What is a Pressure Switch?

A pressure switch is a control device that senses changes in pressure and mechanically opens or closes an electrical circuit at a predetermined pressure point. 

Ashcroft is an industry leader in pressure measurement instrumentation. This article provides a basic foundation for anyone who wants to understand pressure switches, including the different types, common applications, operating principles, key terminology, accuracy and activation methods.

Where are pressure switches used?

Pressure switches are used across many industries to control or monitor equipment when pressure reaches a specified point. Pressure switches are typically used with pumps, boilers, burners, power supplies and other equipment where a pressure change needs to initiate an electrical response.

Figure 1: Pressure Switch Industry Applications

How does a pressure switch work?

A pressure switch works by using process pressure to move an actuator against an opposing spring force until the pressure reaches the switch setpoint and causes the electrical switch to change state.

Here is how the process works:

1. Pressure from the process enters the switch through the process connection.
2. The pressure applies force to the actuator, also known as the diaphragm, causing it to move upward.
3. An opposing force created by the spring and adjusting nut resists the actuator's movement.
4. The force from the pressure source must become greater than the opposing spring force before the actuator assembly can move.
5. The adjusting nut controls the switch setpoint:
   • Raising the adjusting nut decreases the force in the spring, lowering the setpoint.
   • Lowering the adjusting nut increases the force in the spring, raising the setpoint.
6. When the process pressure overcomes the force of the spring, the actuator moves upward. This moves the push rod and causes the switch to change state.

Figure 2: Pressure Switch Operational Elements

What are the key elements of a pressure switch?

The key elements of a pressure switch include the switching element, pressure-sensing element and components that determine how the switch responds to pressure. Understanding these elements can help you determine which pressure switch is appropriate for an application.

1. The switching element

The switching element, also known as a microswitch, changes state when the push rod presses or releases the button on the microswitch. This causes the internal arm to move from one internal contact and connect with another. For more information, read Choosing the Right Microswitch for Your Application.

Figure 3: Switching Element – Normally Open and Normally Closed

2. A normally open (NO) contact

A normally open (NO) contact is open when pressure is below the setpoint and closes when pressure exceeds the setpoint.

For example, a pressure switch with a 10 psi setpoint will be open below 10 psi and close when pressure rises above 10 psi.

3. A normally closed (NC) contact

A normally closed (NC) contact operates in the opposite way: it is closed when pressure is below the setpoint and opens when pressure exceeds the setpoint.

For example, a pressure switch with a 10 psi setpoint will be closed below 10 psi and open when pressure rises above 10 psi.

These NO and NC operating principles apply to pressure switches. For a vacuum switch, which operates at pressure below atmospheric or barometric pressure, the operation is reversed.

What are the different microswitch configurations?

Pressure switches typically use one of two microswitch configurations: Single Pole Double Throw (SPDT) or Double Pole Double Throw (DPDT).

Single Pole Double Throw (SPDT)

An SPDT switching element has one normally open terminal, one normally closed terminal and one common terminal. It allows for one setpoint and can be wired to either a normally open or normally closed circuit.

Figure 4: Single Pole Double Throw (SPDT)

Double Pole Double Throw (DPDT)

A DPDT switching element has two separate contacts, each with:

• One normally open terminal
• One normally closed terminal
• One common terminal

These switches can be wired into two separate loads that activate at the same setpoint. Each circuit can be wired to either the normally open or normally closed contacts. 

To create a DPDT switching element, manufacturers use two SPDT switches mounted in a special bracket that allows both to actuate at the same setpoint.

Figure 5: Double Pole Double Throw (DPDT)

What is the pressure-sensing element in a pressure switch?

The pressure-sensing element moves or displaces in response to the pressure or vacuum being applied. Depending on the pressure range and application, the sensing element may be either a diaphragm or a piston.

How does a diaphragm work in a pressure switch?

A diaphragm responds to process pressure and transfers that force through the pressure switch mechanism. Because the diaphragm is exposed to the air, gas or liquid being measured, its material must be compatible with the application's process media.

