A pressure switch is a simple, decisive device: it watches pressure and flips an electrical contact the instant that pressure crosses a set threshold. Where a transmitter reports a continuous number, a switch answers one question - is pressure above or below the limit - and acts on it directly. This guide explains how a pressure switch works, what set point and deadband mean, and why the PSH and PSL switch family sits at the core of wellsite shutdown and pump-protection logic.
Pressure Switch in one line: A pressure switch is a discrete on-off device that opens or closes an electrical contact when process pressure rises above or falls below a preset value, giving a simple trip signal rather than a continuous measurement. High-pressure switches (PSH) and low-pressure switches (PSL) are used to shut down equipment, sound alarms, and interlock processes, and they differ fundamentally from a pressure transmitter, which reports pressure as an analog signal for control and monitoring.
A pressure switch senses process pressure with a mechanical element - a diaphragm, piston, or Bourdon tube - opposed by a spring whose tension is adjustable. When pressure pushes hard enough to overcome the spring, the element snaps a microswitch or contact to change state; that trip pressure is the set point, and setting the spring is how you dial it in. A high-pressure switch is arranged to trip when pressure rises through its set point, and a low-pressure switch to trip when pressure falls through its set point, so together they bracket a safe operating window.
The second essential parameter is deadband, sometimes called the reset differential: the gap between the pressure at which the switch trips and the pressure at which it resets. Deadband exists on purpose. Without it, a pressure hovering right at the set point would chatter the contact rapidly on and off, and small pulsations would cause nuisance trips. With a defined deadband, once a high switch trips it stays tripped until pressure falls back well below the set point, which stabilizes the action. On many switches the deadband is fixed by design, and on others it can be adjusted separately from the set point.
In field tag conventions, a pressure switch high is a PSH and a pressure switch low is a PSL, and the doubled forms PSHH and PSLL denote the high-high and low-low switches reserved for safety shutdown rather than routine alarm. These devices are the muscle behind a wellsite emergency shutdown system. A PSH set above normal operating pressure catches a blocked outlet or overpressure and trips the ESD to close valves and stop inflow, while a PSL set below normal catches a line break, a leak, or a lost feed and shuts things down before the situation worsens.
Pump-off and equipment protection lean on the same devices. A low-pressure switch on a pump suction or a flowline can shut a pump down when pressure drops - a sign the well or tank has run low or the pump has lost prime - protecting the pump from running dry. A high-pressure switch on a discharge protects against a closed valve or plugged line. Because a switch acts directly on a contact, its trip can be wired straight into shutdown and motor circuits, giving a fast, hardwired protective action that does not depend on a controller reading and interpreting an analog value first. That directness and simplicity is precisely why switches remain standard for safety trips.
A pressure switch and a pressure transmitter answer different questions and often coexist on the same equipment. The transmitter provides the continuous story - the actual pressure, its trend, and how close it is drifting toward a limit - which a cloud SCADA such as Merobix displays and alarms on. The switch provides the hard boundary: an independent, discrete trip that acts whether or not the monitoring system is watching or even powered.
That independence is the point of a dedicated switch. In a well-designed safety scheme, the analog transmitter can raise an early alarm on a rising trend so an operator intervenes before anything trips, while a separate PSHH remains as the final, hardwired backstop that fires regardless of software. Keeping the trip on its own switch rather than deriving it from the transmitter avoids a single point of failure, which is central to how protective layers are built.
For remote monitoring, a switch's change of state is itself a valuable discrete signal. Wiring a pressure switch's contact into an RTU tells the control room the instant a trip occurs, so the SCADA can log and annunciate the event even though the switch, not the software, did the actual shutting down. The result is layered: the transmitter gives operators the continuous picture and the room to act early, and the switch gives the process a fast, reliable last line of defense whose action is still reported upstream.
A pressure switch is a discrete device that flips an electrical contact when pressure crosses a set point, giving a simple trip or alarm signal. A pressure transmitter outputs a continuous analog signal proportional to pressure for control and monitoring. Switches are used for direct, hardwired trips; transmitters are used to read, trend, and alarm on the actual pressure value.
Deadband is the difference between the pressure at which the switch trips and the pressure at which it resets. It prevents the contact from chattering on and off when pressure hovers near the set point and stops small pulsations from causing nuisance trips. It may be fixed by the switch design or adjustable separately from the set point.
PSH is a pressure switch high, which trips when pressure rises above its set point, and PSL is a pressure switch low, which trips when pressure falls below its set point. The doubled forms PSHH and PSLL are the high-high and low-low switches dedicated to safety shutdown. Together they bracket a safe operating window and drive alarms and emergency shutdown actions.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.