Pressure switches

A diaphragm pressure switch is built around a thin, flexible disc — the diaphragm — that separates the process fluid from the switch mechanism. The diaphragm is usually made of metal, rubber-coated fabric, or PTFE, chosen to be chemically compatible with the fluid being measured.

The process pressure acts directly on one side of the diaphragm, while a calibrated compression spring pushes against the other side. At low pressures the spring force wins and the diaphragm sits in its rest position. As process pressure rises, the diaphragm bows toward the spring and pushes a small rod or plunger against it. When the pressure crosses the setpoint, the rod has moved far enough to trip a snap-action microswitch behind it, opening or closing the contact.

The setpoint is adjusted by turning a screw that changes the preload on the spring — more preload means a higher setpoint. Diaphragm switches give a relatively large surface area for the pressure to act on, which makes them ideal for low to medium pressures, gases, and pneumatic systems. They are also the natural choice for media that would foul or damage a piston-style switch.

A piston pressure switch replaces the flexible diaphragm with a small cylindrical piston sliding inside a precisely-machined bore. Process pressure acts on one face of the piston, and a calibrated spring pushes back from the other side. Seals around the piston — usually O-rings — keep the process fluid contained.

When the process pressure exceeds the setpoint, the force on the piston overcomes the spring and the piston strokes against the spring. A rod attached to the piston pushes against a microswitch, changing its state. The setpoint is set by the spring rate and the area of the piston face: a larger piston face produces more force for a given pressure and gives a lower setpoint range.

The big advantage of a piston switch is robustness. Pistons can take pressures that would burst a diaphragm — hundreds or even thousands of bar — and they shrug off pressure spikes and surges that would fatigue a thin sensing element. The trade-off is that the piston seals have some friction and stiction, so piston switches are less precise at low pressures and have a slightly wider deadband than diaphragm types.

A Bourdon-tube pressure switch uses the same elegant mechanism as a Bourdon-tube pressure gauge — but instead of moving a pointer across a dial, the tube's motion drives an electrical contact. The sensing element is a flat-walled metal tube formed into a C-shape, closed at one end and connected to the process at the other.

Because the cross-section is flattened rather than circular, the tube has a natural tendency to try to straighten itself when pressure is applied inside it — a circular cross-section is the most efficient way to contain pressure. The walls flex outward, the flattened tube becomes slightly more circular, and the closed free end moves a small distance along an arc.

That movement is amplified by a small mechanical linkage and used to actuate a snap-action microswitch when the pressure reaches the setpoint. Bourdon switches are mechanically robust, cover a wide pressure range from a few bar up to several hundred, and can be easily calibrated by adjusting the position of the switch relative to the tip. They are common as pressure cut-outs on hydraulic systems, compressed-air receivers, and steam plant.

A differential pressure switch is a clever variation that does not care about the absolute pressure at all — it cares only about the difference between two pressures. It has two pressure ports, often labelled high and low, each connected to its own diaphragm or to opposite faces of a single piston. The sensing element only moves when the pressure at one port differs from the pressure at the other by more than the spring preload.

When both ports see the same pressure — even if that pressure is enormous — the forces on the sensing element cancel out and the switch sits in its rest position. As soon as a difference appears, the resulting net force overcomes the spring and the contact trips. The setpoint is the ΔP value, not the absolute, and the deadband is set in the same units.

This makes differential switches ideal for detecting filter blockage by reading the pressure drop across the filter element, for confirming flow by reading the ΔP across an orifice, for detecting pump failures by reading the ΔP across the pump, and for measuring liquid level in a closed pressurised vessel using a hydrostatic ΔP. The absolute system pressure can vary wildly without bothering the switch.

The setpoint of a pressure switch is the pressure at which the contact changes state on rising pressure. If you set a compressor cut-out at six bar, the switch fires the moment the receiver pressure crosses six bar going up. So far, so simple.

The deadband — sometimes called the differential, the reset value, or the hysteresis — is the gap between the rising trip point and the falling reset point. With a deadband of one bar, that same compressor switch will not turn back on until the pressure has fallen all the way down to five bar. The contact stays in its tripped state through the whole band between five and six bar, regardless of what the pressure does inside that range.

Without a deadband, any small pressure wobble around the setpoint would cause the switch to chatter — flicker on and off many times per second — which destroys the contacts, hammers the compressor with rapid restarts, and pulses the load wildly. The deadband is what makes the switch usable in the real world, and the right value is a balance: enough hysteresis to ignore normal process noise, but not so much that the controlled pressure swings wider than your process can tolerate.