What is the meaning of On-Off differential for Pressure switches & Temperature switches?

How can select this differential for all switches whether Adjustable or fixed?

What is the difference between Adjustable & fixed differential in switches?

 

>What is the meaning of On-Off differential for Pressure switches & Temperature switches?

Differential is more often called 'deadband' to avoid confusion with 'differential pressure', but I hear both terms are used in conversation.

Deadband is the difference between the trip point/setpoint when the switch changes state and the reset point when the switch changes state back to 'normal'.

For example, with increasing pressure, the trip point/setpoint might be 100 units. A switch that trips at 100 units will stay switched until the pressure falls below 85 units. The deadband is 15 units: 100 - 85 = 15.

Deadband is inherent in the mechanical tolerances of a pressure or temperature switch and to some extent, is built in to prevent the switch for chattering when the applied process is at or near or setpoint.

>How can select this differential for all switches whether Adjustable or fixed?

Unless a switch is stated by the manufacturer to have adjustable deadband, the deadband is assumed to be fixed for that model.

Deadband is either published in the switch specifications or in a separate manufacturer publication.

Deadband is always specified as a range of pressures/temperatures, because of mechanical tolerances.

Deadband is almost always a 'narrow' deadband, with the reset point in some single digit or low two digit percentage of span, unless the switch is specifically designed for wide deadband operation, like for pump-up or pump-down operation. Just having adjustable deadband does not mean a switch's deadband can be set across its span range.

>What is the difference between Adjustable & fixed differential in switches?
1) Price. Adjustable deadband costs more
2) Availability.

Be wary of the limited rangeability for 'adjustable deadband'. Do not assume that adjustable deadband will cover the range you needed. Calculate it. See this blog for details:
http://tinyurl.com/75wl32k