Application Guidelines
Differential pressure indicators react to the pressure drop across the filter that is caused by the flow of fluid through the filter housing and element. These devices measure the difference in pressure upstream and downstream of the filter element, regardless of the system pressure. They are utilized in most pressure and in-line return filters. Static pressure indicators measure only the build-up of pressure upstream of the filter element (downstream pressure is ambient - tank vented to atmosphere). Consequently, if any components are located downstream of the filter, the indicator will measure the pressure drop caused by the filter and that component, thus, causing a false reading of Delta P across the filter. As a result, static indicators are recommended only on filters that discharge directly to vented tanks and have minimal backpressure. A filter that incorporates a differential pressure indicator should be used whenever there is a significant impediment to flow in the line after the filter, even when system pressure is relatively low. For example, the filter in the feed line of a lube system requires a differential pressure indicator, although the system pressure may be low.

Differential Pressure
Indicator Operation
As the differential pressure across the filter increases, the piston/magnet assembly is driven down against a spring until the attractive force between the magnet and indicator pin (Type 1) or a switch actuator lever (Type 2) is reduced sufficiently to allow the indicator to trip. In a visual indicator (Type 1), tripping results in the indicator pin rising and giving visual indication that the filter must be serviced. In an electric indicator (Type 2), tripping causes a switch to make or break, permitting a remote indication to warn of the need for servicing. When the Delta P drops below the trip pressure for any reason, (installation of a clean element, heating of the oil, etc.), the piston/magnet assembly returns to its original position. With a visual indicator, the pop-up indicator pin may then respond in one of two ways: (1) With Manual Reset (type BM) the pin remains extended, even after the system is shut down, and must be physically pushed down to reset (2) With Automatic Reset (type B) the indicator pin retracts to its original position along with the piston. With all electric indicators, the circuit is automatically restored to its original normally closed or normally open position once the Delta P drops below the trip setting.

Static Pressure Indicator Operation
Increasing pressure upstream of the filter acts upon a diaphragm in the indicator (Type 3) and causes the indicator pin to overcome an opposing spring force until it trips at a pre-set pressure. The indicator pin automatically resets once pressure is reduced below the trip pressure. (Note: certain indicators have a red/yellow/ green display in addition to, or instead of, the pop-up indicator pin). Electric static pressure indicators, which also operate mechanically, are available as well. These, too, reset automatically.

General Indicator Type Drawings