Pressure Switch: The Complete Guide
A pressure switch is a kind of switch that activates an electrical contact when a predefined fluid pressure on its input is exceeded. The switch may be set to make contact either when the pressure increases or decreases.
The pressure switch opens or closes a matching switch contact when a specified pressure is reached. Depending on the requirements, mechanical or electrical pressure switches may be used.
Pressure switches are extensively used in industry to monitor and adjust systems that utilise pressurised fluids automatically. A pressure switch and a pressure transducer are not the same thing. A pressure transducer converts pressure into an electrical output signal.
Another kind of pressure switch detects mechanical force; for example, a pressure-sensitive mat is used to automatically open commercial building doors. These sensors are also used in security alarm systems, such as pressure sensitive flooring.
Construction
A pressure switch detects fluid pressure by using a capsule, bellows, Bourdon tube, diaphragm, or piston element that deforms or displaces in proportion to the applied pressure. The resulting motion is either directly or indirectly transferred to a series of switch contacts. Because pressure might fluctuate slowly and contacts must respond quickly, an over-center device, such as a tiny snap-action switch, is used to ensure that the contacts respond quickly. One sensitive kind of pressure switch includes mercury switches positioned atop a Bourdon tube; the shifting weight of the mercury provides a useful over-center feature.
The pressure switch may be modified by relocating the contacts or adjusting the tension in a counterbalance spring. A calibrated scale and pointer may be used on industrial pressure switches to show the switch's set point. A pressure switch possesses hysteresis, or a differential range about its setpoint known as the switch's deadband, within which slight pressure changes have no influence on the contacts' state. Differential adjustment is possible in certain kinds.
The pressure-sensing element of a pressure switch may be set to respond to the difference between two pressures. Such switches are useful when the difference is significant, such as when detecting a clogged filter in a water supply system. When the common mode pressure varies, the switches must be designed to respond just to the difference and not to false-operate.
Pressure switch contacts may be rated from a few tenths of an amp to around 15 amps, with lower values encountered on more sensitive switches. A pressure switch will typically drive a relay or other control device, while some versions may directly operate small electric motors or other loads.
Because the internal components of the switch are in contact with the process fluid, they must be chosen to balance strength and life expectancy against compatibility with process fluids. Rubber diaphragms, for example, are often used in contact with water, but would quickly degrade if utilised in a mineral oil-containing system.
To prevent an arc at the contacts from igniting the surrounding gas, enclosures are used on switches approved for use in hazardous areas containing flammable gas. Enclosures for weatherproof, corrosion-resistant, or submersible switches may also be required.
A pressure transducer (strain gauge, capacitive element, or other) is combined with an internal circuit that compares the recorded pressure to a set point in an electronic pressure switch. Such technologies may provide more repeatability, accuracy, and precision than a mechanical switch.
Operating principle
Figure above displays a mechanically operated pressure switch that is a single pole double throw (SPDT) switch. All of the components are housed in the switch case (F), which has one intake pressure port (H). In summary, the input pressure causes a piston (D) to push against a known resistance spring (C). The piston then activates the micro-switch (A), causing it to alternate between the usually closed (NC) and normally open (NO) states through an operational pin (B) and an insulated trip button (E).
Figure above: Pressure switch components comprise a microswitch (A), an operational pin (B), a range spring (C), an operating piston (D), an insulated trip button (E), a switch box (F), a trip setting nut (G), and an intake pressure gauge (H) (H)
The trip-setting nut (G) determines the pressure level at which the micro-switch switches between NC and NO by adjusting the spring pocket depth. This variation in depth allows the spring resistive force to increase or fall, according to a predefined pressure to activate the micro-switch. The intake pressure (H) exerts pressure on the working piston (D), resulting in a force that resists the range spring (C). When the force of the intake piston surpasses the force of the opposing spring, the operational pin (B) is pushed into the insulated trip button (E). By pushing this button, the microswitch is shifted from the NC to the NO position. If the pressure drops below the spring force, the button, pin, and piston move away from the microswitch, breaking the connection. The connection is then changed from the NO to the NC status.
Types
Pneumatic
Uses of pneumatic pressure switches include:
- When water is taken from the pressure tank, an automated residential well water pump is activated.
- An electrically driven gas compressor is shut off when a certain pressure in the reservoir is attained.
- A gas compressor is shut off when there is no supply in the suction stage.
- Turning on a warning light in the cockpit of an aeroplane if cabin pressure (depending on altitude) is critically low.
- Air-filled tubes activate switches when automobiles travel over them. It is often used for traffic counts and at gas stations.
Hydraulic
Hydraulic pressure switches are used in automobiles for several functions, such as alerting if the engine's oil pressure falls below a safe level and managing automatic transmission torque converter lock-up. Prior to the 1960s, a pressure switch in the hydraulic braking circuit was used to control power to the brake lights; modern vehicles have a switch that is activated directly by the brake pedal.
When the air pressure in the header of a dust control system (bag filter) is less than what is necessary, a pressure switch on the header raises an alarm. A differential pressure switch may be placed across a filter element to detect rising pressure drop and warn the user when the filter needs to be cleaned or replaced.
Mechanical pressure switches - standard
If a mechanical pressure switch detects the necessary pressure and is activated, the switch will send an electrical signal. It does not need any more electricity for this. One of the advantages of mechanical pressure switches is their reliability.
