| 摘要 |
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.
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