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The choking of fuel filters in most services is caused by soft organic trace constituents of the fuel, and is independent of the extraneous contaminants over the range of concentrations normally encountered in service It is possible for a filter which is reasonably efficient initially to deteriorate in service to such an extent that most of the damaging contaminants applied pass through the filter. Such a filter, when tested with hard particles only, can show a continuously improving efficiency due to the filtering action of the bed of retained particles. The choking life, as determined by the quantity of hard particles it retains before reaching a chosen limiting resistance, is a measure of the effective filtering area and the volume available for retained contaminants rather than of the life which would actually be experienced in service. It is therefore important in testing fuel filters that the service choking process should be reproduced A combined choking and particle transmission test is described which gives information on the service life to be expected and on the particle transmission of the filter at any stage during that life. The methods of preparation and specification of the powders and choking medium used in this test are also described

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