While developing a standard for the determination of safety characteristics for hybrid mixtures the authors discovered, that, beside the ignition source, the mixing procedure is the main difference between the single-phase standards for dusts and gases. The preparation of hybrid mixtures containing a flammable gas and a flammable dust in the 20 L-sphere can be realized in different ways. Either the flammable gas is filled only in the sphere or only in the dust container or in both. In previous works, almost always the first method is applied, without giving any information on the accuracy of the gas mixtures. In this work the accuracy of the gas mixtures and the results of the tests applying two methods of mixing were studied. No significant influence of the mixing method itself on the safety characteristics explosion pressure p_ex and the normalized rate of pressure rise (K-value) was found. Obviously, homogenization of the gas mixtures can be obtained sufficiently by the turbulence that is caused during the injection from the dust container into the explosion chamber within a short time. However, the mixing procedure has a great influence on the accuracy of the gas amount of the mixtures obtained. Without modifying the 20 L-sphere by installing precise pressure sensors, assuring its tightness and performing gas analysis, it must be expected, that the accuracy of the gas mixtures is very low. This has a significant influence on the measured safety characteristics and may lead to unsafe facilities or unnecessary expensive safety measures.

This article focuses on determination of the minimum ignition energy of dust. For the measurement of the minimum ignition energy of dust are available device from different manufacturers. In this article, the comparison device from three manufacturers - Chilworth, Kühner and Anko are executed. For the experimental measurement of the minimum minimum ignition energy was chosen 5 dust samples so that they are represented sample of organic dust, synthetic organic dust and coal dust. The article briefly introduces each apparatus for determining the minimum ignition energy and there is a comparison of the results obtained with individual devices. Finally, it is an assessment the results obtained and used test equipment.