Pressure Switches: Types, Operation, and Specs

What Is a Pressure Switch?

A pressure switch is a device that detects when pressure reaches a specified threshold and then changes the state of an electrical circuit. Rather than continuously reporting pressure, it performs a discrete action: on/off, open/closed, or changeover between two circuits.

For example, a pressure switch may start a pump when tank pressure falls below a set level, stop a compressor when pressure rises to its upper limit, or activate an alarm when a filter becomes blocked.

This differs from related pressure instruments:

A pressure gauge displays local pressure for an operator.
A pressure transmitter converts pressure into a continuous output signal, such as an analog current, voltage, or digital value.
A pressure switch provides a switching output when pressure crosses a configured setpoint.

Pressure switches can be designed for positive gauge pressure, vacuum, absolute pressure, or differential pressure between two process points.

Where Are Pressure Switches Used?

Pressure switches are used wherever equipment must respond automatically when pressure reaches a defined level. The switch may be part of a simple control circuit or a protective shutdown system.

Common applications include:

Pumps and pressure-boosting systems
Air compressors and pneumatic equipment
Boilers, burners, and combustion systems
Hydraulic power units
Water-treatment equipment
Filters and strainers
Refrigeration and HVAC systems
Power supplies and utility equipment
Process skids and industrial machinery

The electrical action depends on the connected circuit. A pressure change can trigger a motor starter, solenoid valve, warning light, audible alarm, shutdown relay, burner interlock, or control-system input.

For instance, a low-pressure switch on a lubrication system can shut down machinery if oil pressure drops below a safe operating threshold. Conversely, a high-pressure switch can stop a compressor before pressure rises beyond the intended control limit.

How Does a Pressure Switch Work?

In a conventional mechanical pressure switch, process pressure enters through a pressure connection and acts on a sensing actuator. The actuator may be a diaphragm, piston, bellows, or another pressure-responsive component.

The pressure-generated force acts against an opposing spring force. While process pressure remains below the configured switching point, the spring holds the mechanism in its initial position. As pressure rises or falls in the intended direction, the balance of forces changes.

When the pressure force reaches the required level, the sensing element moves enough to operate the electrical switching mechanism. This causes the contacts to open, close, or transfer from one terminal to another.

A simplified relationship is:

Higher process pressure produces greater force on the sensing element.
Greater spring compression requires more pressure to move the mechanism.
Reducing spring compression lowers the switching pressure.

Many adjustable mechanical switches use an adjustment screw, nut, or similar mechanism to change spring compression. This changes the pressure setpoint, although the available adjustment range and procedure depend on the switch design.

Movement from the diaphragm or piston is typically transferred through a push rod, lever, or linkage. That movement presses or releases the actuator on a microswitch or related contact mechanism.

Electronic pressure switches work differently internally. They use an electronic sensor and circuitry to measure pressure, compare it with programmed thresholds, and operate a transistor, relay, or other output. However, the basic control purpose is the same: change an electrical output when pressure crosses a selected point.

What Are the Main Components of a Pressure Switch?

The core functional elements of a pressure switch are the pressure-sensing element and the electrical switching mechanism. Supporting components commonly include the pressure port, housing, setpoint adjustment hardware, springs, seals, terminals, and environmental protection features.

Mechanical design affects several practical characteristics:

Sensitivity to changing pressure
Switching direction and reset behavior
Pressure range
Deadband
Repeatability
Resistance to vibration and pressure pulsation
Compatibility with process media
Suitability for temperature, moisture, dust, or hazardous environments

A switch that works well on clean compressed air may not be appropriate for corrosive chemicals, hot steam, viscous oil, or a hydraulic circuit with rapid pressure spikes. Component construction is therefore central to proper selection.

The Switching Element

The electrical switching element in a mechanical pressure switch is often a microswitch. The pressure-responsive mechanism moves a push rod or linkage, which presses or releases the microswitch actuator.

Inside the microswitch, electrical contacts transfer between terminals as the switch changes state. Depending on the contact configuration, the switch can open a circuit, close a circuit, or change a common terminal from one circuit path to another.

The electrical rating of the switching element matters. Contact ratings must be suitable for the voltage, current, load type, and expected switching frequency. Inductive loads such as motors, coils, and contactors can place different demands on contacts than low-power signal circuits.

Normally Open (NO) Contacts

A normally open, or NO, contact is open below its configured actuation point under the stated reference condition. When pressure crosses the setpoint in the intended actuation direction, the contact closes.

