Instrumentation · Pressure Measurement · Process Control
What Is a Pressure Switch? Working Principle, Types and When to Use Them Right
A complete guide to pressure switches: how they work, the four sensing element types, NO vs NC contacts, mechanical vs electronic, differential pressure switches, setpoint and deadband explained, selection guide and common wiring mistakes.
A pressure switch is one of the simplest and most reliable instruments in process control. It monitors pressure and opens or closes an electrical contact when the pressure reaches a preset value. No 4-20 mA signal, no DCS input card, no PLC analogue channel needed. When pressure reaches the setpoint, the switch acts: directly starting a pump, stopping a compressor, sounding an alarm or triggering a shutdown.
Pressure switches are found in virtually every industrial facility in the world: on compressors, hydraulic systems, boilers, water pumps, fire suppression systems, pneumatic circuits, HVAC units and process pipelines. Despite their simplicity, they are frequently misapplied, miswired or misunderstood. Many field engineers confuse Normally Open with Normally Closed contacts, set the wrong deadband, or select a sensing element unsuited to the process fluid.
This guide covers everything you need to understand and correctly apply a pressure switch: how the sensing mechanism works, the four sensing element types, NO vs NC contact logic, deadband and setpoint, mechanical vs electronic versions, differential pressure switches, selection criteria and the most common wiring mistakes. For context on how pressure switches compare to continuous 4-20 mA transmitters, see our article on pressure gauge and pressure transmitter differences.
What Is a Pressure Switch?
A pressure switch is an electromechanical or electronic device that converts a pressure level into an electrical switching action. It contains a pressure sensing element that responds to process pressure and a set of electrical contacts that change state when the pressure reaches or falls below a preset threshold called the setpoint.
In simple terms: the pressure switch watches the pressure. When the pressure crosses the setpoint going up, the switch triggers. When the pressure crosses the setpoint going down (accounting for the deadband), the switch resets. Each trigger and reset causes the contacts to either close or open an electrical circuit, which in turn can start or stop a motor, energise a solenoid, activate an alarm or send a digital input signal to a PLC or DCS.
Figure 1: A pressure switch directly connects process pressure to an electrical action. When pressure crosses the setpoint, the contacts change state and the connected device (pump, alarm, solenoid) responds immediately without any controller in between.
The Four Pressure Sensing Elements
The sensing element is the mechanical heart of a pressure switch. It is the component that physically responds to process pressure and transmits that movement to the electrical contacts. There are four standard sensing element types, each suited to different pressure ranges and process conditions.
1. Piston (Plunger) Type
- A piston with a calibrated spring. Process pressure pushes the piston against the spring. When force overcomes the spring, the piston actuates the microswitch.
- Pressure range: Medium to high (1 bar to 700 bar)
- Best for: Hydraulic systems, pneumatic circuits, high-pressure air and oil applications
- Advantages: Very robust, tolerates pulsations well, long mechanical life
- Limitations: Less sensitive at very low pressures. Requires clean, non-viscous fluid to prevent piston sticking.
2. Diaphragm Type
- A flexible diaphragm separates the process fluid from the switch mechanism. Pressure deflects the diaphragm, which actuates the microswitch through a push pin.
- Pressure range: Low to medium (0 to 70 bar typical)
- Best for: Corrosive fluids, dirty or viscous media, low-pressure applications, sanitary processes
- Advantages: No direct contact between process fluid and switch mechanism. Diaphragm material can be selected for chemical compatibility (stainless, PTFE, Hastelloy).
- Limitations: Less suitable for very high pressures. Diaphragm can fatigue over time with large pressure cycles.
3. Bourdon Tube Type
- A curved C-shaped or helical tube that straightens as internal pressure increases. The tip movement is linked mechanically to the electrical contacts.
- Pressure range: Wide range (0.5 bar to 4,000 bar)
- Best for: Clean gases and liquids, moderate to high pressures, applications where a visual gauge is also required
- Advantages: Simple, proven design. Can be combined with a Bourdon gauge for both local indication and switching in one unit.
- Limitations: Not suitable for viscous, crystallising or dirty fluids that would block the Bourdon tube. Sensitive to vibration.
4. Bellows Type
- A corrugated metallic capsule (bellows) that expands and contracts with pressure changes. Very sensitive to small pressure changes.
- Pressure range: Very low pressure (vacuum to 10 bar)
- Best for: Low differential pressure, very low gauge pressure, vacuum applications, draft measurement in ducts
- Advantages: Extremely sensitive to small pressure changes. Can measure very small pressures accurately.
