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This article will provide comprehensive insights on pressure
switches. Read further to learn more about:
Definition and Principle of a Pressure Switch
Parts of a Pressure Switch
Different Types of Pressure Switches
And much more…
Chapter 1: What is a Pressure Switch?
A pressure switch is a mechanical or electronic device activated
by the pressure of fluids, air, or gas when the fluids, air, or
gasses reach a threshold or setpoint. The designs of pressure
switches include bourdon tubes, pistons, diaphragms, or
membranes that move or deform with the amount of pressure
exerted by the system.
The components of a pressure switch are connected to one or more
contacts in the switch. With enough force, a contact closes or
opens the switch depending on its configuration. Although
pressure switches have a variety of methods used to detect
pressure, they can be primarily categorized as electromechanical
Pressure switches are used in most industries employing
compressed gas systems, HVAC, instrumentation systems, pumping
systems, and so on.
A typical pressure switch has a piston with one side subjected
to the fluid pressure. The other side is usually in atmospheric
pressure. The force exerted by the fluid pressure is countered
by force from a preloaded spring. The surface area in contact
with the fluid and the spring constant is carefully designed so
that the piston only moves when a certain pressure is reached.
The spring is pre-compressed by the setpoint screw. The setpoint
screw is adjusted to set the activation pressure higher or
Cut-in and Cut-out
Pressure switches generally have two operating points: the
cut-in and the cut-out pressure. In pump and compressor systems,
the switch is activated when the fluid pressure goes below a set
level. This starts the motor of the pump or compressor, which
returns the system to normal levels. The switch does not
deactivate instantly when the pressure goes above the set point.
There is a form of hysteresis or differential that prevents
sudden tripping. This allows pressure to build up until the
higher end of the pressure range is reached. When the higher
setpoint or cut-out is reached, the switch deactivates.
Chapter 2: Parts of a Pressure Switch
This chapter discusses the main parts of a pressure switch. Note
that each type or proprietary design may include additional
components. The parts mentioned below are only applicable to
mechanical pressure switches.
Process (Inlet) Port:
The inlet port is the part that connects the pressure switch
assembly to the process unit. Pressure switches are
installed on nozzles connected to a tank or pipe. The
typical connection is threaded fittings. In rare cases,
bolted or welded connections are used. It is important that
the fitting type and its pressure rating are compatible with
the fluid pressure.
Pressure Sensing Element:
Mechanical pressure switches are classified according to
their pressure sensing element. This is the main part of the
switch that mechanically actuates the switch from the
pressure of the fluid. The area of the piston or diaphragm
on the fluid side is designed to transfer sufficient force
from the expected fluid pressure. The larger the area, the
larger the actuating force and spring force required. Note
that only a small force is needed to actuate the switch.
Much of the pressure is countered by the spring.
The spring counters the force from the fluid. It is
preloaded to match the operating pressure of the fluid. The
switch only activates when the force from the fluid pressure
exceeds the force applied by the spring.
Setpoint Adjustment Screw:
Integrated with the spring is the setpoint adjustment screw.
The setpoint adjustment screw is used to increase or
decrease the activation pressure.
This is used to widen or narrow the operating pressure range
of the switch. The common design mostly seen in pumping
systems is a set of spring and adjustment screws which are
visibly smaller than the setpoint adjustment. Tightening or
loosening this screw modifies only one end (higher or lower
end) of the pressure range while the other end remains the
Diaphragm (Diaphragm-piston Assembly), Seals, and O-rings:
The diaphragm, along with the other sealing parts, protects
the internals of the switch from the process fluid. It is a
flexible material usually made of polymers, elastomers, or
metal alloys. The type of diaphragm material is selected
based on the type of fluid and its temperature. Common
diaphragm and sealing materials are:
Nitrile or NBR (Buna-N):
These materials are highly resistant to oils or
petroleum-based fluids but can degrade in the presence
of ozone and ketones. Nitrile diaphragms and seals have
a good balance of cost and physical properties, making
them suitable for most neutral fluids. Its operating
temperatures can range from -30°C to 100°C.
