Precision Sensors
 
Glossary

WHY SPECIFY INCREASING OR DECREASING PRESSURE:

The "increasing/decreasing pressure" notation is important to help us specify the switch because of the deadband of the switch. The deadband of the switch is a result of hysteresis in the switching mechanism. See example below::

I need to know when the pressure in the line reaches 35 psig.

Is this 35 psig on increasing (the system starts operating above 35 psig)?
OR
Is this 35 psig on decreasing (the system starts operating below 35 psig)?

These may be two different things:

Switch "A": In this example, the normal process is at 50 psig, however at 35 psig the process won't work. In this instance, the switch would want to be set to indicate that the process pressure has decreased below 35 psig; and the switch would be set to actuate on increasing pressure at 39 psig maximum and deactuate on decreasing pressure 35±1 psig. In this scenario, when pressure increases the switch is indicating things are OK, when pressure decreases it indicates the problem condition.

Switch "B": The opposite condition would be a process that normally runs at 10 psig but has a problem if the pressure gets above 35 psig. In this case, the switch would be set to actuate on increasing pressure at 35±1 psig and deactuate on decreasing pressure at 31 psig minimum. In this case the switch would say things are OK until the process goes too high.

As you can see at this pressure there is about a 4 psig deadband that needs to be considered. If switch "A" was used in the switch "B" example the pressure could achieve 39 psig before the switch actuated, this may be a problem for the application.

Typically the specifying engineer will know if they are worried about losing pressure (decreasing pressure) or if they need to know that they have achieved pressure (increasing pressure). This information helps Precision Sensors to ensure the correct switch is specified for the application.

 
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