Examples of diaphragm materials for different applications include:

• Buna-N or Viton™: Hydraulic, oil or gas applications
• Teflon™: Chemical applications
• Stainless steel: Steam, water or wastewater applications
• Monel®: NACE-related applications

Each material has a specific temperature and pressure range that it can withstand without damage.

Figure 6: Diaphragm Materials Temperature and Pressure Ranges

When is a piston used in a pressure switch?

Piston sensing elements are typically used for higher-pressure designs of 1,000 psi or greater. Pistons are usually made of stainless steel and incorporate an O-ring seal.

Both the stainless steel and O-ring materials should be checked for compatibility with the process media.

What are proof pressure and burst pressure?

Proof pressure and burst pressure define different pressure limits for a pressure switch and are important considerations when determining whether a switch can safely handle an application.

What is proof pressure?

Proof pressure is the maximum pressure that can be applied to a pressure switch without causing damage. It is determined under strict laboratory conditions, including a controlled rate of pressure change and temperature, so the value is for reference only.

Be sure to consult the factory when a switch must operate at pressures above its nominal range or reference temperature. Understanding a switch's proof pressure may also identify applications where the switch can safely withstand pressures beyond what might otherwise be assumed from its nominal range.

What is burst pressure?

Burst pressure is the maximum pressure that can be applied to a pressure switch without causing leakage or rupture. Typically, it is at least 400% of the switch range. Exposure to pressure greater than the specified burst pressure can permanently damage the switch.

What is the difference between differential and static pressure?

Differential pressure is the difference in pressure between two points in a system, while static pressure is the system's normal operating pressure. This distinction is particularly important in applications involving filters and strainers, where differential pressure can indicate when a filter needs to be cleaned or replaced.

As fluid passes through a filter, accumulated debris can cause a pressure drop from one side of the filter to the other. For example:

• 20 psi: Static pressure, representing normal pressure through the system on both sides of the filter.
• 5 psi: The desired setpoint representing a 5 psi pressure difference between the two sides of the filter.

A pressure switch can monitor pressure across the filter. If the downstream pressure drops to 15 psi while the upstream pressure remains at 20 psi, the resulting 5 psi differential reaches the setpoint. The switch then closes a circuit to alert the operator that the filter should be cleaned or replaced.

Figure 8: Monitoring Flow with Pressure Switches

What does pressure switch repeatability mean?

Repeatability describes a pressure switch's ability to repeatedly open and close at the same setpoint. It indicates how consistently the switch activates when the same pressure condition occurs.

For most Ashcroft switches, including B-Series products, accuracy is ±1% of span.

For example, a switch with a 100 psi range has an accuracy of ±1 psi. If its setpoint is 50 psi, the switch should close each time at a pressure between 49 and 51 psi.

What is deadband in a pressure switch?

Deadband, also known as hysteresis or differential, is the difference between the pressure at which a pressure switch trips and the pressure at which it resets. Because pressure switches are mechanical devices, the switch does not reset immediately when pressure moves back across the setpoint.

As pressure rises, the switch trips at its setpoint. When pressure begins to fall, it must move beyond the reset point before the switch changes back to its original state. The difference between these points is the switch's deadband.

For more information, see What Is Deadband on a Pressure Switch and How Does It Work?

Figure 9: Deadband Setpoint and Reset Point

It is important to note:

• On an increasing setpoint, the switch resets at a lower pressure.
• On a decreasing setpoint, the switch resets at a higher pressure.

The pressure range, diaphragm material and switching element all affect the deadband of a pressure switch. The combination of these variables determines how large or small the deadband will be.

What approvals can pressure switches require?

Pressure switches may require agency approvals depending on the application and operating environment. Because pressure switches are electrical devices and may be installed in explosion-proof, watertight or safety-related applications, relevant approvals can include:

• Electrical safety approvals
• Hazardous-location approvals
• Safety Integrity Level (SIL) certification

Where can you learn more about pressure switches?

Once you understand the basic operation and components of pressure switches, the next step is learning how switch characteristics affect product selection for a specific application. If you have additional questions about pressure switches, you can refer to the related resources below or contact us to speak with a product expert.