This test method describes a procedure for determining the insoluble contamination level of the downstream side of filter elements. Results of this procedure represent the particulate release rate of the tested filter element under the prevailing conditions of the test and may be used for comparative evaluation of the effectiveness of various cleaning methods or cleanliness of elements as received from manufacturers. Because of the variety of conditions which may exist even under the provisions of this procedure, it is difficult to correlate data from one testing agency to another. The data obtained by this procedure do not necessarily indicate qualitatively or quantitatively, the contamination which may be released by a filter element into the operating fluid during service. When properly conducted, however, the procedure will show marked differences between various cleanliness levels of filter elements. Because of the probability of extraneous contamination caused by conducting this test in an uncontrolled atmosphere, all operations should be conducted in an environment equivalent to that for FED-STD-209, Class 100,000.