Differential Pressure Indicators and Switches

Estimated Reading Time: 3 minutes, 15 seconds.

Do you know when to change your filters?

Differential pressure (∆P) indicators and switches are instruments that provide a visual reading of the pressure on the inlet versus the outlet side of the filter element in order to notify the operator of the filter condition. This allows a replacement filter to be installed before filter element bypass occurs. As a filter becomes plugged with particulate matter and other various contaminants, the flow restriction through the element increases, resulting in reduced efficiency and increased energy consumption. These indicators must be read under a flow condition to get an accurate reading.





∆P across the filter increases as contaminant is trapped within the industrial filtration medium. A ∆P indicator actuates at P1, signalling the need for element change before the bypass relief valve opens at P2. The bypass valve protects the filter and system from excessive differential pressure. Without a bypass valve, continued operation at higher ∆P risks degradation of filtration performance (point A) and filter element collapse (point B) where the integrity of the filter element is lost.





Technical principle of the mechanical indicators:



Differential pressure indicators operate by sensing the ∆P between ports upstream and downstream of the filter element. When the ∆P across the internal piston/magnet assembly reaches a preset value, determined by the range spring, the piston assembly moves downward, reducing the attractive force between the magnet and indicator button. The indicator button spring then overcomes the reduced magnetic force and releases the button to signal the need for element change. Activation can be visual using a button as shown here or electrical using a microswitch.



Electrical Switches



Electrical switches have an automatic reset function that allows them to be used as system warm-up monitors. High fluid viscosity associated with low temperature start-ups will cause a higher pressure drop across the filter element. Electrical switches will signal high differential pressure until the system reaches operating temperature.


As the system warms up, the differential pressure is reduced, and the range spring returns the piston assembly to its normal position, thus restoring the magnetic force between the magnet and the actuator lever.  The restored magnetic force is sufficient to overcome the switch spring force, allowing the single pole double throw switch (SPDT) to signal a differential pressure below the switch setting.


Single Pole Double Throw Switch


The SPDT switch allows this device to be used in a typical green-red light circuit.  When the green light signals pressure drop below the actuation pressure, there is element life remaining.  The red light signals pressure drop above the preset value... either a cold start or the need for an element change. The two-light circuit also serves as a continuity check; one of the two lights should always be on.





Visual Indicators






All visual indicators show whether a preset differential pressure has been exceeded at any time – even after the system is shut down.  When the factory-set differential pressure is exceeded, the red button pops up and stays up until it is manually reset. 


When the differential pressure is reduced, the range spring returns the piston assembly to its normal position; however, the gap between the magnet and the now extended indicator button is too great for magnetic force to overcome the button spring force.  The button, therefore, remains in the extended position until differential pressure is reduced and the button is manually reset.


Thermal Lockout


An optional bimetallic thermal lockout prevents actuation below 32 °F and allows actuation above 80 °F system temperature.  Between 32 °F and 80 °F the thermal lockout device is in transition.  Above 80 °F the indicator will actuate when the element is plugged and the differential pressure exceeds the pre-set limit.