How a pressure switch works Animation. Mechanical pressure switch & Electronic pressure switch.

As the name indicates pressure switch is a switch that operates using pressure.

Pressure switch is a device that turns an electrical circuit connected to it ON or OFF when a preset pressure is reached on its input.

The switch may be designed to make contact either on pressure rise on pressure fall.

In the case of a light switch when they are switched on the switch makes contact closes the circuit and the light turns on.

When they are switched off the circuit is broken and the power turns off but in the case of a pressure switch instead of being manually switched the switch closes or opens the contact when the input pressure reaches above or below a preset value, this preset pressure is known as a set point of the pressure switch.

Output of the pressure switch is digital, that means output can have only two states that is on or off.

Parts of a Pressure Switch.

A pressure switch mainly consists of a Transducer which measures the pressure and an Electrical Switch. The transducer is the part that comes in contact with the process pressure.

Some switches have the switch terminals open when no pressure is connected which is called normally open switch (NO) , when enough pressure is applied the switch closes.

On the other hand some switches have the switch terminals closed when no pressure is connected which is called normally closed switch (NC) and when enough pressure is applied the switch opens.

All pressure switches have two operating points known as Set Point and Reset Point.

Set point is a pressure at which the switch activate and the reset point is the pressure at which the switch goes back to its normal position.

The variation in pressure do not change the status of the switch until the Set point is reached, when the set point is reached the switch changes its state from NO to NC or NC to NO and remains in the changed state until the pressure drops below the Reset point.

The difference in pressure value between the Set point and the reset point is known as Differential or Dead Band of a Pressure Switch.

In most switches differential can be adjusted and some switches have fixed and not adjustable Differential.

Types of pressure switch.

There are two main types of pressure switches,

1.Mechanical pressure switch

2.Electronic pressure switch.

Parts of a Mechanical pressure switch

Process Inlet Port:

The Inlet Port is the part connects the pressure switch into the process unit.

Pressure Sensing Element:

This is the main part of the mechanical pressure switch which mechanically deforms according to the fluid pressure and actuates the switch. Diaphragm or piston or bourdon tube is usually used as pressure sensing element.

Diaphragm piston assembly:

The Diaphragm along with the other sealing parts protects the internal parts of the switch from the process fluid.

The type of diaphragm material is selected based on the type of fluid and its temperature. Range spring, the range 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 on the fluid pressure exceeds the force applied by the spring.

Setpoint Adjustment Screw:

Setpoint Adjustment Screw is used to increase or decrease the activation pressure.

Switch Housing:

The switch housing protects the switch and other internal parts from the external environment.

Micro Switch:

The micro switch is used to make or break an electrical circuit when the pressure switch operates.

We have already discussed about it in one of our previous chapter.


This is where the circuit controlled by the pressure switch is connected

Working of a Mechanical pressure switch.

Consider that we have a gas container and we need a green light to switch on when the pressure inside the container is below 100 PSI and a red light to switch on when the pressured reaches 100 PSI.

For this we install a pressure switch on this container and adjust the set point of this pressure switch at 100 PSI by turning the setpoint adjustment screw.

Now we wire green light to normally closed in NC terminal of the switch and red light to the normally open NO terminal of the pressure switch. Power is connected to the common terminal of the switch.

The inlet pressure from the container is applied to the bottom of the operating piston during normal operation when the pressure inside the container is below 100 PSI the normally closed circuit will be powered and green light will be ON.

We had already adjusted the tension of the range spring using setpoint adjustment screw. so that it gets compressed when the inlet pressure reaches the set point.

So when the inlet pressure reaches 100psi the piston is forced upwards by the inlet pressure compressing the range spring and the piston will hit the trip button on the micro switch and change it over. Now the normally open contact will get power and the red light glows while the green light switches off.

if this pressure switch has a differential of 5 PSI then reset pressure will be 95 PSI which means that when the pressure reduces to 95 PSI then the switch will come back to its normal state that is green light will be on and red light will be switched off.

Electronic pressure switch

An Electronic pressure switch is a pressure sensing instrument which uses a pressure sensor and conditioning electronics which provides a switch output that is open or closed circuit using either a relay or NPN or PNP output.

Most electronic pressure switches have analogue capabilities this means they are not limited to open or closed position but are also capable of sending a continuous variable signal that is used for more accurate monitoring.

They often have a display from which the measured pressure can be read.

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