Liquid Conductivity Testing

BS EN 60079-32-2

The liquid conductivity test is conducted to establish whether a liquid will allow an electrical charge to migrate through it or not and therefore evaluate its ability to accumulate and retain a charge.

The liquid to be tested is slowly charged into a cylindrical, measuring test cell comprising of an inner pole and a concentric outer electrode. A voltage is applied, and the current is measured from one electrode to the other. Using the current value measured, a liquid conductivity is calculated using the test cells constant, determined from its geometry.

The test classifies a material using the following standardised boundaries:

The level of charge accumulation in a particular liquid, and therefore the electrostatic hazard that static electricity can be created, is strongly dependent upon its electrical conductivity and dielectric constant (relative permittivity), εr. To describe the possible hazards and associated means of prevention. As per PD IEC/TS 60079-32-1:2018 – Explosive atmospheres. Electrostatic hazards guidance, liquids may be classified according to their conductivity as follows:

• high conductivity > 10 000 pS/m;
• medium conductivity between 25 × εr pS/m and 10 000 pS/m;
• low conductivity < 25 × εr pS/m.

For liquids with a dielectric constant of around 2, (e.g. hydrocarbons), these classifications reduce to:

• high conductivity > 10 000 pS/m;
• medium conductivity between 50 pS/m and 10 000 pS/m;
• low conductivity < 50 pS/m.

For liquids with a dielectric constant that is substantially higher than 2 or for liquids whose dielectric constant is unknown, the border limit for low conductivity is usually set to 100 pS/m. The upper border limit of medium conductivity remains at 10 000 pS/m.

Although the material may be a self-heating substance, if transported within specified volume restrictions, exceptions can be made and therefore huge costs are avoided. These exemptions can only be determined through testing.

The conductivity dictates how efficiently charge migrate through a material by electrical conduction. The lower the conductivity value, the more resistive the liquid is. Low conductivity liquids will accumulate and retain charge presented to it in all situations. high and most mid-range conductivty materials (conductive and static dissipative) will dissipate charge, providing it has a good path to earth. This can be achieved by handling these liquids in a well earthed environment (earthed conductive or static dissipative containers and plant equipment). The build-up and retention of charge on liquid or equipment (conductive or insulating) creates a hazard if the charge is suddenly released in the form of a discharge which can cause an ignition of a flammable atmosphere. Many solvents have extremely low minimum ignition energies, meaning most types of electrostatic discharges will ignite their vapours.