Isothermal basket test

Self heating materials

A surprisingly large number of materials exhibit self heating properties. In order to safely transport, store and handle such materials without risk of fire or spontaneous combustion, laboratory scale oven tests have been developed that allow scale up to much larger industrial size applications. Two tests are available:

Isothermal basket test

Laboratory oven tests are carried out according to BS EN 15188 (Determination of the spontaneous ignition behaviour of dust accumulations). The material under test is filled into a cube shaped basket ranging in size from 25mm to 100mm and made of stainless steel mesh. The basket is suspended in an air circulating oven, preheated to a known temperature, with thermocouples both inside out outside the basket to record temperature. Data logging is used to plot temperature against time during the test which may last up to 48 hours. Two types of behaviour are observed: sub-critical, where the sample temperature increases above oven temperature, but then slowly drops back after reaching a peak; super-critical, where the sample temperature continues to rise above oven temperature and heats to ignition. The division between these two behaviours is quite sharp with most self heating materials and is usually determined to an accuracy of 2K. By plotting the logarithms of the volume to surface area ratios of differently sized samples versus the reciprocal values of the respective self ignition temperatures, a straight line is produced. The line may then be extrapolated to give insight into the self ignition behaviour of material held in much larger storage and process conditions.

UN 4.2 Classification -

substances liable to

spontaneous combustion

The UN 4.2 classification is a similar isothermal basket test, but groups materials based on ignition / non ignition in two sizes of basket and at three different oven temperatures. Testing is required for dangerous materials under the UN Recommendations on the Transport of Dangerous Goods.