Abstract: An intake filter (1) or a coalescing filter (4) of a compressor or vacuum pump, or an in-line filter has its identity recorded in a transponder (50, 54), memory chip or other device such as a magnetic or magnetostrictive strip. Interrogation e.g. by means of antennae (52, 56) enables a control unit (55) to verify the identity of the filter and to store the result in memory (57). A record can therefore be maintained of whether approved filters are being used and whether they are exchanged at approved intervals.

Abstract: A filter element for gas or liquid comprises a layer of glass, ceramic, metal, mineral wool or organic fibers containing, wholly within the layer, an apertured sheet, for example of steel or aluminum, the fibers being bound to one another throughout the layer and through the apertures, as well as to the sheet itself, by a synthetic resin such as silicone resin or polyurethane. The filter element is formed by mounting the apertured sheet a selected distance above a filter surface on which the fibers build up while a slurry containing the fibers is drained through the filter surface. When the fibers have built up to a predetermined distance above the sheet, the fibers containing the sheet are removed and the assembly impregnated with a synthetic resin and cured. Two or more grades of slurry mixture may be used in succession to produce a density grading through the thickness of the fibrous layer.