What is a Pressure Switch : Working & Its Applications

We know that a device used to make and break the connection within an electric circuit is known as a switch. There are different types of switches available in the market where their classification can be done based on the requirement. The different types of switches are limit, joystick, speed, temperature, selector, proximity, and pressure switch. Each type of switch has its own functionality and applications. So this article discusses one of the types of switches a pressure switch.


This is one kind of device that activates an electrical contact once a fixed liquid pressure is reached. Generally, this switch has mainly two functions where the primary function is to maintain the pressure otherwise reservoir levels of the system and the secondary function is to guard equipment against running at less efficiency or harm.

What is a Pressure Switch?

The pressure switch definition is; a mechanical switch that activates through the pressure by reaching a set point is known as a pressure switch. This switch is used to control an electrical circuit by turning ON and OFF. Here the pressure point that triggers the switch is known as the setpoint whereas the pressure threshold that disables the switch is known as the cut-out point. With sufficient force, a contact can open or close the switch based on its configuration. The pressure switch symbol is shown below.

Symbols
Pressure Switch Symbol

Construction

The construction of a pressure switch can be done by using different components like a diaphragm, adjustment spring, lever, electrical contacts, and terminals.

Construction
Pressure Switch Construction

A diaphragm in this switch works like the detection element, so it is used to detect pressure. This element is usually made with a pliable material that is responsive to pressure.

An adjustment spring will change the set or cut-out points. Some switches have separate springs which are used for controlling the set & cut-out points.

An AUTO/OFF lever is used to activate or deactivate the switch manually. This lever is very helpful in turning off the switch throughout fitting or maintenance. Sometimes, instead of a lever, the knob is used; however, its working principle is the same.

Electrical contacts allow the flow of current through them once they touch an exterior power source.

Terminals of the switch are used for connecting the exterior power source to the contacts.

The working principle of a pressure switch is to operate an electrical contact once a fixed liquid pressure is attained. This switch will make electrical contact on either pressure increase or pressure drop from a fixed preset pressure level.

Specifications

The specifications of the pressure switch include the following.

  • The range is -1 to 250 Bar.
  • Contact rating is 5a, 10a, 15a at 250/125 Vacs.
  • The process connection is ¼”,½”, ¾”, 3/8” Bsp or npt Threaded (m/f).
  • Switch type is 1spdt or 2spdt or dpdt.
  • The enclosure is die-cast aluminum.
  • Protection is Weather Proof, Ip66, or Flameproof.
  • The maximum working temperature is 180°c.
  • Mounting is Field/ On-Line/wall/Back Panel.
  • Type of sensing element is bellow/capsule/diaphragm/bourdon/piston.

Pressure Switch Types

There are two types of pressure switches available in the market mechanical and electronic.

Mechanical Pressure Switches

As compared to electronic pressure switches, these switches are most widely used because of their lower cost & simplicity. These switches are available with a mechanical pressure detecting element that deforms based on the pressure of the fluid. These switches are classified based on the different methods used for detecting pressure as either electronic or electromechanical. The different types of mechanical pressure switches are discussed below.

Mechanical Pressure Switch
Mechanical Pressure Switch

Piston Pressure Switch

This switch is most popular and also widely used in different applications. Once the pressure of liquid varies, then it moves the piston axially so that it triggers the switch. This switch can detect the pressure of liquid directly or indirectly. So directly detecting mainly involves seals like O-rings to avoid the liquid from getting into the components whereas indirect detecting mainly involves a flexible diaphragm that divides the piston from the liquid.

Piston Pressure Switch
Piston Pressure Switch

Diaphragm Pressure Switch

These are high-quality switches, mainly developed for safety-critical applications. The main benefit of these switches is that no voltage supply is necessary for the process of switching. This switch includes a metal membrane that is connected directly to the soaked part of the switch and the diaphragm that activates the switch. This switch is used for monitoring pressure & control of processes in different industries like chemical, petrochemical industries, gas, oil, etc.

Diaphragm
Diaphragm

Bourdon Tube Pressure Switch

This is an elastomeric or flexible metallic tube that is connected to one end of the switch whereas another end is free to move. Once liquid pressure is enhanced within the tube, then it tends to make a level, then this movement is mainly used to activate the switch. These switches are suitable for different applications like chemical, conventional power stations, and petrochemical wherever the operating force is middle-high.

Bourdon Tube
Bourdon Tube

Differential Pressure Switch

This switch is very useful in evaluating the forces between two points within a system. The two points within the system are simply connected to two process ports which are the upside or downside of the equipment. If the difference in pressures between the two sides increases a certain threshold, then a switch can be triggered. These switches are applicable in monitoring pressure fall across screens, filters & tank levels.

Differential
Differential Pressure Switch

Electronic Pressure Switch

Electronic pressure switches are available with a pressure transducer like a strain gauge. These types of switches have analog capabilities, so they are not restricted to an open or closed position but also transmit a continuous and variable signal for monitoring more precisely. Therefore, these are not only switches but also measuring instruments or transmitters. Some additional features of these electronic switches are switching function, on-site programmability of time delay, hysteresis, setpoint, etc.

Electronic Pressure Switch
Electronic Pressure Switch

Difference between Flow Switch Vs Pressure Switch

The difference between a flow switch and a pressure switch includes the following.