- Limitations: Not suitable for high-pressure applications. Bellows can be damaged by sudden pressure spikes (water hammer).
Normally Open vs Normally Closed Contacts: The Most Important Concept
The most commonly misunderstood aspect of pressure switches is the difference between Normally Open (NO) and Normally Closed (NC) contacts. Getting this wrong results in a safety trip that does nothing when it should, or an alarm that is always on even in normal conditions.
Figure 2: NO contact is open at rest (below setpoint) and closes when pressure reaches the setpoint. NC contact is closed at rest (below setpoint) and opens when pressure reaches the setpoint. The key is knowing what the switch does at rest, not during normal plant operation.
| Contact type | State below setpoint | State above setpoint | Use when you want to | Typical application |
|---|---|---|---|---|
| NO (Normally Open) | Open (circuit broken) | Closed (circuit complete) | Activate a device or alarm when pressure rises above setpoint | High pressure alarm, high pressure interlock to shut down a pump or valve |
| NC (Normally Closed) | Closed (circuit complete) | Open (circuit broken) | Deactivate a device when pressure rises above setpoint, or run a device until pressure reaches a limit | Compressor high pressure trip, cooling fan runs until pressure exceeds limit |
| SPDT (Single Pole Double Throw) | Common terminal connected to NC terminal | Common terminal switches to NO terminal | Activate one device and deactivate another simultaneously at the same setpoint | Start standby pump while alarming; switch between two control paths based on pressure |
Setpoint and Deadband (Switching Differential) Explained
Two terms appear on every pressure switch datasheet and must be understood clearly before commissioning or adjusting any pressure switch in service: setpoint and deadband.
Figure 3: The switch trips when pressure rises to the setpoint. It does not reset until pressure falls to the reset point, which is the setpoint minus the deadband. Without deadband, a pressure fluctuating around the setpoint would cause rapid contact chattering.
| Term | Definition | Example |
|---|---|---|
| Setpoint (trip point) | The pressure at which the switch contacts change state. Also called the actuation point, trip point or operating point. This is what you set when adjusting the switch. | A compressor protection switch set to 15 bar: contacts trip at 15 bar as pressure rises. |
| Deadband (switching differential) | The pressure drop required below the setpoint before the contacts reset to their normal state. The switch will NOT reset until pressure drops by the full deadband amount below the setpoint. This prevents contact chattering when pressure fluctuates near the setpoint. | Same switch with 1 bar deadband: contacts trip at 15 bar rising but only reset when pressure falls to 14 bar. |
| Reset point | The pressure at which the contacts return to their normal state. Reset point = Setpoint minus Deadband (for high pressure switches). | Setpoint 15 bar, deadband 1 bar: reset point = 14 bar. |
| Contact chattering | Rapid opening and closing of contacts when pressure oscillates around the setpoint with no deadband. Chattering destroys contacts and causes connected equipment to rapidly cycle on and off. Always ensure adequate deadband is set. | If deadband is zero and pressure bounces between 14.8 and 15.2 bar, the contacts will open and close many times per second. |
Mechanical vs Electronic Pressure Switches
Mechanical Pressure Switch
- Sensing element movement directly actuates a microswitch via a mechanical linkage
- No external power supply needed (the switch contacts are passive)
- Simple, proven design with decades of reliability data
- Fixed setpoint adjusted by spring tension (screwdriver or locknut adjustment)
- Setpoint not visible. Must be measured by test or adjusted by feel
- Typically handles higher switching currents (up to 15A or more)
- Susceptible to vibration (can cause contact chattering on spring-loaded types)
- Limited switching accuracy (typically ±1% to ±5% of setpoint)
- Lower cost per unit
- Best for: simple on/off control where setpoint stability and exact adjustment are not critical
Electronic (Smart) Pressure Switch
- Uses a piezoelectric or strain gauge pressure sensor with electronic signal processing
- Requires external DC power supply (typically 10-30V DC)
- Digital display shows actual measured pressure in real time
- Setpoint adjustable via push buttons on the unit. Setpoint value visible on display
- Multiple setpoints and window functions available on same unit
- Solid-state output (PNP or NPN transistor) for low current switching
- Excellent vibration and shock resistance (no moving parts in sensing element)
- High switching accuracy (typically ±0.5% or better)
- Often includes 4-20 mA analogue output as well as switching output
- Best for: precision control, multiple setpoints, when setpoint visibility is important, harsh vibration environments
| Feature | Mechanical | Electronic |
|---|---|---|
| Power supply needed | No (passive contacts) | Yes (DC supply required) |
| Setpoint adjustment | Mechanical (spring tension) | Digital (push button, displayed) |
| Switching accuracy | ±1% to ±5% | ±0.5% or better |
| Switching current | High (up to 15A direct) | Low (solid state, typically under 1A) |
| Vibration resistance | Moderate (spring can chatter) | Excellent (no moving parts) |
| Multiple setpoints | No (one setpoint per unit) | Yes (typically 2 to 4 setpoints) |
| Analogue output option | Not available | Often included (4-20 mA) |
| Cost | Lower | Higher |
Differential Pressure Switches
A standard pressure switch measures gauge pressure (pressure relative to atmosphere) or absolute pressure at one process connection. A differential pressure switch has two process connections and measures the difference in pressure between them. The contacts change state when this pressure difference reaches the setpoint.
