Automatic or manual reset— All Hydac indicators with electric switches reset automatically to their original position when the pressure across the filter drops below trip pressure. This is true for our Type B visual indicator. However, on the type BM visual indicator with manual reset, the signal arm extends once the trip pressure is exceeded and remains that way until physically reset. The advantage, here, is that the indicator signals that the element is dirty even after the system is shut down, thus, simplifying maintenance.

Thermal lockout— When mobile and other equipment is started in cold weather, the hydraulic fluid is likely to be highly viscous until it approaches normal operating temperature. The high pressure drop created by a highly viscous fluid can trip the indicator and falsely signify that the element is clogged. An optional thermal lockout device, available on many of our electric indicators, prevents the indicator from tripping until the fluid reaches a certain specified temperature. The device consists of a switch in series in the indicator circuit, which is actuated by a bimetal strip that changes shape according to temperature. The thermal lockout feature may be chosen so that the indicator is deactivated at a fluid temperature less than 100˚F ±5˚. Because electric indicators automatically reset once the fluid heats up, thermal lockout is necessary only when a false signal of filter condition during cold start-up poses a problem.

SPDT switches — Hydac’s differential pressure indicators and most static pressure electrical indicators contain single-pole, double-throw (SPDT) switches. These switches come with normally open (NO) or normally closed (NC) contacts when the pressure differential is below the trip point. The choice between  NO or NC contacts is determined by the requirements of control circuitry.

Magnetic coupling — Most of our indicators use magnetic coupling, which separates the fluid from the actuating device. The benefit is that there is no need for a dynamic seal. Therefore, far less chance exists for fluid leakage under high system pressure.

Application guidelines

Differential pressure indicatorsreact to the pressure drop across the filter caused by the flow of fluid through the filter housing and element. They do this by sensing the difference in pressure upstream and downstream of the filter element, regardless of the system pressure. They are used in most pressure and in-line return filters.

Static pressure indicatorsmeasure only the buildup 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 ∆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 resistance 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 lubrication system requires a differential pressure indicator, although the system pressure may be low.

∆P indicator operation

As the differential pressure across a filter increases, a 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 ∆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 pin may then respond in one of two ways. With manual reset, the signal pin remains extended, even after the system is shut down, and must be physically pushed down to be reset. With automatic resetthe signal pin reverts back to its original position along with the piston. With electric indicators, the circuit is automatically restored to its original normally closed or normally open position once the differential pressure drops below the trip setting.

Static pressure indicators

With static pressure indicators, increasing pressure upstream of the filter acts on a diaphragm in the indicator (Type 3) and causes the signal 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. Electric static pressure
indicators, which also operate mechanically, are available as well. These, too, reset auto-


Jeff Ruth is product manager - industrial filtration at HYDAC Technology Corp., Bethlehem, Pa. For more information, visit Click here to download a PDF brochure describing HYDAC’s filter clogging indicators.