Ethylene Propylene Diene Monomer or EPDM:
This is another elastomer widely used for
high-temperature water and steam service. Its operating
temperatures can reach up to 482 ° F (250°C). It
is resistant to ozone, ketones, mild acids, alkalis, and
other oxidizing chemicals. They are not used in
petroleum service since EPDM can absorb oils and fuels,
which causes them to swell.
Fluorocarbon or FKM (Viton):
Viton is a proprietary material with properties similar
to NBR. This material is resistant to petroleum-based
fluids and solvents. They are not suitable for fluids
containing ketones as well. Viton has superior operating
temperatures that can reach up to 200°C.
PTFE is rarely used as a diaphragm membrane over the
previous materials due to its polymeric chain structure.
It is not as elastic as elastomers and is prone to
creep. They are only considered for very high
temperatures (up to 500°C) and corrosive or high
abrasion service. A popular PTFE diaphragm is made of
Teflon (PTFE) with a Kapton Layer (polyimide).
The switch housing protects the switch and other internal
parts from the external environment. An important
specification of the switch housing is its protection
rating. Typical enclosure specifications are IP, NEMA, and
ATEX ratings. IP and NEMA ratings describe the protection
level against the ingress of solid and liquid foreign
objects. ATEX rating is for environments with risks of fire
The contacts are one of the conductive parts of the switch.
Separating or linking the contacts will de-energize or
energize the electrical circuit. Switch contacts are made of
materials with high corrosion resistance and electrical
conductivity, such as copper, silver, gold, or brass. In
terms of its connections, contacts can be NO, NC, or CO. NO
is for initially de-energized circuits which cut in at the
setpoint. NC performs the opposite by being initially
energized. CO switches serve two connections or circuits,
one open and one closed, which are commonly used for control
interlocking or more complicated circuits. For simple
control activation, NO or NC is enough.
The terminal where the control or instrumentation circuit is
connected. Most pressure switches have markings on their
nameplate about the configuration of the terminals with
respect to the contacts. The nameplate includes schematics
or diagrams to determine the correct terminal connection in
the circuit. Like the contacts, the terminals must be
resistant to corrosion and highly conductive.
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Chapter 3: Types of Pressure Switches
There are two main types of pressure switches: electronic
pressure switches and mechanical pressure switches. Electronic
pressure switches are solid-state switches that do not require
actuation from the pressure-sensing element to operate the
switch. They operate indirectly by using other properties, such
as resistance and capacitance. Mechanical pressure switches are
further divided according to the form and construction of the
Electronic (Solid-state) Pressure Switch
An electronic pressure switch has a pressure transducer,
typically a strain gauge, with additional proprietary
electronics that amplify and convert signals into a readable
display. Some electronic pressure switches have analog
capabilities, which means they have switching capabilities and
can transmit continuous, variable signals that represent the
pressure reading. Additional features of electronic pressure
switches are on-site programmability of time delay, switching
function, setpoint, and hysteresis.
High Pressure Switches
High pressure switches have high pressure-proof pressure limits
and can operate from 1 psig up to over 10,000 psig, with 4500
psig and 7500 psig being the average. They can be actuated using
diaphragms, pistons, or piezoelectric crystals. The most common
form of high pressure switch is diaphragm activated, which is
actuated by pressure changes. As with all diaphragm pressure
switches, actuation is triggered when the flow exceeds the set
Although most pressure switches will fail under certain
conditions, high pressure switches continue operating and
maintain pressure control, regardless. Due to their ability to
provide continuous pressure control, high pressure switches are
used as explosion-proof and waterproof pressure switches under
intense pressure conditions.
High pressure switches have high durability and tensile strength
and are made of aluminum, stainless steel, Monel, Hastelloy, or
steel. Depending on the type of alloyed metal, some high
pressure switches are corrosion-resistant.