Flow Switch

Pressure Switch

A flow switch is used to check the rate of water flow & pressure of air, and liquids through a system, duct, or loop. A pressure switch is used to turn ON/OFF an electrical circuit through pressure.
This switch is also called a flow indicator or flow sensor. This switch is also called a pressure sensor.
Flow Switches are triggered by the water flow recognition. Generally, the pressure is lower than 500 PSI then 1 to 2 gallons of water drives through before the pressure switch turns ON. These switches are triggered once you reach pressure between 500 to 700 PSI. Once the pressure reaches this threshold, then this switch can be used to trigger the burner for hot water.
These switches are suitable for engine-driven hot water pressure washer machines. These switches are suitable for lower hot water pressure applications.

Pressure Switch Circuit Diagram

The pressure switch circuit with a force-sensing resistor is shown below. Generally, lighting circuits or switches activated by sensors are very simple to design. Here we will explain how to design a pressure switch for AC appliances. This circuit is used in different applications like auto room lighting, theft alarms, pressure monitor, etc.

Pressure Switch Circuit using FSR
Pressure Switch Circuit using FSR

To design this pressure switch circuit, we need a sensor like a Force sensing resistor or FSR. Generally, FSR is a resistor that exhibits different resistance values once force is applied to it. But as soon as force is applied to the force-sensing resistor then its resistance will drop to 100kilo ohms. Further, this resistance will drop to 100 ohms high force is applied onto the sensor plate

This circuit starts working when FSR detects the force applied to its plate. Here, FSR is connected in series with R1 so that a voltage divider can be formed.

The output voltage from this divider is given to the non-inverting terminal of the comparator. Alternatively, another voltage divider pair can be formed through R3 & variable resistor (RV1). So, the output voltage of this divider can be given to inverting terminal of the comparator. Here, a variable resistor is used to regulate the pressure or triggering voltage in the circuit.

Whenever no force is applied, the FSR output voltage & R1 pair will be less as compared to the output voltage of RV1 & R3. Similarly, once force is applied to the resistor then its resistance will be reduced thus voltage divider pair output will enhance. Now at the noninverting terminal of the comparator, the voltage will be high as compared to inverting terminal. So the output of the comparator will be high which activates the transistor connected to it like a switch.

After that, a 12V relay is used to trigger any AC appliance in 12A current rating. Here, to activate the relay, the LM193IC output is low so, 2N2222 is used to trigger the relay. The ‘D1’ Diode works as a safety element for the rest of the circuit from the reverse current which supplies once the relay gets turned off.

Calibration of Pressure Switch

Pressure switch calibration is required like many other instruments to ensure their reliability & accuracy. As compared to transmitters, switch calibration is difficult. So, the wrong type of calibration can cause several errors in the result. So here we are going to discuss how to calibrate pressure switches properly. The calibration procedure of this switch can be done using the following steps.

Pressure Switch Calibration
Pressure Switch Calibration

In the first step, connect the switch to the source of pressure. For instance; the pressure source like air supply is connected through a hand pressure regulator & test gauge as shown in the above diagram.

In the second step, a digital multimeter or an ohmmeter is used to check continuously whether the switch contacts are indicated as normally closed (NC) or normally opened (NO).

In the third step, connect the ohmmeter in between two switches like NO & NC of the switch. The ohmmeter must read as “open circuit”. So now the hand pressure regulator has to be adjusted for increasing the pressure toward the switch’s setpoint until the switch contacts change over.

Now, the ohmmeter should read “short circuit”. So not down the reading of pressure because this pressure is the set point of the switch for increasing pressure.

In the fourth step, increase the air pressure toward the switch to its highest rating. Now reduce the pressure slowly until the switch changes from closed to normally open. Now note down the reading of the pressure because this is the setting of the switch for reducing pressure.

In the final step, from the obtained readings, check the pressure difference among the rising & falling pressure settings. This is known as the dead band of the switch and the dead band calculated must be equivalent to or below the dead band of manufacturers. Generally, the highest dead band value is stated by the manufacturer. If the maximum dead band value is above the manufacturer’s recommendation then the switch is unserviceable.

Applications

The applications of pressure switches include the following.

  • These switches are used in a wide range of applications like HVAC equipment, pumping systems, compressed air systems, process equipment, etc.
  • These types of switches are extensively used in a range of manufacturing & industrial applications like pumps and compressors.
  • These are used in the industries for equipment control like plastic molding injection, welding machines, and press machines.
  • Pneumatic & Hydraulic switches are used for controlling the air bellows within trucks & brake pressure within trains.
  • These switches are applicable in various automotive applications for engine oil monitoring, transmissions & power steering.
  • These switches are used in Medical equipment like oxygen delivery systems for monitoring the incoming gas pressure.
  • These switches are used to monitor the levels of oxygen within gas tanks, used for dispensing liquid within drinks machines, filtration systems, blockage detection in various systems, compression units within medical facilities, HVAC boiler room controls, Medical breathing devices like ventilators, machinery in agriculture field like crop spraying, water levels in washing machines, DNA test machines for solution level, etc.

Thus, this is all about an overview of a pressure switch like a circuit, it’s working, and its applications. This is one kind of industrial sensor, used to detect a fixed amount of pressure to open or close an electrical contact. So these are frequently used as ON/OFF switches to control electrical elements within a system by actively monitoring the air pressure or fluid within a system. Here is a question for you, what is the limit switch?

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