Answer / aftab ahmed mazari

Differential pressure switch has two sensing ports. Hi and
Lo, Suppose set point of differential pressure switch is
30 PSI. Connect the pressure source at hi side of switch.
Leave Lo side (No any connection or any supply to low side)
Connect multimeter at switch terminals. Apply pressure at
high side. When pressure will reach 30 PSI, Switch will
operate and will change its contacts and also will be
indication at meter.
Suppose switch Low side is faulty, then apply 30 PSI at
High side and also apply low side pressure (5 to 10 PSI),
switch contacts should change, which indicate switch is
working.
aftabmazari@yahoo.com

Answer / suresh m (fcri)

In most pressure switches, the set-point is adjustable.
During calibration, pressure is applied slowly near the set-
point, and usually a continuous reading Reference sensor
(or a calibrated dial-gauge)is used. During increasing
pressure, when set-point is reached (or crossed), if the
switch does not show contact change, then the knob (or
spring / bolt) is adjusted for correct action.
The test is repeated to confirm the switching of contacts
at desired pressure.
The pressure is raised slowly to a value above the set-
point during the test even as we observe switch-over
pressure. Later the switch change-over back to initial
state is also confirmed as we lower the pressure below the
set-point pressure.

Positioner usage.

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