Low Pressure Switches
Light or low pressure switches are designed to respond to
reduced or small fluctuations in pressure. They are a protection
method that prevents loss of pressure in a line that could
damage or harm a system. If flow or pressure is absent in a
line, low pressure switches will turn off equipment, activate an
alarm, or provide a pressure reading.
Much like high pressure switches, low pressure switches come in
several different operational methods, including diaphragms,
pistons, and piezoelectric crystals. They are commonly used with
hydraulic and pneumatic systems, where constant pressure is
necessary. The exceptional sensitivity of low pressure switches
allows them to react in correlation to pressure changes in a
Differential low pressure switches work by measuring the
pressure between two points with different pressures and actuate
in accordance with their set point. Positive low pressure
switches work by converting a positive pressure signal into
electrical output in response to changes in positive pressure.
Conversely, negative low pressure switches convert negative
pressure signals into electrical output when there is a change
in negative pressure.
Other Types of Pressure Switches
There are an endless number of pressure switches each of which
is designed to perform a specific function to aid in the
completion or protection of a process. Added to the
classifications of mechanical and electronic are specialized
Adjustable Pressure Switches: An adjustable pressure
switch allows users to determine the pressure level at which the
switch will activate. They are used where pressure levels vary,
such as air compressors, hydraulic systems, irrigation systems,
and HVAC systems.
Air Pressure Switches: Air pressure switches control air
in pneumatic systems, air compressors, HVAC systems, power
tools, and machinery.
Gas Pressure Switches: As with air pressure switches, gas
pressure switches monitor and control gas pressure in home
appliances such as furnaces, boilers, and hot water heaters.
They are used in industrial applications to control pressure in
Oil Pressure Switches: Oil pressure switches are found in
engines, compressors, and hydraulic systems. They perform the
same function as gas and air pressure switches in regard to the
use of oil. Oil pressure switches are critical to hydraulic
systems since oil is the driving force of the hydraulic process.
Hydraulic Pressure Switches: Hydraulic pressure switches
serve as a safety measure for hydraulic systems that operate
under high pressure. Their main function is to prevent damage to
equipment as well as protection of workers. Hydraulic pressure
switches are found in any industry that uses hydraulic power.
Vacuum Switches: Vacuum switches measure negative
pressure and monitor the status of a vacuum in an open or closed
connection. They come in several versions, which include electro
mechanical, solid state, and pneumatic. There are any number of
configurations, designs, and types of vacuum switches, each of
which varies according to being normally open or closed, single
poles, double poles, and throw type of vacuum.
Well Pressure Switch: A well pressure switch is designed
to turn a well pump on or off based on the pressure in the well.
They have a cut on and cut off pressure that decides when the
switch turns the pump on or off. Well pressure switches are a
safety device and control method that ensures that a well is
always at the correct level, not too empty and not too full.
Chapter 4: Pressure Switch Selection Criteria
Like any other measuring or monitoring device, pressure switches
have several selection criteria that need to be considered.
Selecting the right pressure switch for a given application
leads to lower costs and longer service life of the device.
The chemical properties of the process fluid determine the
type of material required for the wetted parts. The wetted
parts are the ports, seals, and the pressurized side of the
pressure sensing component. These parts must be capable of
withstanding any chemical or physical attack from the
process fluid. Mechanisms of part degradation can be through
corrosion, oxidation, or erosion. The most commonly used
materials for the rigid parts are steel, brass, stainless
steel, PTFE, or PP; while the elastic pressure sensing parts
and seals use NBR, EPDM, and FKM.
The operating temperature influences the material used.
Certain materials degrade at high temperatures. Materials
suitable for high-temperature service are FKM and stainless
steel 316. The temperature of the media being measured must
be within the manufacturer’s specified temperature range for
The effect of temperature on accuracy must also be
considered. When a pressure switch is configured at room
temperature, the setpoint may need to be readjusted if the
process is at a higher temperature. Fitting connection sizes
can range from 1/8 to 1/2 NPT.
