Understanding Pressure Switches and Their Functionality

Pressure Switch - How They Work

Pressure switches

Figure 1: Pressure switches

A pressure switch is a device that controls an electrical contact when a preset fluid pressure is reached (pressure rise or fall from a certain preset pressure level). Pressure switches are used in various industrial and residential applications like HVAC systems, well pumps, and furnaces. They come in two main types - mechanical and electric, each suited to different applications and offering unique advantages. This article explores the working mechanisms of pressure switch types, their typical selection criteria, and applications.

Types of pressure switches

Mechanical pressure switch

Mechanical pressure switch

Figure 2: Mechanical pressure switch

A mechanical pressure switch (Figure 2) operates based on the physical movement of its internal components, primarily a spring and either a diaphragm or piston, to activate an electrical micro-switch at predetermined pressure levels.These pressure switches usually have three different types of contact: normally open (NO), normally closed (NC), and changeover (SPDT) contacts. Read our article on pressure switch installation for a step-by-step installation process.

Operating principle

The components of a pressure switch include: micro-switch (A), operating pin (B), range spring (C), operating piston (D), insulated trip button (E), switch case (F), trip setting nut (G), inlet pressure (H)

Figure 3: The components of a pressure switch include: micro-switch (A), operating pin (B), range spring (C), operating piston (D), insulated trip button (E), switch case (F), trip setting nut (G), inlet pressure (H)

The structure of a mechanical pressure switch is designed to monitor and respond to the pressure levels in various systems.

  1. Micro-switch (A): The microswitch is responsible for opening or closing the electrical circuit. It activates when the pressure switch detects that the fluid pressure has reached the preset level.
  2. Operating pin (B): The operating pin connects the mechanical movement of the pressure switch's internal components (like the operating piston) to the micro-switch. When the pressure moves the piston, the operating pin translates this motion into the action of the micro-switch.
  3. Range spring (C): The range spring is adjustable and determines the pressure range within which the switch operates. By adjusting the tension of the spring (using the trip setting nut), one can set the pressure level at which the switch will activate the micro-switch.
  4. Operating piston (D): The piston is a movable component that reacts to changes in pressure. When the pressure within the system reaches a certain level, it pushes against the piston. This movement is then translated into an electrical signal by the microswitch.
  5. Insulated trip button (E): This feature allows for manual testing or resetting of the pressure switch. It's insulated to ensure safety during operation.
  6. Switch case (F): The switch case houses all the internal components, protecting them from external elements and ensuring the durability of the switch.
  7. Trip setting nut (G): The trip setting nut is used to adjust the range spring, allowing users to set the desired pressure level at which the switch will activate.
  8. Inlet pressure (H): This is where the fluid pressure enters the pressure switch. The pressure level at this point is what the switch monitors and responds to.

In short, the inlet pressure exerts pressure upon the operating piston, generating a force opposing the range spring. Once the inlet pistons force is higher than the opposing spring force, it pushes the operating pin into the insulated trip button. This button then moves the micro-switch from the NC position to the NO position. If the pressure decreases below the spring force, the button, pin, and piston move away from the micro-switch, breaking the connection. The connection then goes from the NO position to the NC position. Other crucial components in a mechanical pressure switch include:

  • Adjustment screw: This component is used to calibrate the pressure switch. By turning the adjustment screw, operators can set the pressure at which the switch activates or deactivates. This adjustability is crucial for controlling the switch's response to the specific needs of the system.
  • Electrical connection: The electrical connection is where the switch interfaces with the system's circuitry. The switch transmits its signals by either opening or closing an electrical circuit, based on the pressure it detects.
  • O-ring: The o-ring is a small but essential component that ensures a tight seal at the connection points of the switch. It prevents leaks and contamination, which could otherwise compromise the switch's accuracy and the system's integrity.
  • Connection port: This is the point at which the pressure switch is attached to the system. It is typically threaded to allow for a secure attachment, and it is through this port that the switch senses the system's pressure.

Electronic pressure switch

Electronic pressure switch

Figure 4: Electronic pressure switch

An electronic pressure switch (Figure 3) monitors the pressure of a fluid and activates an electrical output when the pressure reaches a specified level. It combines the functions of pressure sensing and electrical switching into a single unit, offering a more sophisticated and versatile approach to pressure control compared to mechanical pressure switches. Electronic pressure switches offer advantages over mechanical pressure switches:

  • Greater accuracy
  • The ability to handle a wide range of pressures
  • Programmability
  • Digital outputs for integration with modern industrial control systems

The following parameters can typically be adjusted by the user according to the requirements:

  • Switch point
  • Output signals
  • Hysteresis (discussed later)
  • Delay time

Electronic pressure switches are suitable for automated and controlled equipment systems that require programmable function, digital display, flexibility, accuracy, ingress protection, and stability.