Differential pressure switches are one of the most important instruments in process plant maintenance. Their most common application is monitoring filter and strainer condition. As a filter accumulates dirt, the pressure drop across it increases. When the differential pressure reaches the setpoint, the switch triggers an alarm telling the operator it is time to change the filter element.
| Application | What it monitors | Typical setpoint action |
|---|---|---|
| Filter or strainer condition | Differential pressure across filter housing | High DP alarm when filter is blocked and needs replacement |
| Pump or fan health | Differential pressure across pump (inlet vs outlet) | Low DP alarm if pump loses prime or impeller is worn |
| Duct or room pressure | Differential between a clean room and corridor or between two zones | Low differential alarm if room loses positive pressure containment |
| Heat exchanger fouling | Differential pressure across tube bundle | High DP alarm when fouling requires cleaning |
| Flow detection (no flow switch) | Differential across an orifice or venturi inline | Low DP indicates no flow condition when flow should be present |
Differential pressure switches should not be confused with differential pressure transmitters. A DP transmitter produces a continuous 4-20 mA signal proportional to the pressure difference and connects to a DCS or PLC for monitoring. A DP switch simply produces an on/off output when the pressure difference crosses a setpoint, with no continuous signal output. For applications requiring trending and historical data, use a DP transmitter. For simple alarm or pump control at a fixed limit, a DP switch is more cost-effective and simpler to install.
Pressure Switch vs Pressure Transmitter: When to Use Each
| Requirement | Use a pressure switch | Use a pressure transmitter |
|---|---|---|
| Output type needed | On/off digital action at one or two fixed setpoints | Continuous 4-20 mA signal representing the full pressure range |
| DCS or PLC required? | No. Switch directly controls a motor, solenoid or alarm. | Yes. Transmitter output connects to DCS/PLC analogue input card. |
| Trend or log pressure over time? | No. Only gives state change information. | Yes. Full range continuous signal can be logged and trended. |
| Multiple alarm setpoints? | Requires separate switches for each setpoint (or electronic switch with multiple outputs) | Single transmitter. DCS configures unlimited alarms at any value. |
| Accuracy requirement | ±1% to ±5% typical for mechanical types | ±0.1% to ±0.5% typical for smart transmitters |
| Cost | Lower. No I/O card, no DCS engineering, no loop calibration needed. | Higher total installed cost due to I/O card, wiring, configuration and commissioning. |
| Typical application | Pump start/stop, compressor protection, filter change alarm, fan interlock | PID pressure control loop, process monitoring, safety SIS input |
Pressure Switch Selection Guide
- Define the process pressure range. The switch must be rated for the maximum system pressure including any surge or water hammer peaks. Never select a switch rated exactly at the maximum operating pressure. Select one rated to at least 1.5 times the maximum expected pressure.
- Check media compatibility. Every wetted part of the switch (process connection, sensing element, seals) must be chemically compatible with the process fluid. Stainless steel 316L suits most water and chemical service. PTFE or Hastelloy diaphragms are needed for aggressive acids or solvents.
- Choose the correct sensing element. Use piston type for high-pressure hydraulic and pneumatic. Use diaphragm type for corrosive, viscous or dirty fluids. Use Bourdon tube for clean gases and moderate pressures. Use bellows for very low pressure or vacuum applications.
- Determine NO or NC contact requirement. For safety trips and alarms, always prefer NC contacts so that wiring failure causes a safe state. For start-on-demand applications where a device should activate at a pressure threshold, NO contacts are typically correct.