The pressure range defines the limits for adjusting the
cut-in and cut-out pressures. This is often termed the
working range of the pressure switch. It is recommended to
have the setpoint at 40 to 60% of the pressure range to
anticipate of any adjustment or field changes.
Type of Pressure:
Pressure switches are often used in positive pressure
systems. But there are also cases where they are used in
vacuum applications. For negative pressure systems, pressure
switches specified for vacuum and compound pressure must be
Switches can be characterized according to the number of
poles and throws. The pole refers to the number of circuits
a switch can control, while the throw is the number of
connections the switch can make. Both pole and throw can be
single or double. The switching function classifications
Single Pole, Single Throw (SPST):
This is the basic on and off switch. It can only be
either NO or NC.
Single Pole, Double Throw (SPDT):
This is the most common due to its versatility. It can
be used as a NO, NC, or CO switch. It can also have
three positions, with the center being an off position
for a CO switch. This is referred to as single pole,
triple throw and is rarely used for pressure switches,
which typically have only two positions.
Double Pole, Single Throw (DPST):
This is the same as two SPST switches connected in a
Double Pole, Double Throw (DPDT):
This is the same as two SPDT switches controlled by a
Differential, Deadband, or Hysteresis:
This is the difference between the cut-in and cut-out
pressures. Pressure switches can have either adjustable or
fixed deadbands. Adjustable deadbands are widely used for
water pumping services. Fixed deadbands, on the other hand,
are seen in packaged equipment and alarm systems where
modifications are not necessary or avoided to prevent any
inadvertent modifications of the system. Diaphragm and
bourdon tube pressure-sensing elements generally have a
narrower deadband compared than pistons.
Proof pressure is the maximum pressure the switch can
withstand without causing any change to its properties or
performance. This is also known as the over-range capacity
or maximum system pressure. Identifying the proof pressure
considers any pressure spikes or surges occurring in the
This is the maximum positive or negative deviation from the
setpoint or specified characteristic curve under specific
conditions and operations. Accuracy is a more important
factor in selecting analog pressure sensors and electronic
pressure switches. For these devices, having a higher
accuracy significantly increases the cost of the pressure
switch. Accuracy is specified in terms of a percentage of
the full scale (FS) value. Typical pressure switch
accuracies for diaphragm and bourdon tube is ±0.5%, while
piston pressure switches have an accuracy of ±2%. On the
other hand, electronic pressure switches have better
accuracies of ±0.2 to 0.5% depending on the manufacturer.
Repeatability is the deviation between measurements or
activations at the same pressure. This is different from
accuracy because a device can have high repeatability but
low accuracy. A pressure switch can repeatedly activate at a
certain pressure, but the activations are far from the
setpoint. Like accuracy, repeatability is specified in terms
of full-scale percentage.
This is the expected period between two activations. This
factor must be considered since continuous deformation of
the pressure sensing element subjects it to constant
fatigue, lowering its service life. Piston and bourdon tubes
operate on the principle of deformation and are suitable for
low cycling applications. For high cycling, piston and
electronic pressure switches are used. A piston pressure
switch experiences less fatigue since the actuation relies
only on the movement of the piston or plunger. An electronic
pressure switch also experiences the same since deformations
in a strain gauge are much smaller than mechanical sensing
This is directly influenced by the speed of cycling. The
service life is the expected number of times the switch can
activate and deactivate before failure. Since electronic
pressure switches are solid-state devices that have no
moving parts, they have better service life which is
expected to be above a million cycles. Among the mechanical
pressure switches, piston switches have better service life
than bourdon tube and diaphragm switches.
Control System Voltage:
This specifies the electrical characteristics of the control
circuit. The power switch must be rated to the same current,
voltage, and frequency. Otherwise, the switch, particularly
electronic switches, may not activate or may have poor
accuracy. Control circuits that use pressure switches are
usually in DC. However, in some instances, AC voltages are
also used. Common DC voltages are 8, 12, 24, and 30 volts,
while AC voltages at 60Hz are 24, 120, 240, and 480 volts.