Operating principle

The working principle of an electronic pressure switch involves several key components and steps:

  1. Pressure sensor: The core component that detects pressure changes. It converts the physical pressure into an electrical signal. Common types of sensors include piezoelectric, strain gauge, and capacitive sensors.
  2. Signal processing circuitry: This includes amplifiers and analog-to-digital converters that condition the sensor signal, making it suitable for analysis by the control unit.
  3. Control unit: Often a microcontroller or digital circuit that interprets the sensor signal based on programmed thresholds (setpoints). It decides when to activate or deactivate the output switch.
  4. Output switch: This can be a relay or a solid-state component that opens or closes an electrical circuit in response to the control unit's commands, thereby controlling external devices like pumps, valves, or alarms.
  5. User interface: Many electronic pressure switches feature a user interface, which can range from simple dials for setting pressure thresholds to digital displays and keypads for programming and monitoring.

Read our digital switches article for more information on the various sensing mechanisms used in an electronic pressure switch.

Mechanical vs electrical pressure switches

When choosing between mechanical and electrical pressure switches, consider the application's specific needs, including accuracy, response time, and integration capabilities. Read our digital pressure switch article for more information.

Selection criteria

Consider the following parameters while selecting a pressure switch:

  1. Type of media: The type of media should be compatible with the housing and seal material. Nitrile butadiene rubber (NBR) is suitable for use with air and hydraulic/machine oil. Ethylene propylene diene monomer rubber (EPDM) is suitable when water is the medium. Common media used with pressure switches are:
    1. Hydraulic oil
    2. Heating oil
    3. Turpentine
    4. Petrol/gasoline
    5. Air
    6. Water

    Read our chemical compatibility chart for more information on the compatibility of different materials with various media.

  2. Pressure: The pressure switch must be able to withstand the maximum working pressure. Low pressure switches typically use a diaphragm as the sensing element, while high pressure switches use a piston design.
  3. Temperature: The pressure switch should work well within its maximum and minimum temperature range.
  4. Repeatability: Accuracy refers to how close the switch's activation point is to the true pressure value, while repeatability is the switch's ability to consistently activate at the same pressure point over multiple cycles. The range of accuracy required determines the selection of the pressure switch for the application. Diaphragm designs generally provide more accuracy than the piston design.
  5. Hysteresis: Hysteresis is the difference between the switch point and the reset point. The switch stays active for a long time if the reset point is too large. If the reset point is too short, the switch will flip between on/off states frequently. Hysteresis is configurable in an electric pressure switch but preset by the manufacturer in a mechanical pressure switch.
  6. Process connection: The size and type of the process connection should match the system's piping or equipment. Common types include NPT, BSP, and flange connections.
  7. Approvals: Choose pressure switches with ATEX certifications for use in a potentially explosive atmosphere.
  8. Electric or mechanical pressure switch: An electric pressure switch is more expensive but comes with more control over the settings, like pressure setpoint and hysteresis, compared to a mechanical pressure switch. Some applications may require the ability to adjust the set point, reset point, or pressure range of the switch. Determine if you need a switch with adjustable settings to accommodate changes in system requirements.

Common applications

A pressure switch is used in a wide range of domestic and commercial applications as listed below:

  • HVAC, gas cylinders, air pumps, etc. use air compressor pressure switches to monitor and control the systems air pressure. Read our air compressor pressure switch adjustment article to learn more about how to adjust them.
  • Oil pressure switches are used by engines as an actuator or sensor to determine when the engine's oil pressure has dropped below the preset level.
  • Furnace pressure switches act as safety devices for industrial as well as residential purposes. They detect the negative pressure during the furnace start-up and shuts down the furnace if there is low air pressure.
  • Well pump pressure switches are used in residential and commercial buildings to bring water from the well and ensure that there is enough water pressure in the system to provide water without being over-pressurized.
  • Water pump pressure switches in residential, commercial, and agricultural applications auto-regulate water flow.
  • Vacuum pressure switches measure vacuum or negative pressure in the system. They are in residential boilers, electric heaters, air compressors, and transmission systems.

Read our pressure switch symbol article for more details on pressure switch symbols and diagrams.


What does a pressure switch do?

A pressure switch monitors the system's fluid pressure and either opens or closes an electrical connection based on a preset pressure level.

How does a mechanical pressure switch work?

A mechanical pressure switch senses pressure changes and sends an electrical signal to affect a system to keep it running safely and correctly.

How to adjust a pressure switch?

For a mechanical pressure switch, turn the nut/knob clockwise to increase and counterclockwise to decrease the switch point. An electric pressure switch has a keypad for adjustments.

What is the difference between a pressure switch and a pressure sensor?

Pressure switches operate electrical switches at a preset pressure level, while pressure sensors read the system pressure and convert it into an electrical signal.