- Check the required enclosure rating. For outdoor, washdown or hazardous area installations, verify the enclosure IP rating. IP65 is the minimum for most outdoor industrial locations. For hazardous areas, check that the switch carries the required ATEX, IECEx or UL/CSA hazardous area certification.
- Consider the switching current. Mechanical switches typically handle 5A to 15A directly. Electronic switches typically handle much lower currents and require an intermediate relay to switch inductive loads such as motor starters and solenoid valves.
- Specify the correct process connection. Common connections are 1/4 inch NPT, 1/2 inch NPT and G1/4 BSP. Always match the thread standard (NPT for North America, BSP for Europe and most of Asia) to the process connection on the pipe or equipment.
Common Pressure Switch Wiring Mistakes to Avoid
| Mistake | What happens | How to avoid it |
|---|---|---|
| Connecting to the wrong terminals (NO instead of NC or vice versa) | The circuit behaves completely opposite to what was intended. A safety trip does nothing. An alarm is permanently on. Equipment starts when it should stop. | Always check the terminal markings on the switch body before wiring. Most SPDT switches mark terminals C (Common), NO and NC. Wire to C and the correct NO or NC terminal. Test before commissioning by simulating the trip condition. |
| Connecting a large inductive load directly to an electronic pressure switch output | The solid-state transistor output of an electronic switch is damaged by the inductive kick from motor starters, solenoids or relay coils. Switch fails within days or weeks. | Always use an intermediate relay between an electronic pressure switch output and any inductive load. Use a relay with the correct coil voltage for the switch output (typically 24V DC) and contacts rated for the load current. |
| No deadband set on a mechanical switch near a pulsating line | The contacts chatter rapidly as pressure pulses through the setpoint. Contact welding, equipment damage and nuisance alarms result. | Set an appropriate deadband. On mechanical switches, the deadband is often factory-set and not adjustable. Select a switch with the correct fixed deadband for the application. On electronic types, set the deadband via the menu. Install a pulsation dampener on the impulse line if pressure pulses are severe. |
| Installing on a pulsating line without a syphon or snubber | Pressure pulsations from pumps and compressors reach the switch mechanism directly, causing premature fatigue failure of the sensing element and false trips from pressure peaks. | Install a pressure snubber (sintered metal pore insert) in the process connection or use a needle valve to dampen pulsations. For steam service, install a pigtail syphon to protect the sensing element from steam temperature. |
| Selecting the wrong IP rating for the environment | Moisture ingress into the switch enclosure causes corrosion of contacts and terminals. False trips, signal loss and switch failure. | For indoor cabinets: IP54 minimum. For outdoor locations: IP65 minimum. For washdown or submersion risk: IP66 or IP67. For hazardous areas: verify the applicable ATEX or IECEx certification. |
Further Reading and External Resources
- RealPars: Pressure Switch Explained: Types and Working Principle. A well-illustrated beginner guide covering electromechanical and solid-state types with circuit symbols from a respected industrial automation education platform.
- IQS Directory: Principles, Parts and Types of Pressure Switches. Detailed technical reference on all sensing element types, selection criteria and industrial applications.
- Omega Engineering: Pressure Switch Selection Guide. Practical selection guidance with technical specifications from one of the leading instrumentation manufacturers.
- What Is Piping: Pressure Switch Working Principle, Types and Selection. Engineering-focused reference covering mechanical operation, selection parameters and media compatibility.
Frequently Asked Questions: Pressure Switch
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- What Is SIL (Safety Integrity Level)? A Beginner's Guide
- NAMUR NE43 Standard: Signal Range and Fault Detection
What we learn today
- A pressure switch converts a process pressure level into an on/off electrical contact change at a preset setpoint. It directly controls a device without a DCS or PLC in between, making it simple, robust and cost-effective for pump start/stop, compressor trips and filter alarms.
- Normally Open (NO) contacts are open at rest and close when pressure reaches the setpoint. Normally Closed (NC) contacts are closed at rest and open at the setpoint. Always use NC contacts for safety trips because a broken wire causes a fail-safe open circuit.
- The deadband (switching differential) is the pressure drop below the setpoint required to reset the switch. Always set an adequate deadband to prevent rapid contact chattering when process pressure fluctuates near the setpoint.
- Use a pressure switch for simple fixed-setpoint on/off control without a controller. Use a pressure transmitter when continuous monitoring, PID control, trending or multiple alarm setpoints are needed from a DCS or PLC system.
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