For example, consider a pressure switch configured to close an NO contact as pressure rises. If its setpoint is 10 psi, the contact may remain open below the actuation point and close when pressure rises to approximately 10 psi. Its actual reset pressure may be lower because of deadband.

“Normally” describes the switch condition before actuation under the manufacturer’s defined reference condition. It does not necessarily mean the contact will always be open whenever a system is not operating.

Normally Closed (NC) Contacts

A normally closed, or NC, contact is closed below its configured actuation point under the stated reference condition. When pressure crosses the setpoint in the intended actuation direction, the contact opens.

Using the same rising-pressure example, an NC contact may remain closed while pressure is below the setpoint and open when pressure reaches the configured trip point.

Vacuum-switch terminology requires particular care. Whether a contact opens or closes as vacuum increases depends on the pressure reference, the switch construction, the selected terminals, and the manufacturer’s stated switching convention. Always confirm terminal behavior, setpoint direction, and reset behavior in the device documentation.

What Microswitch Configurations Are Available?

Pressure switches commonly use single-pole double-throw (SPDT) or double-pole double-throw (DPDT) contact configurations. The chosen arrangement determines how many circuits can be switched and what wiring options are available.

A switch may also have other output arrangements, especially in electronic models. Examples can include relay outputs, solid-state switching outputs, or programmable switching points. For mechanical control circuits, however, SPDT and DPDT configurations remain common.

Single-Pole Double-Throw (SPDT)

An SPDT pressure switch has one common terminal and two switched terminals:

Common (COM)
Normally open (NO)
Normally closed (NC)

It provides one changeover function. In one state, the common terminal is connected to the NC terminal. When the switch actuates, the common terminal transfers to the NO terminal.

This arrangement allows a user to wire either an NO-controlled circuit or an NC-controlled circuit. It can also support a simple indication arrangement, such as one light showing a normal condition and another showing an alarm condition.

Double-Pole Double-Throw (DPDT)

A DPDT pressure switch provides two separate sets of changeover contacts. Each pole has its own common, normally open, and normally closed terminals.

Because the two poles operate at the same pressure setpoint, a DPDT switch can control two loads or circuits simultaneously. For example, one pole may operate an alarm circuit while the second pole sends a signal to another control device.

A DPDT assembly may use two mechanically linked switching elements. Although both poles respond to the same pressure mechanism, their electrical circuits remain separate. Users should still verify contact ratings and wiring diagrams for the specific model.

What Is the Pressure-Sensing Element?

The pressure-sensing element is the component that moves in response to applied positive pressure, vacuum, or pressure difference. Its motion is converted into mechanical or electrical switching action.

Diaphragms and pistons are common sensing-element designs. The best choice depends on the pressure range, process medium, temperature, contamination risk, pressure cycling, and required service life.

A sensing element must withstand normal operating pressure as well as expected transients within the product’s rated limits. It must also remain compatible with the process fluid or gas and maintain reliable movement over repeated cycles.

How a Diaphragm Works in a Pressure Switch

A diaphragm is a flexible membrane that deflects when pressure acts on one side of it. That deflection transfers force to a push rod, spring assembly, or switching mechanism.

Diaphragm designs are often useful where relatively small pressure changes must produce measurable movement. They may be used with gases, liquids, vacuum service, and differential-pressure applications, depending on construction.

Material compatibility is essential. The diaphragm must be suitable for the process medium and the operating environment. Selection should account for:

Chemical resistance
Operating temperature
Pressure range and pressure cycling
Corrosion potential
Fluid contamination
Permeation or swelling risks
Cleanliness requirements

Broad material categories may include elastomeric diaphragms, fluoropolymer-based materials, and metallic diaphragms. Elastomeric materials may be suitable for some oils, hydraulic fluids, and gases, while fluoropolymer materials can be considered for certain chemical services. Metallic diaphragms may be used where higher temperature, water, steam, wastewater, or specific corrosive-service requirements apply.

Suitability must always be confirmed against the actual process medium, concentration, temperature, pressure, and the complete switch construction—not only the diaphragm material.

When Is a Piston Used in a Pressure Switch?

Piston sensing elements are commonly used in pressure-switch designs intended for higher-pressure service. A typical arrangement uses a piston that moves within a bore against a spring. Process pressure acts on the piston area, producing a force that shifts the mechanism when the setpoint is reached.

Piston-based switches often include corrosion-resistant metallic components and an O-ring or similar seal. Their robust construction can be useful in applications involving hydraulic pressure, lubrication systems, and other higher-pressure fluid circuits.

