LIMITING OXYGEN CONCENTRATION
In general, nitrogen is used as an inert gas, therefore the following test conditions are based on nitrogen only.
The required nitrogen/air mixtures can be produced easily with the partial – pressure procedure.
Prior to 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 vol %), a second series will be done at e.g. 10% O2 in N2 over a wide range of dust concentrations.
If the tests are positive, a possible value of the limiting oxygen concentration can be estimated by extrapolation of the two Kmax-values to 0 m·bar/s. 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.
In this manner further tests have to be done until dust explosions are no longer possible.
To establish no ignition, there must be at least three trials with the same dust concentration.
The oxygen concentration which will just not allow an explosion of the dust/air/inert gas mixture in three ignition tests is stated as the limiting oxygen concentration LOC.
With decreasing oxygen concentration, the optimum dust concentration is shifted to lower values. The tests must therefore also be performed especially in this concentration range (e.g., 30, 60 g/m³).