Pressure differential indicator with non-mechanical thermal lockout

Abstract

An excessive pressure differential indicating device for use with a filter in a liquid circulating system. The device has a reciprocable piston that reciprocates between a high pressure location, when the pressure differential is not excessive, and a low pressure location, when the pressure differential is excessive. The device has an indicator button with a magnetically conductive element, and the button is retained in a non-indicating position when the reciprocable piston is at the high pressure location by magnetic attraction between a permanent magnet carried by the reciprocable piston and the magnetically conductive element, a spring being provided to bias the indicator button to an indicating position when the reciprocable piston moves with the permanent magnet away from the indicator button to the low pressure location. A thermal ferrite ring, which is magnetic at low temperatures and substantially less magnetic at high temperatures, is provided to retain the indicator button in its non-indicating position at low temperatures, even if the reciprocable piston moves to the low pressure location, since the excessive pressure differential which causes such movement of the reciprocable piston can, at low temperature, be caused solely by the increased viscosity of the circulating liquid, as opposied to an excessively clogged condition of the filter.