A pressure switch can help ensure safe, reliable measurements for an application, but only if you fully understand the physics of its mechanics and the various nuances such as the setpoint, reset point and deadband.
But what does deadband mean and how does it apply to a pressure switch?
This article will explain the meaning of deadband and how it works so you can help ensure continued accuracy for your application.
What is Deadband?
The term deadband in pressure switches (also referred to as hysteresis, differential or reset) is the physical mechanical properties that dictate the difference between the pressure at which the switch activates (the setpoint) and the pressure at which the switch deactivates (the reset point).
Pressure switches can have two different types of deadbands: fixed or adjustable. Switches that are fixed have a deadband value that is determined by the mechanical properties of the switch. Adjustable switches have deadbands that can be adjusted or selected within a specific range to meet a customer's specific application requirements.
Deadband parameters and/or limitations apply to all mechanical pressure switches in all applications. It is normally expressed in the units of the actuating variable or engineering unit of measure.
Figure 1: Switch Deadband Diagram
Since pressure switches are mechanical devices, the physics of the device do not allow the switch to reset as soon as the pressure drops below the setpoint. In the case of a switch with an increasing setpoint, as pressure rises, the switch trips at the setpoint. Then when the pressure drops below the setpoint, the switch does not reset immediately.
The illustration below (Figure 2) shows a pressure switch that utilizes a straight-line actuator assembly to mechanically open and close a microswitch. When the pressure entering the process connection exceeds the setpoint of the switch and the force of the spring pushing down on the push rod and diaphragm, the push rod moves up and mechanically closes the button on the microswitch changing the state of the microswitch.
Figure 2: Switch Cross Sectioned View of Switch Assembly
The difference in pressure from when the microswitch engages and disengages is known as the deadband. This is the actual travel of the plunger of the microswitch moving up and activating the microswitch and moving down and deactivating the microswitch.
Figure 3 below shows the example of pressure overcoming the spring force and activating the microswitch by pushing on the plunger. Then the pressure is reduced and the microswitch plunger is allowed to return to its original state, which causes the microswitch to deactivate.
Figure 3: Microswitch Activation
The below graph shows the example above for a switch with a particular diaphragm material/actuator and pressure range. The deadband of the switch is 3 psi, so for a 60 psi increasing setpoint, the pressure will have to drop to 57 psi in order to reset.
Figure 4: Deadband Example
Factors that Affect Deadband
It is important to note that:
- On an increasing setpoint, the switch will reset at a lower pressure point.
- On a decreasing setpoint, the switch will reset at a higher pressure point.
The force to move the plunger of the microswitch depends on many factors like the pressure range (spring force), diaphragm material stiffness, microswitch actual plunger travel distance and any other possible restriction like dual microswitch elements.
These are all variables that dictate the deadband of a switch. How big or small the deadband is dependent on these combined factors.
We don’t like to pressure you, but we have more information.
Now that you better understand deadband on a pressure switch and how it works, you can find the best solution for your application.
If you want to learn more about pressure switches, we have a video available on the basic calibration instruments for Ashcroft pressure switches.
You can also read our related articles about pressure switches:
- Choosing the Right Microswitch for your Application
- Class and Division Codes on Explosion-Proof Pressure Switches
- When Should You Use an Electronic Pressure Switch?
- Choosing a Pressure Switch: 9 Factors to Consider
- What is SIL Certification for Pressure Switches?
Ashcroft can help you find the right pressure switch for your process. Contact us today to talk to one of our industry experts and get your questions answered.
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