Deadband is a region of pressure where a change in pressure produces no change in measurement output or control signal.
Many types of pressure sensing devices have a region slightly above and below zero pressure where the output does not vary. For example a pressure sensing diaphragm is considered to be at rest when pressure is equal on both sides of a diaphragm, which is the case when venting a gauge reference or differential pressure measurement instrument. If the pressure is increased or decreased, the measurement output will not respond until the mechanical slackness of the diaphragm assembly has been removed by the increasing pressure difference. The threshold of positive and negative pressure around zero where no change in output is detected is called the dead band. The dead-band is what is responsible for the dog-leg shaped characteristic that is noticed in the measurement output when the pressure changes from positive to negative and vice versa, as it passes through the null point of the diaphragm.
Another example of a pressure dead-band is how the hysteresis of a pressure switch is used to create a process control deadband. A basic mechanical pressure switch opens and closes at different pressure points. Take a pressure switch which is set to close when the pressure is increased to 3 bar pressure. When the pressure is reduced the switch re-opens at 2.7 bar. The pressure difference of 0.3 bar between the opening and closing of the switch is called the deadband and this is caused by the inherent pressure hysteresis of the switch technology.
The deadband produced by a pressure switch is a very important to its operation, since it provides a way of stabilising control of a process without the need for additional dampening filters. Electronic pressure switches which utilise pressure sensing technology that have much smaller hysteresis will include the ability to adjust the open/close deadband via the electronics.
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