A device for indicating the dirtiness of a filter, by implication from the press. drop across it, consists of a diaphragm enclosed between two chambers and attached to a small coaxial piston in a sealed chamber. The upward movement of the diaphragm is recorded by a rod which remains in the highest position to which it is pushed by the piston. A restriction is placed upstream of the filter and the pressures before and after the restriction are applied to the upper and lower sides of the diaphragm. The press. downstream of the filter is applied to the piston. The movement of the diaphragm is not strongly dependent on the fluid flow rate through the filter.
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