The type of fitting connection on the pressure switch must
match with the process stub connection or pressure port.
Male and female threaded connections are widely used in
mounting pressure switches. Fitting connection sizes can
range from 1/8 to 1/2 inches. Aside from the size and type,
the material of the fittings is also specified according to
the type of environment and matching connection. The main
reason is to prevent corrosion, either from the atmosphere
or from galvanic processes.
Enclosure Protection Rating:
This determines the environment the switch housing can
withstand. Since pressure switches are widely used in almost
all industries, there are various enclosure designs to
balance robustness and cost. Enclosure protection ratings
are specified through NEMA and IP numbers. Generally, a
higher NEMA number indicates a better protection level. IP
numbers, on the other hand, have two digits. The first
indicates the solids or particulates protection rating,
while the second is for liquids. For general-purpose indoor
use, NEMA 1 to 2 or IP 10 to 11 is used where only
protection from personnel contact is required. NEMA 3S to
NEMA 4X or IP 54 to IP 64 is sufficient outdoors to protects
against dust, rain, and snow. In the case of occasional
washdown and immersion, NEMA 6 and IP 68 are commonly used.
Aside from solid and liquid protection, enclosures are also
rated according to their compatibility with an explosive
environment. ATEX and IECEX markings specify the hazardous
applications of pressure switches and other electronic
devices. Before requiring an ATEX rating, first look into
the type of hazardous area where the pressure switch will be
used. It is important to specify this accurately since
having higher protection ratings greatly increases the cost
of the device. Also, having a higher rating does not mean a
higher level of protection for a particular application.
Certifications assure that the product conforms with the
general safety standards mandated by national and
international organizations. This is especially significant
for pressure switches used in applications that directly
affect consumer health and safety, such as food
manufacturing, fire protection, flammable gas handling, and
so forth. Underwrite Laboratories (UL Listed or Recognized),
CSA, FM, and CE are widely accepted certifications.
Chapter 5: Applications
There are two main functions of a pressure switch. One is to
maintain the pressure or reservoir levels of the system. The
other is to protect equipment from damage or from running at low
Water Pumping Systems:
This may be the most common use of pressure switches.Water
pumps use pressure switches to cut in power into the motor,
which drives the pump in case of low level or low line
pressure. Upon reaching the set pressure, power is cut out.
Compressed Air Systems:
This is similar to water pumping systems. Pressure switches
are used to cut-in power to the compressor motor when low
pressure is detected. This maintains the pressure of the
compressed air system.
Pneumatic and Hydraulic Systems:
These are control systems that use pneumatic and hydraulic
actuators. Pumps and compressors maintain reservoir pressure
and level through pressure switches.
Air Conditioning and Refrigeration:
In a refrigeration system, the thermostat provides the
controlling feedback signal. However, if there is a problem
in the system, the thermostat will only sense the
temperature in the cooled space but not the state of the
equipment. A pressure switch serves as a safeguard that
trips the compressor motor in case of overpressure. Another
use of a pressure switch in a refrigeration system is
protection on the low-pressure, indicating a possible
Furnace and Boiler Systems:
The pressure switch in a furnace or boiler serves as a
safety interlock to prevent the igniter from operating in
case of a problem with the draft system. This prevents the
combustion chamber from operating, which can result in
Filtering and Screening Equipment:
A differential pressure switch is used to measure or monitor
the pressure drop across filters and screens. The pressure
switch triggers an alarm or notification to indicate that
the filter is blocked or clogged and is due for maintenance,
cleaning, or replacement.
