Limiting Oxygen Concentration Test (LOC)
- Relevant standards
ASTM E2931-13 (earlier: ASTM WK1680).
2. Differences between standards:
Sample particle size rules:
EN 14034-4: it is recommended that the particle size of the sample should be determined by ISO 4225 (generally maximum particle size will not exceed 500 µm),
ASTM E2931-13: it is recommended that the test sample be at least 95% minus 75 µm.
Specifications concerning the moisture content of the sample:
EN 14034-4: the oxygen limit concentration decreases with decreasing moisture content, therefore the moisture content of the tested powder sample must be determined and recorded in the test report;
ASTM E2931-13: the moisture content of the test sample must not exceed 5%, otherwise the results of the test will be affected.
Applicable ignition source:
EN 14034-4: the recommended ignition source is two chemical igniters each having an energy of 1000 J.
ASTM E2931-13: the recommended ignition source is a 2500-J pyrotechnic ignitor.
Specification for the criterion of ignition or explosion (so-called “go or no go” test):
EN 14034-4: pmax-pignition≥0.5 bar,
ASTM E2931-13: PR=(pmax-Δpignitor)/pignition≥2, where:
§ (pmax): the maximum explosion pressure (absolute) reached during the test,
§ pignition: the absolute pressure at the time the ignitor is activated,
§ : the value of the pressure rise in the chamber due to the ignitor by itself in air at atmospheric pressure,
§ PR: pressure ratio.
3. Measuring equipment:
an instrument for measuring oxygen concentration.
Hollow sphere testing chamber with a volume of 20 litres manufactured by Kühner
4. Amount of sample required
500 g (or 1000 g, if the severity of the explosion is also measured).
5. Purpose of the measurement
To determine the highest oxygen concentration at which ignition of dispersed dust at ambient temperature and pressure is not possible.
6. The measurement procedure
The test must be carried out in a 20-liter spherical explosion chamber. This equipment is also suitable for determining the maximum explosion pressure of powders and Kst value.
The first step of the study is the analysis of the received sample in the received state in terms of moisture content and mass median diameter. If the moisture content is above 10 m/m%, the sample must be dried. If the median value of the particle size of the sample is greater than 63 µm, the sample shall be shredded.
The conditions of the test are the follows:
ignition energy: IE=2 x 1 kJ,
ignition delay: tv=60 ms,
initial pressure: pi=1013 mbar,
initial temperature: Ti=20 °C (water cooling).
The required nitrogen/air mixtures can be produced easily with the partial-pressure procedure. Before the tests, it is recommended to check the composition of these nitrogen/air mixtures with suitable instruments.
After the first test series in normal air (O2 = 20,8 % V/V) a second series will be done at e.g. 10% O2 in N2 over a wide range of dust concentrations.
7. Result of the measurement
If the tests are positive, a possible value of the limiting oxygen concentration can be estimated by extrapolation of the two Kmax-values. At this extrapolated value the explosion tests will be continued over a wide range of dust concentrations.
If positive tests are observed, they have to be repeated at a 1 vol% lower value of O2 in N2.
If negative tests are observed, they have to be repeated at a 1 vol% higher value of O2 in N2.
The test must be continued until the concentration of O2 where no explosion occurs is found. Three consecutive tests at the same dust concentration are required for a reliable result.
8. Limits of the test
The test is for solid samples only and cannot be used (directly) for gases and vapours (although a similar test is available for these).
The test considers only ambient pressure and temperature conditions.