electrostatic properties of your materials to ensure they are handled correctly to prevent discharges and potential ignition of flammable atmospheres.
When to perform Surface Resistivity
It is recommended to perform surface resistivity testing of liners, laminates and sheets of materials such as clothes used for PPE’s where accumulation of electrostatic charge is to be controlled. Typically, the higher the surface resistivity value, the more resistive or less conductive the material is. High resistivity or Low conductivity materials tend to accumulate and retain charges. Low resistivity materials or static dissipative materials tend to conduct or dissipate charges easily.
Why work with Prime Process Safety Center
- Prime Process Safety Center is a leader in process safety testing with very experienced laboratory personnel. At Prime Process Safety Center our goal is to provide accurate, reliable and defensible data that meets industry and regulatory standards. We understand the need for the quality of your data, and we work assiduously to achieve just that.
- We are knowledgeable and experienced in performing surface reisitivity test, ensuring accurate and reliable results.
- We have state-of-the-art surface resistivity testing equipment, providing precise and sensitive measurements.
- We follow strict testing protocols and quality control measures to ensure consistent and reliable test results.
- Our team can interpret and analyze the data obtained from the tests, providing valuable insights and recommendations for your specific application or research.
FAQs
What is Surface Resistivity?
Answer: Surface resistivity is the measure of a material’s ability to resist or conduct electrical current across its surface when an electrical potential difference is applied.
How is Surface Resistivity Different from Volume Resistivity?
Answer: Surface resistivity refers to the resistance along the surface area, while volume resistivity is the resistance through the volume of a material. Surface resistivity deals specifically with the material’s surface conductive properties.
What Units are Used to Express Surface Resistivity?
Answer: Surface resistivity is commonly expressed in ohms per square (Ω/sq), representing the resistance between two opposite edges of a square sample of the material.
Why is Surface Resistivity Important?
Answer: Surface resistivity is crucial for assessing a material’s suitability for electrical insulation, static dissipation, and determining its conductive or insulative properties on the surface.
How is Surface Resistivity Testing Performed?
Answer: Surface resistivity testing involves applying a known voltage across the material’s surface and measuring the resulting current to calculate the resistance. The resistance and electrodes’ length and distance determine the surface resistivity.
What Factors Influence Surface Resistivity?
Answer: Material composition, surface treatments, temperature, moisture content, contaminants, and environmental conditions affect surface resistivity.
What Materials Have High Surface Resistivity?
Answer: Insulating materials such as ceramics, glass, and certain plastics often exhibit higher surface resistivity, making them suitable for electrical insulation purposes.
Where is Surface Resistivity Testing Applied?
Answer: Surface resistivity testing is used in various industries for selecting materials in electronics, manufacturing of antistatic products, and applications requiring controlled conductivity or insulation.
How Does Surface Resistivity Affect Electrical Systems?
Answer: Understanding surface resistivity helps in selecting materials for electrical components, systems, or applications where controlled surface conductivity or insulation is essential for proper functioning and safety.
What Standards Govern Surface Resistivity Testing?
Answer: International standards from organizations like ASTM (American Society for Testing and Materials) and IEC (International Electrotechnical Commission) provide guidelines for surface resistivity testing methods.
