In this video,
we're going to talk about different types of pressure switches and how they operate.
Ok, Let’s go
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 video the 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.
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 transmitter has a transducer
just like a pressure switch.
That’s where the similarity ends.
The output of a Pressure Transmitter is an analog electrical voltage
or a current signal
representing zero to one hundred percent of the pressure range
sensed by the transducer.
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.
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.
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.
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,
or NEMA, and International Electrical Commission, or IEC.
This 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.
This 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.
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 fifteen psi.
In this example,
the Brake Oil Filter pressure switch is normally closed
and will open on a decrease in pressure below the setpoint.
The setpoint is not stated in the schematic.
One more thing before we wrap up.
What is Pressure Switch Deadband?
We said that the Brake Oil Filter pressure switch is normally closed
and will open on a decrease in pressure below the setpoint.
Let's assume the setpoint is fifteen psi
and the switch has been operated,
meaning the pressure has dropped below fifteen psi.
If the switch does not close again until the pressure rises to seventeen psi,
then the Deadband is two psi.
So, what is Deadband?
Deadband is the difference between the setpoint
and the point where the switch reactuates.
If you want to learn more,
you might want to review two of our other videos
“What is a Pressure Sensor”
and “Pressure Transmitter Explained Working Principle”.
You can find the links to these videos in the description.
Ok. let’s summarize what we’ve discussed today
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 reactuates.
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