What is a pressure switch?
Switches are safety devices that determine the process parameter reaching a certain point and activate the status of alarms or safety interlocks. Pressure switches are set to a specific process pressure and activate an electrical circuit when the pressure reaches the set point. In the case of differential pressure switches, the pressure difference between the two process pressures is monitored and the switch is activated when this difference reaches a predetermined value.
The two terms pole and path are used to describe how switches work. The number of routes is equal to the number of separate locations that the switch can specify. A single-way switch is a connector that can be closed or open. The two-way switch has a connector that can connect the input to one of the two outputs.
Pressure sensor family
Pressure switches are in the group of pressure sensors. Some people will tell you that a pressure sensor is a device for measuring the pressure of gases or liquids. Pressure converters or pressure transmitters are among the other names in this category of pressure sensors.
This can be a bit confusing because in the world of instrumentation and process control we define a sensor as a device that detects changes in physical, electrical, or chemical properties and responds to changes in electrical output.
And just to add more confusion, unfortunately the terms pressure sensor, pressure transducer and pressure transmitter are used interchangeably in the industrial world.
What is a pressure switch?
First of all, let’s start by defining what a pressure switch is. The pressure switch is a two-part device that consists of a sensing converter and an electrical switch.
The electrical switch opens and closes at a certain pressure, often called a setpoint. Depending on the switch, the switch pressure setting may be fixed or adjustable.
Depending on the switch and vendor, the switch pressure setting may be fixed or adjustable.
A transducer is a device that converts one type of energy into another. Therefore, the pressure switch is a component that contacts the process under test. Then somehow the converter has to work the electrical switch.
Vendors use different types of converters and different types of pressure switches.
Pressure switch against pressure transmitter
Before we go any further, let’s clear up any confusion about the difference between a pressure switch and a pressure transmitter. A pressure switch has a converter just like a pressure transmitter. This is where the similarity ends.
The output of the pressure switch is the digital on / off signal. This signal can only have two on and off modes. The output of a pressure transmitter is an analog electrical voltage or current signal that represents 0 to 100% of the pressure range sensed by the converter. Therefore, the main difference between a pressure switch and a pressure transmitter is the type of output signal. The former gives you a digital signal at the output, while the latter gives you an analog signal.
Types of pressure switches
There are two different types of pressure switches: electromechanical and solid state.
Electromechanical pressure switches All have traditional style mechanical switches with moving parts. There are several types of converters used for Swiss mechanical work, such as diaphragms and the Bourdon type.
Diaphragm switches use a metal diaphragm to operate the switch.
– Bourdon pipe switches use Bourdon pipe to operate the switch.
2) solid state
Solid state pressure switches do not have any moving parts. Electrical switching is done by working with a semiconductor device such as a bipolar junction transistor or a Field Field transistor.
A typical converter used in a solid state pressure switch is a Wheatstone Bridge sphygmomanometer.
2-1) Electronic (solid state)
The term electronic pressure switch is slowly replacing the term solid state pressure switch. The electronic pressure switch is solid but much more versatile. It offers options such as programmable functions, LED display, multiple output switch settings and adjustments such as normal open mode, depending on normal mode or both.
Electromechanical or solid state?
Which type of pressure switch is better, the electromechanical type or the solid state type? Like all instruments, the choice between the two depends on the application. Solid state pressure switches have several advantages over electromechanical switches including longer life, better accuracy, impact and vibration resistance. In general, electromechanical switches are able to change higher currents and are not voltage dependent.
Pressure switch symbols
Like other electrical devices, there are several different symbols in the diagrams to indicate pressure switches. Let’s look at the pressure switch symbols recognized by the two industry standard organizations. Here are two symbols from the National Electricity Producers Association (NEMA) and the International Electricity Commission (IEC). Below is the symbol of a typical SPST closed pressure switch.
The left switch is NEMA and the right switch is IEC. This sign indicates that the switch opens as the pressure increases. Below is the symbol of an SPST open pressure switch normally shown.
Again the left switch is NEMA and the right switch is IEC. This sign indicates that the switch is closing with increasing pressure. It’s a good time to mention that all the pressure switches will be shown in a schematic diagram in a state without energy, or in other words, a position on a shelf.
Pressure switches in operation
For example: The PS101 is a normally closed switch that opens when the pressure increases by more than 15 psi.
Example 2: In this example, the Brake Oil Fltr pressure switch is normally closed and opens by reducing the pressure below the set point. The setting point is not expressed in the schematic.
What is Deadband Pressure Switch?
We said that the Brake Oil Fltr pressure switch is normally closed and opens when the pressure drops below the set point. Suppose the set value is 15 psi and the switch is working, ie the pressure is below 15 psi. If the switch is not closed again until the pressure reaches 17 psi, it is Deadband 2 psi.
So we have to say that Deadband is the difference between the set point and the point where the switch is reactivated.
A pressure switch is a two-part device that consists of a sensor component and an electrical switch that opens or closes a contact at a specific pressure. – There are two types of pressure switches: electromechanical and solid state – There are several different known signs for pressure switches – Deadband is the difference between the declared pressure and the actual pressure at which the switch is reactivated.
The following table shows the common types of switches in terms of performance:
|Complete switch phrase||Types of pressure switches|
|single pole, single throw||SPST|
|single pole, double throw||SPDT|
|double pole, single throw||DPST|
|double pohe, double, throw||DPDT|
Pressure switches are used to determine absolute, gauge and differential pressures. The sensor element can be a bellows, a tube, a diaphragm or other elastic sensors.
The pressure range in which the set point can be set is called the adjustable range. The switch can change position when the pressure at the set point increases or decreases. The set point is the pressure that the switch activates repeatedly. The difference or dead band is the difference between the set point and the switch operation point again.
Manufacturers of pressure switches include Vika, Danfoss, EBB, Squardi, UE, Delta.
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