Knowledge · Hydraulics
Filter clogging indicator on hydraulic filters: function and selection
A clogging indicator tells you when the filter element is loaded and must be changed, before the bypass valve opens and unfiltered oil reaches the circuit. To do so it measures the pressure drop across the element, not the cleanliness of the oil. Choosing an indicator means deciding three things: the measuring principle, the readout and the trip point. All three follow from the system itself, the measuring principle from the mounting position, the readout from the monitoring, the trip point from the bypass valve.
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What the indicator measures
The clogging indicator measures the differential pressure across the filter element and trips when it reaches a set value. A clean element offers the oil little resistance, so the pressure drop is low; with every captured particle the flow resistance rises, and with it the differential pressure between inlet and outlet. The trip point sits deliberately below the opening pressure of the bypass valve. That way you change the element while it is still filtering, and before unfiltered oil flows past the element into the circuit.
What matters is what the indicator does not measure. It shows the loading of the element via pressure, not the cleanliness class of the oil. Oil cleanliness is stated as a cleanliness class per ISO 4406 and determined by particle counting, not by the clogging indicator. A tripped indicator therefore means that the element is loaded, not that the oil is contaminated by a particular class.
How much dirt the element holds before tripping depends on its dirt-holding capacity, determined in the multipass test per ISO 16889. The indicator is thus a warning at the end of that capacity, not an instrument for the condition of the oil. It sits where the pressure drop arises: on the pressure filter between pump and consumer, on the return filter ahead of the tank, or on the suction filter ahead of the pump. That mounting position decides whether differential pressure or back pressure is the right measuring principle.
Types
Clogging indicators differ along two dimensions: by the measuring principle, that is differential pressure or back pressure, and by the readout, that is optical or electrical. The two can be combined. There are therefore also optical-electrical indicators that show a colour field on site and at the same time send a signal to the controller.
| Type | Working principle | Readout or signal | Typical use |
|---|---|---|---|
| Differential pressure, optical | Pressure difference across the element | Colour field or pin, by visual check | Pressure and return filters, manned systems |
| Differential pressure, electrical | Pressure difference via a pressure switch | Contact to a warning lamp or controller | monitored and automated systems |
| Back pressure (return line) | Upstream pressure against tank or atmosphere | optical or electrical | Return and suction filters, unpressurised outlet side |
Reading and reacting
An indicator is only worth as much as the response to it. The sequence is always the same, whether you read a colour field or see a signal at the controller.
- Read at operating temperature. Cold, viscous oil drives up the differential pressure and can trip the indicator without the element being loaded.
- Check under flow, that is in operation under load, not at standstill. Without flow no meaningful differential pressure builds up.
- If the indicator trips at operating temperature, change the element. Merely resetting the indicator does not fix the cause.
- After the change, reset or check the indicator and document the change so that element life can be tracked.
- If the indicator keeps tripping early, clear the cause: heavy dirt ingress, aged oil or too fine a filter choice. Changing more often is not a solution.
Selection
Selection follows two questions: where does the filter sit, and how is the system monitored? The mounting position sets the measuring principle, the monitoring the readout. The remaining points make sure the indicator matches the element, the controller and the housing.
- Mounting position and filter type: pressure, return or suction filter decides between a differential-pressure and a back-pressure indicator.
- Readout: optical for manned systems with a visual check, electrical for remote monitoring and PLC connection.
- Trip point: must match the element and the bypass opening pressure so the indicator warns before the bypass.
- Cold-start suppression: useful where the system starts cold, so the cold start does not trigger a false alarm.
- Signal type and connection: normally open or normally closed, with a connector to suit the existing controller.
- Compatibility: connection and form must match the filter housing.
Filters and clogging indicators from stock
Whether pressure, return or suction filter: PTS hydraulics carries filters and matching clogging indicators from stock. Choose by mounting position and monitoring, and if the filtration sizing is unclear, engineering clears the cleanliness requirement before you order.
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