However, piston and seal materials must both be compatible with the process medium. Seal performance can be affected by temperature, fluid chemistry, pressure cycling, particulate contamination, and lubricant condition. There is no universal pressure threshold at which a piston is always required; product limits vary by manufacturer and model.

What Do Proof Pressure and Burst Pressure Mean?

Proof pressure and burst pressure are distinct limits that help indicate how a pressure switch may tolerate abnormal overpressure conditions. Both are important when assessing equipment protection and application safety.

These ratings should not be treated as normal operating setpoints. A switch should ordinarily be selected so that normal pressure, expected surges, and system transients remain within the manufacturer’s stated operating limits.

Proof Pressure

Proof pressure is the maximum pressure a switch can withstand without damage under specified test conditions. Depending on the product documentation, this may mean the device continues to meet defined performance requirements after the test.

The stated value is established under controlled conditions, which can include defined temperature, pressure-rise rate, duration, and test procedure. Conditions in the field may differ substantially, especially where pressure spikes, rapid cycling, vibration, or elevated temperature are present.

If a switch will operate beyond its nominal range, near its limits, or outside its reference-temperature conditions, manufacturer documentation should be consulted.

Burst Pressure

Burst pressure is the maximum pressure the device can withstand without leakage or rupture under specified test conditions. Pressure above the stated burst rating can cause permanent damage, leakage, loss of containment, or other hazardous failure.

Burst pressure is not an acceptable operating limit or a substitute for proper system pressure protection. Its value varies with product design, materials, connection type, sensing-element construction, and applicable testing requirements.

What Is the Difference Between Differential and Static Pressure?

Differential pressure is the pressure difference between two points in a system. Static pressure is the normal operating pressure at a location or within a line.

For a differential measurement:

Differential pressure = upstream pressure − downstream pressure

For example, if pressure upstream of a filter is 20 psi and pressure downstream is 15 psi, the differential pressure across the filter is 5 psi.

As a filter or strainer becomes loaded with debris, resistance to flow often increases. This can increase the pressure drop across the component. A differential-pressure switch can activate an alarm, maintenance signal, bypass valve, or shutdown circuit when the selected differential setpoint is reached.

A differential-pressure switch must also be suitable for the static pressure present on both sides of the sensing element. A relatively small differential pressure can exist within a system that has much higher line pressure.

What Does Pressure Switch Repeatability Mean?

Repeatability describes how consistently a pressure switch changes state at or near the same setpoint over repeated cycles under equivalent conditions.

A repeatability specification indicates expected variation in switching performance. It does not necessarily describe every aspect of accuracy, including setpoint calibration, thermal effects, long-term drift, or the influence of installation conditions.

When repeatability is expressed as a percentage of span, it must be interpreted against the full pressure range. For example, a switch with a 100 psi span and a repeatability specification of ±1% of span would correspond to a switching variation of ±1 psi under the stated test conditions.

Actual performance depends on the applicable model, pressure range, operating direction, temperature, vibration, cycling history, and process conditions. The relevant datasheet should be used for selection and performance expectations.

What Is Deadband in a Pressure Switch?

Deadband, also called hysteresis or differential, is the separation between the actuation pressure and the reset pressure. A mechanical pressure switch does not necessarily reset at the same pressure at which it actuates.

For a switch that actuates as pressure rises, the reset point is typically lower than the rising-pressure trip point. For a switch that actuates as pressure falls, the reset point is typically higher than the falling-pressure trip point.

Deadband is often useful because it prevents rapid cycling when pressure fluctuates around the setpoint. Without enough separation between trip and reset points, a pump, relay, or valve could repeatedly switch on and off as pressure varies slightly.

The amount of deadband can be influenced by the switch range, sensing-element material, spring system, and switching mechanism. Terminology and pressure-direction conventions can vary, so the manufacturer’s definitions should be confirmed for the selected switch.

What Approvals May a Pressure Switch Require?

Required approvals depend on the installation environment, equipment function, local regulations, and the consequences of switch failure.

Relevant considerations may include:

Electrical-safety requirements
Hazardous-location suitability
Ingress protection against dust and water
Environmental ratings for temperature, vibration, or corrosion
Functional-safety requirements
Equipment-specific certification requirements
Local authority and site rules

For example, a switch installed in a potentially explosive atmosphere may require an appropriate explosion-protected design and certification for the classified area. Outdoor, washdown, or marine installations may require suitable enclosure protection. A pressure switch used as part of a safety-instrumented function may require documented performance data and an appropriate safety lifecycle approach.

Approvals and markings should be verified against current local regulations, applicable standards, authority requirements, installation conditions, and the manufacturer’s documentation.