Test Method for Minimum Ignition Energy (MIE)
1. Related standards:
2. Differences between standards:
Test according to EN 13821: The sample must not contain particles larger than 500 μm and the median must be less than 63 μm. If the moisture content of the operating sample is not known, it shall be dried to equilibrium at 50°C in a vacuum or 75°C at ambient pressure.
Test according to ASTM E2019: 95% of the sample must be less than 75 μm. According to the standard, the ignition energy decreases with increasing moisture content in food powders. No specifications are given, but efforts should be made to remove moisture.
3. Test Apparatus:
The device used for the measurement is a Hartman tube made of transparent material and a volume of 1 liter (Figure 12).
Figure 8.: Test apparatus of MIE
4. Amount of sample required:
5. Purpose of the measurement:
Determination of the minimum external energy required to ignite a dust sample capable of sputtering. This is not a material property, as it is a value that strongly depends on the particle size distribution, the shape of the particles, and the moisture content of the sample.
6. The measurement procedure:
The sample must be tested as received.
The sample is placed in a dish at the bottom of the tube, from where compressed air will pulverize it and create a dust cloud that passes through an electric arc.
Two brass electrodes are inserted into the Hartman tube, between which sparks of known energy are emitted.
The measurement involves testing dust clouds of different concentrations with different spark discharges of known energy. The dust concentrations for the tests are greater than the minumum explosive concentration.
The test should be carried out until flammation is no longer present. In this case, there should be twenty repeated negative results.
The ignition sparks will be caused by the discharge of the manually set capacitors. The energy of the spark can be determined from the capacitance (C) of the capacitor, the charging voltage (V1), and the discharge voltage (V2).
7. Result of the measurement:
The result of the measurement is the minimum ignition energy (the MIE itself), which is the energy required to ignite the most sensitive dust concentration. It should always be given as a range, where the lower limit is the maximum energy where no explosion has yet occurred and the upper limit is the minimum energy where ignition is already occurring. An example of the evaluation of the result is shown in Figure 8.
8. Limits of the test:
The test procedure mentioned above is only applicable to dusts, not to vapours or gases (for these there is a similar specification, but not the same as presented here).
The test must be carried out at room temperature (20°C) because the increased temperature drastically reduces the value of the minimum energy.
In the case of dust samples containing volatile solvents, the test does not provide reliable results. In this case, the MIE test must be performed on the solvent vapour, not the dust, according to the vapour specifications.