Chapter 6: History of Pressure Vessels
Scientists used these building blocks to invent and develop
pressure switches, starting in the 1800s. In 1843, French
scientist Lucien Vidie invented and assembled the first aneroid
barometer, which used a spring balance to measure atmospheric
pressure. While under pressure, the spring extension would
mechanically amplify on an indicator system. Based on
Vidie‘s methods, Eugene Bourdon patented the Bourdon tube
pressure gauge in 1849. This was the first well-known mechanical
pressure measurement device, which is still used today.
The Bourdon tube was then combined with a mercury switch,
facilitating the creation of one of the first pressure switches.
From this originated the basic concept of all electromechanical
pressure switches, which use a sensing element like the Bourdon
tube and a switch.
While the Bourdon tube pressure switch was a revolutionary
invention, it was not without its drawbacks. Due to the tube
being a tracing-type sensing element, they experience shorter
service life. Additionally, the design did not perform well in
applications with pump ripple or surge pressure, vibration, or
ambient temperature changes. These influences could be lessened
by using a higher quality tube; however, they are expensive to
manufacture. This led others to seek out a better pressure
In 1956, Roy Dunlap became aware of some oil workers that needed
an accurate pressure switch to prevent their oil tanks from
overflowing. For help, Roy contacted Ben Brown, a physics
professor at the University of Kansas, and together they created
the Static "O" Ring® pressure switch. The
design‘s sensing element used a force balanced
piston-actuated assembly sealed by a flexible diaphragm and a
static o-ring. Fluid pressure against the diaphragm counteracts
the force of the range spring, moving the piston shaft only a
few thousandths of an inch to directly actuate the electrical
snap-action switching mechanism. Their simple design worked
flawlessly, and because the o-ring was static, wear and tear
were virtually eliminated. Roy started manufacturing and selling
Static "O" Ring® pressure switches and changed the
company name to Static "O" Ring®, which later became
Mechanical pressure switches were the only type available until
1930 when engineers began experimenting with transduction
mechanisms with sensing device movements as part of an
electrical quantity. These were the first pressure transducers.
Then, in 1938, engineers at the Massachusetts Institute of
Technology and California Institute of Technology independently
developed bonded strain gauges. They raced to the finish line,
and E.E. Simmons of Caltech was the first to apply for a patent.
The development of strain gauges was an important step in the
right direction for solid-state pressure switches, which were
widely introduced in 1980 by Barksdale Inc. At the time, they
featured a bonded strain gauge sensor combined with a triac
Today, solid-state pressure sensors are very popular and feature
digital displays, digital and analog outputs, full
programmability, and between one and upwards of four switch
Although the enhanced features of electronic pressure switches
might lead one to believe that mechanical pressure switches
serve no purpose, this could not be further from the truth.
Electronic pressure switches require a power source to operate,
and if power to the device is lost, it will no longer actuate
when fluid pressure reaches the setpoint. This could result in
monetary damages or harm to human life. Electromechanical
pressure switches do not require power to operate and are simply
acting as a pair of contacts to make or break a circuit.
Many industries use pressure switches as a redundant safety
measure. If power is lost in the primary instrument, such as a
pressure transmitter, the pressure switch is a backup to actuate
when the setpoint is reached. Their lower instrument cost and
lack of power supply give mechanical pressure switches a much
lower cost of ownership than their electronic counterparts and
are one of the many reasons they are still in use today.
A pressure switch is a type of switch activated by the
pressure of the process fluid upon reaching a certain
threshold or set point. A pressure switch can have a bourdon
tube, piston, diaphragm, or membrane that moves or deforms
according to the amount of pressure exerted by the system.
There are two main types of pressure switches: mechanical
pressure and electronic pressure switches. A mechanical
pressure switch has a mechanical pressure-sensing part that
deforms according to the fluid pressure.
Electronic pressure switches are solid-state switches that do
not require actuation from the pressure-sensing element to
operate the switch. They operate indirectly by using other
properties, such as resistance and capacitance.
There are two main functions of a pressure switch. One is to
maintain the pressure or reservoir levels of the system. The
other is to protect equipment from damage or from running at
Leading Manufacturers and Suppliers
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