Pressure Switch Explained | Types of Pressure Switches
In this article, we’re going to talk about different types of pressure switches and how they operate. Ok… Let’s go…
Pressure sensor family
Depending on who you talk to, pressure switches fall under the Pressure Sensor category.
Some people will tell you that a Pressure Sensor is a device for pressure measurement of gases or liquids. Included in this Pressure Sensor category along with pressure switches are pressure transducers, pressure transmitters, and pressure senders, among other names.
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 produces an electrical output in response to that change.
And just to add even more confusion, the terms pressure sensor, pressure transducer, and pressure transmitter are unfortunately used interchangeably in the industrial world.
Now that we’ve got you scratching your head, let’s dig into the device which is the focus of this article: the pressure switch.
What is a pressure switch?
First of all, let’s start by defining what a pressure switch is. A Pressure switch is a two-part device consisting of a sensing transducer and an electrical switch.
The electrical switch opens and closes at a specific pressure often referred to as the Setpoint.
Depending on the switch and the vendor, the switching pressure setpoint may be fixed or adjustable.
A transducer is a device that converts one form of energy to another. So, the transducer part of the pressure switch is the piece that comes in contact with the process under test. Then somehow, the transducer has to operate the electrical switch.
Vendors produce pressure switches using different types of transducers and different types of switches.
Pressure switch vs pressure transmitter
Before we go any further, let’s clear up any confusion you might have about the difference between a pressure switch and a pressure transmitter.
A pressure switch has a transducer just like a pressure trasmitter. That’s where the similarity ends.
The output of a pressure switch is a digital on-and-off signal. This signal can only have two states of on and off.
The output of a pressure transmitter, however, is an analog electrical voltage or a current signal representing 0 to 100% of the pressure range sensed by the transducer.
So the main difference between a pressure switch and a pressure transmitter is the type of output signal. The first one gives you a digital on-and-off signal in the output, while the second one gives you an analog signal.
Pressure switch types
Ok… now that we’ve got that cleared up, let’s get back to the pressure switch. 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 a variety of different types of transducers that are used to mechanically operate the switch such as Diaphragm and Bourdon type.
– Diaphragm switches use a metal diaphragm to operate the switch.
– Bourdon tube switches use a bourdon tube to operate the switch.
Solid-state pressure switches have no moving parts. Electrical switching is accomplished by operating a semiconductor device such as a bipolar junction transistor or a Field Effect transistor.
A typical transducer used on a solid-state pressure switch is a strain gauge Wheatstone Bridge.
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-state but much more versatile. It offers options such as Programmable functions, LED displays, multiple and adjustable output switch configurations such as normally open, normally closed, or both.
Electromechanical or solid-state?
Which type of pressure switch is better, the electromechanical type or the solid-state type? As with all instrumentation, the choice between the two depends upon the application.
Solid-state pressure switches provide several advantages over electromechanical switches such as longer life, better accuracy, shock and vibration resistance.
Generally speaking, electromechanical switches are capable of switching higher currents and are not voltage-dependent.
Pressure switch symbols
As with other electrical devices, there are several different symbols used on schematics to represent pressure switches.
Let’s look at pressure switch symbols as designated by two industry-recognized Standards organizations.
Here are two symbols from the National Electrical Manufacturers Association, (NEMA) and International Electrical Commission (IEC).
Below is the symbol for an SPST normally closed pressure switch.
The switch on the left is NEMA and the switch on the right is IEC. This symbol indicates that the switch will open with an increase in pressure.
Below is the symbol for an SPST normally open pressure switch.
Once again, the switch on the left is NEMA and the switch on the right is IEC. This symbol indicates that the switch will close with an increase in pressure.
This is a good time to mention that all pressure switches on a schematic diagram will be shown in their De-energized condition, or in other words, the condition they would be in sitting on a shelf.
Pressure switches in action
Let’s examine a couple of pressure switches on schematics…
PS101 is a normally closed switch that will open on an increase in pressure above 15 psi.
In this example, the Brake Oil Fltr pressure switch is normally closed and will open on a decrease in pressure below the setpoint. The setpoint is not stated in the schematic.
What is Pressure Switch Deadband?
One more thing before we wrap up… What is Pressure Switch Deadband?
We said that the Brake Oil Fltr pressure switch is normally closed and will open on a decrease in pressure below the setpoint.
Let’s assume the setpoint is 15 psi and the switch has been operated, meaning the pressure has dropped below 15 psi. If the switch does not close again until the pressure rises to 17 psi, then the Deadband is 2 psi.
So… what is Deadband? Deadband is the difference between the setpoint and the point where the switch re-actuates.
Ok… let’s review…
– A pressure switch is a two-part device consisting of a sensing 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 recognized symbols for pressure switches
– Deadband is the difference between the stated pressure and the actual pressure where the switch re-actuates.
If you want to learn more, you might want to review two of our other articles:
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