Specific powder resistivity


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Specific powder resistivity

Powders can become electrostatically charged when bulk materials are handled, e.g. during pneumatic transport or even during simple decanting processes.

This can lead to dangerous discharges, e.g. in the form of cone discharges. However, these cone discharges can only form if the dust or powder is electrostatically insulating. If, on the other hand, the specific powder resistivity is  < 1010 Ωm, cone discharges can be excluded.

Identifying the powder resistivity allows the electrostatic properties of the powder to be analyzed and suitable safety precautions to be defined and taken.

The specific powder resistance of bulk material is identified in accordance with DIN EN 60079-32-2 (December 2015) – Electrostatic hazard test method using a Tera Ohmmeter TO3 by means of a powder electrode. To measure resistance, the test substance is placed in a powder measuring cell. This measuring cell consists of two conductive metal electrodes facing each other at a distance of 10 mm, and which are embedded in a Teflon body. The test subject is poured into the space between the metal electrons and scraped with a spatula.

Next, the powder electrode is closed with a lid and the measurement is performed. The bulk density is identified from the volume of the space between the electrodes and the mass of the test substance. Depending on the specific resistance, the measuring voltage can be, for example, 100 V or 500 V. The resistance of the bulk material is determined and the temperature and humidity are noted.

Taking the volume of the measuring cell (height, length, distance between the electrons) into consideration, the specific powder resistivity is identified.

A substance or material is classified according to its specific resistivity ρ in accordance with TRGS 727 as follows:

Conductive:               ρ ≤ 104 Ωm

Dissipative:                 104 Ωm < ρ <109Ωm

Insulating:                   109 Ωm ≤ ρ

The following limit values apply to bulk materials as defined in TRGS 727:

Low specific resistance:                          ρ ≤ 106 Ωm

Medium specific resistance:                   106 Ωm < ρ ≤1010 Ωm

High specific resistance:                         1010 Ωm < ρ