from a specified value. A permanent magnet in the
differential. Bring the pressure to the value at which the
switch provides a positive snap when the contacts are
circuit is to be closed or opened. Turn the range
opened or closed. This snap action prevents excessive
adjustment screw slowly clockwise or counterclockwise
arcing at the contacts. The pressure at which the switch
until the contacts operate. This adjustment will set the
operates is adjustable within ranges, such as 0-15,
open or closing pressure.
15-50, and 50-100. This adjustment is made by using the
You have now brought the pressure to the point
range adjustment screw.
where the circuit will be activated. Since the differential
adjustment screw is now set at minimum, the circuit will
probably operate at a lower pressure than desired.
T h e r e f o r e , turn the differential adjustment screw
Normal maintenance performed on pressure
clockwise to widen the differential until the desired
switches is accomplished by using the PMS. This
activating pressure is obtained. Refer to the
maintenance includes inspecting the switch for damage,
manufacturer's technical manual for detailed
corrosion, and proper operation.
instructions on adjusting these switches.
When you inspect a pressure switch, look for any
physical external damage, such as loose-fitting covers,
d a m a g e d pressure and electrical connectors, and
missing parts. Check inside the metal case for corrosion
and the presence of water or oil. If water or oil is present,
After adjusting the operating range of pressure
check the condition of the rubber gasket in the cover and
switches, check the operation through at least one
replace it if necessary.
complete cycle. If you find a variation from the desired
During PMS, sometimes you must adjust the switch
operating values, go through the entire adjustment
to bring the operating values back in tolerance. To set a
procedure again. In an emergency, you can use the
pressure switch, you first have to set a known pressure
pressure of the system in which the switch is installed
in the working range of the switch. Normally, you do
instead of using the test setup. In this case, however, you
this by using a test setup, such as the one shown in figure
should reset the switch as soon as possible using the test
6-2. To set the operating range of the switch, turn the
setup. After adjusting the switch, watch the operation
differential adjustment screw (see fig. 6-1)
through another complete cycle. If you cannot adjust the
c o u n t e r c l o c k w i s e against the stop for minimum
switch, you must troubleshoot it.
Figure 6-2.--Setup for calibrating a pressure device.