Pressure Switch - How They Work

Pressure switches

Figure 1: Pressure switches

A pressure switch is a device that operates an electrical contact when a preset fluid pressure is reached. The switch makes an electrical contact on either pressure rise or pressure fall from a certain preset pressure level. Pressure switches are in a wide range of industrial and residential applications like HVAC systems, well pumps, furnaces, etc. Read our pressure switch symbol article for diagram information.

A pressure switch should not be confused with a pressure transducer. A pressure transducer converts pressure into an electrical output signal. Read our pressure transducer technical article to learn more about them.

Table of Contents

Types of pressure switches

Mechanical pressure switch

Mechanical pressure switch

Figure 2: Mechanical pressure switch

A mechanical pressure switch uses a spring and a diaphragm, or piston, to control at what pressure the micro-switch is triggered. The spring is the opposing force to the inlet pressure and the springs pretension is adjusted with a set screw or knob. The spring pretension directly correlates to the pressure at which the switch makes an electric contact. When the pressure drops, the switch resets to its original state.

The difference between the switch point and reset point is called hysteresis. Often this is expressed as a percentage of the switch point value, for example, 20%. The manufacturer defines the hysteresis, and for most mechanical switches, it is not configurable by the user.

These pressure switches usually have three different types of contact: normally open (NO), normally closed (NC), and changeover (SPDT) contacts. Changeover contacts are for both NO and NC operation.

The mechanical pressure switch is better suited for handling high voltages and amperages than an electronic pressure switch. They can be used to make a contact change for an increase or decrease in pressure. We have an article on how to adjust mechanical pressure switches. Figure 2 is an example of a mechanical pressure switch.

Electronic pressure switch

Electronic pressure switch

Figure 3: Electronic pressure switch

An electronic pressure switch uses an electrical pressure sensor to measure the change in inlet pressure. They have digital displays to set up the switching function. The switch point can be manufacturer set or can be programmed on-site according to the application requirements. Switch point, output signals, hysteresis, delay time, etc. are some functions that can be adjusted by the user according to the requirements.

Electronic pressure switches are suitable for automated and controlled equipment systems that require programmable function, digital display, flexibility, accuracy, ingress protection, and stability. Figure 3 is an example of an electric pressure switch.

Operating principle

The pressure switch shown in Figure 4 is an example of a single pole double throw (SPDT) switch, which has a mechanical operation principle. All of the components are inside the switch case (F), and it has one inlet pressure port (H). In short, the inlet pressure pushes a piston (D) against a spring (C) that has a known resistant force. Then, the piston triggers the micro-switch (A), moving it between normally closed (NC) and the normally open (NO) position through an operating pin (B) and an insulated trip button (E).

Pressure switch components

Figure 4: 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)

To set the pressure level at which the micro-switch switches between NC and NO, the trip-setting nut (G) changes the spring pocket depth. This depth change allows the spring resistant force to increase or decrease, which correlates to a set pressure to trigger the micro-switch. The inlet pressure (H) exerts pressure upon the operating piston (D), generating a force opposing the range spring (C). Once the inlet pistons force is higher than the opposing spring force, it pushes the operating pin (B) into the insulated trip button (E). 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.

Selection criteria

Consider the following selection criteria for your application:

  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:
    • Hydraulic oil
    • Heating oil
    • Turpentine
    • Petrol/gasoline
    • Air
    • Water
  2. Pressure: It must be able to withstand the maximum working pressure. A diaphragm design works well for vacuum and low-pressure applications. For high-pressure applications, use a piston design.
  3. Temperature: It must be able to work well within its maximum and minimum temperature range.
  4. Repeatability: Repeatability, or accuracy, is the ability of the device to accurately switch back to the same set point for every repetition. The range of accuracy required will determine the selection of the pressure switch for your 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 state frequently. Hysteresis is configurable in an electric pressure switch but preset by the manufacturer in a mechanical pressure switch.
  6. Type of pressure switch: For low pressure and vacuum applications, use one with a diaphragm design. For high-pressure applications, use one with a piston design.
  7. Approvals: Our pressure switches are ATEX zone 1, 2, and 22, certified 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.

Common applications

A pressure switch can be 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.
  • Oil pressure switches are used by engines as an actuator or sensor to determine when the engines 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.

FAQ

How to adjust a pressure switch?

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

How to test a pressure switch?

Unplug electric power from the pressure switch and connect the multimeter to its terminals. For a normally open switch, the multimeter should read open circuit. Turn on the circuit to let the fluid pressure into the pressure switch. If the multimeter does not read 0 ohms, replace the switch. Opposite for a normally closed switch.

What does a pressure switch do?

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

How to tell if a pressure switch is bad?

The pressure switch can have issues if the fluid is leaking, the pressure is too low, pressure changes too frequently, and incorrect initial set pressure.

How does a mechanical pressure switch work?

When the inlet pressure force is greater than the springs pre-tensioned force, it switches the electrical contact. An NC contact will open, and an NO contact will close. Once the pressure reduces, the contacts will return to their normal state.

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.


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