Abstract: A liquid filter (10) housing an annular filter element (16) has an annular space or inlet chamber (24) between the filter element and the housing and viewable through the housing such that an operator can see the level of liquid in the inlet chamber as an indication of when to replace the filter element, the higher the level of liquid in the inlet chamber the greater the pressure drop across the filter element. A change interval plugging indicator (102), preferably part of the filter element, is provided by a gas trap and pressure responsive release mechanism (104) trapping gas in the upper section (23) of the inlet chamber until a designated release pressure is reached, corresponding to a desired terminal pressure, to prevent premature plugging indication otherwise indicated by rising liquid level in the inlet chamber. Accurate and reliable calibration is enabled by separating the filtration function and the plugging indication function.

Abstract: A liquid filter (10) housing an annular filter element (16) has an annular space or inlet chamber (24) between the filter element and the housing and viewable through the housing such that an operator can see the level of liquid in the inlet chamber as an indication of when to replace the filter element, the higher the level of liquid in the inlet chamber the greater the pressure drop across the filter element. A change interval plugging indicator (102), preferably part of the filter element, is provided by a gas trap and pressure responsive release mechanism (104) trapping gas in the upper section (23) of the inlet chamber until a designated release pressure is reached, corresponding to a desired terminal pressure, to prevent premature plugging indication otherwise indicated by rising liquid level in the inlet chamber. Accurate and reliable calibration is enabled by separating the filtration function and the plugging indication function.

Abstract: An annular space or inlet chamber 24 between the filter element 16 and housing of a liquid filter is viewable such that an operator can see the level of liquid in the inlet chamber as an indication of when to replace the filter element: the higher the liquid level, the greater the pressure drop across the filter element. A change interval plugging indicator includes a porous outer wrap 50 that traps gas in an upper section 23 of the inlet chamber below a designated release pressure and releases trapped gas upon attainment of the release pressure causing a rapid rise in liquid level within the housing and corresponding to a desired terminal pressure. The indicator prevents premature plugging indication otherwise indicated by a slowly rising liquid in the chamber. Accurate and reliable calibration is enabled by separating the filtering and indicating functions.

Abstract: A liquid filter (10) housing an annular filter element (16) has an annular space (24) between the filter element and the housing and viewable through the housing such that an operator can see the level of liquid in the annular space as an indication of when to replace the filter element, the higher the level of liquid in the annular space the greater the pressure drop across the filter element. The rise in liquid level in the annular space is delayed for applications where the filter element is otherwise changed prematurely and has a longer life than otherwise indicated by the rising liquid level in the annular space. The delay is provided by a liquid and vapor impermeable sleeve (50) trapping vapor. In a further aspect, the sleeve is provided with apertures (70, 72, 74, 90, 92) filled by liquid-soluble buttons (76, 78, 80, 94, 96) providing filter element change interval indication, including advance sequential indication.

Abstract: A liquid filter (10) housing an annular filter element (16) has an annular space or inlet chamber (24) between the filter element and the housing and viewable through the housing such that an operator can see the level of liquid in the inlet chamber as an indication of when to replace the filter element, the higher the level of liquid in the inlet chamber the greater the pressure drop across the filter element. A change interval plugging indicator (102), preferably part of the filter element, is provided by a gas trap and pressure responsive release mechanism (104) trapping gas in the upper section (23) of the inlet chamber until a designated release pressure is reached, corresponding to a desired terminal pressure, to prevent premature plugging indication otherwise indicated by rising liquid level in the inlet chamber. Accurate and reliable calibration is enabled by separating the filtration function and the plugging indication function.