It is determined if a powder sample is explosible or not by means of several tests with different dust concentrations in the modified Hartmann apparatus.

Before performing more specific tests, it is sometimes recommended to perform a screening test to determine if the powder sample can cause a dust explosion.

The modified Hartmann apparatus consists of a vertical cylindrical glass tube with a volume of 1.2 litre and a diameter of 7 cm. By means of pressurized air a quantity of the powder sample is suspended in the presence of an ignition source. The ignition source used for the tests is a continuous spark generated by a high voltage transformer between two standardized electrodes placed at the bottom of the cylindrical tube. The energy content of the spark corresponds to an equivalent energy of about 10 Joule of a discharge (temporary) spark. On top of the glass tube a hinged cover allows for pressure relief in the case of a dust fire or explosion in the tube. The opening of the lid is monitored and gives an indication of “0” = no opening or lift of the cover, “1” = small lift or opening of the cover or “2” = complete opening of the cover. Concentrations between 36 mg and 3000 mg per 1.2 litre (30 g/m³ to2500 g/m³) are tested.

A powder is considered to be explosible if dust fires (with or without opening of the lid) or explosions are observed during the tests. If no dust fires or explosions are observed in three series of tests for any concentration, the powder is considered to be not explosible at the conditions of the test (modified Hartmann apparatus, continuous spark, high voltage transformer, 10 Joule). In that case, a confirmation test in a closed vessel (e.g. 20 l sphere) with a higher ignition energy (2000 Joule, pyrotechnical igniters) needs to be executed before the powder can be considered to be not explosive. Powders that are explosible and for which a maximum indication of “0” or “1” is observed over a wide range of concentrations belong to dust explosion class “St 1” (Kst < 200 bar.m/s).

The test is normally performed on the fraction of the powder with a particle size < 63 micron.

“NOT EXPLOSIBLE” means that there are no explosions observed with an ignition energy of approximately 10J. This result needs to be confirmed in a closed vessel test with a higher ignition energy (e.g. 20 l sphere, ignition energy 2 kJ cfr. EN 14034 Annex C) before the powder can be considered as being not explosible with reference to ATEX.

• EN ISO/IEC 80079-20-2:2016: Explosive atmospheres Part 20-2: Material characteristics – Combustible dusts test methods, Section 7.1
• VDI-Richtlinien 2263, Blatt 1: Untersuchungsmethoden zur Ermittlung von sicherheitstechnischen Kenngrössen von Stäuben (1990)
• Kühner AG, Dust explosion test: Modified Hartmann Apparatus
• W. Bartknecht, Staub Explosionen: Ablauf und Schutzmassnahmen (1987)
• R.K. Eckhoff, Dust explosions in the process industries (1997)