A mechanical pressure switch uses a spring and a diaphragm, or piston, to regulate when a microswitch is engaged. A set screw or knob may be used to adjust the tension of the spring, which serves as an opposing force to the intake pressure. The pressure required to make an electric contact via the switch is directly proportional to the spring pretension. The switch returns to its original position when the pressure drops.
The difference between the switch and reset points is known as hysteresis. This is frequently expressed as a percentage of the switch point value, for example, 20%. For most mechanical switches, the hysteresis is set by the manufacturer and cannot be changed by the user.
Typically, these pressure switches have three contact types: usually open (NO), normally closed (NC), and changeover (SPDT). Changeover connections enable both NO and NC operation.
Mechanical pressure switches can withstand higher voltages and amperages than electronic pressure switches. They may be used to generate a contact change in reaction to a pressure increase or decrease.
Mechanical pressure switches - explosion-protected
Mechanical pressure switches are available in Ex-ia or Ex-d configurations for usage in hazardous environments.
Electronic pressure switches
In an electronic pressure switch, an electrical pressure sensor is utilised to detect changes in intake pressure. They have digital screens on which the switching function may be programmed. Therefore, not only is the needed switch point identified, but the actual measured pressure is also given.
Depending on the application requirements, the switch point may be pre-programmed by the manufacturer or programmed on-site. The switch point, output signals, hysteresis, delay duration, and other properties may all be changed by the user.
An electronic pressure sensor is the building block for an electronic pressure switch. In addition to the electrical signal for switching the electrical circuit, many electronic pressure switches give an additional analogue signal proportional to the pressure.
Electronic pressure switches often have a display that shows the measured pressure as well as customisable settings and programming. Switch points, output signal, delay period, hysteresis, and a range of other parameters may all be customised for the application.
Electronic pressure switches are suitable for automated and controlled equipment systems that need programmable functionality, digital display, flexibility, accuracy, ingress protection, and stability. An electric pressure switch is seen below.
Examples
Selection criteria
Consider the following selection criteria for your application:
Type of media: The media should be compatible with the housing and seal material. Both air and hydraulic/machine oil may be utilised with nitrile butadiene rubber (NBR). When the medium is water, ethylene propylene diene monomer rubber (EPDM) is acceptable.
Pressure: It must be able to withstand the greatest amount of working pressure. A diaphragm design is appropriate for vacuum and low-pressure applications. For high-pressure applications, use a piston design.
Temperature: It must be able to operate normally within its maximum and minimum temperature ranges.
Repeatability: The capacity of a device to return to the same fixed point on each repetition, also known as repeatability or accuracy, is referred to as repeatability. The pressure switch for your application will be selected depending on the required accuracy range. Diaphragm designs are often more precise than piston designs.
Hysteresis: The difference between the switch point and the reset point is known as hysteresis. If the reset point is set too high, the switch stays active for an indefinite length of time. If the reset point is too close to the switch, it will often cycle between on and off states. The hysteresis of an electronic pressure switch may be changed, but the hysteresis of a mechanical pressure switch is fixed by the manufacturer.
Type of pressure switch: For low-pressure and vacuum applications, use a diaphragm design. For high-pressure applications, choose one with a piston design.
Electric or mechanical pressure switch: An electric pressure switch is more expensive than a mechanical pressure switch, but it gives you more control over characteristics like pressure setpoint and hysteresis.
Common applications
A pressure switch can be used in a wide range of domestic and commercial applications as listed below:
- Air compressor pressure switches are used to monitor and regulate the air pressure in HVAC, gas cylinders, air pumps, and other systems.
- Oil pressure switches are used in engines as a sensor to detect when the engine's oil pressure goes below a specified threshold.
- Furnace pressure switches are used to provide safety in both industrial and domestic environments. They detect low air pressure and turn off the furnace if it is detected during the furnace's start-up process.
- Well pump pressure switches are used in residential and commercial buildings to draw water from a well while also ensuring that the system has enough water pressure to provide water without being over-pressurised.
- In residential, commercial, and agricultural uses, water pump pressure switches automatically adjust water flow.
- The existence of vacuum or negative pressure in the system is detected using vacuum pressure switches. They are often found in boilers, electric heaters, air compressors, and transmission systems.
FAQ
How to adjust a pressure switch?
To increase the switch point of a mechanical pressure switch, turn the nut clockwise, and to lower it, turn the nut counterclockwise. To make modifications to an electric pressure switch, a keypad is utilised.
How to test a pressure switch?
Unplug the power supply from the pressure switch and connect the multimeter to its terminals. For a normally open switch, the multimeter should read open circuit. To enable fluid pressure to enter the pressure switch, turn the circuit on. If the multimeter does not read 0 ohms, replace the switch. A typically closed switch is the converse of a normally open switch.
What does a pressure switch do?
A pressure switch monitors the fluid pressure in the system and based on a predefined pressure level, either opens or closes an electrical connection.
How to tell if a pressure switch is bad?
The pressure switch may fail if the fluid is leaking, the pressure is too low, the pressure swings too often, or the initial set pressure is incorrect.
How does a mechanical pressure switch work?
When the intake pressure force exceeds the spring's pretensioned force, the electrical contact is switched. A NO contact will be closed, whereas an NC contact will be opened. Once the pressure is released, the contacts will return to their normal state.
What is the difference between a pressure switch and a pressure sensor?
At a specified pressure level, pressure switches activate electrical switches, while pressure sensors monitor system pressure and convert it to an electrical signal.
