WHY SPECIFY INCREASING
OR DECREASING PRESSURE:
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
Is this 35 psig on decreasing (the system starts operating below 35
These may be two
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