applied to the circuit, never clean the contacts or apply
lubricants to the contact surfaces.
PRESSURE AND TEMPERATURE
SWITCHES
Pressure and temperature controls have been
grouped together because the switching mechanism is
the same for both controls; the difference is in the
operation.
Figure 2-37.--A float switch mechanism.
Pressure-controlled switches (fig. 2-38) are
operated by changes in pressure in an enclosure such as
(fig. 2-37). Collars C and D on the guide rod are held
a tank. On the other hand, temperature-controlled
in position by setscrews so that their positions can be
switches operate from changes in temperature that take
changed to set the operating levels to the desired
place in an enclosure or the air surrounding the
positions.
temperature-sensing element. Actually, both switches
As the float goes up and down, corresponding to the
are operated by changes in pressure. The temperature
liquid level, it does not move the operating red, E, until
element is arranged so that changes in temperature cause
contact is made with either collar. When the float comes
a change in the internal pressure of a sealed-gas or
in contact with either collar, the external operating arm
air-filled bulb or helix, which is connected to the
of the switch is moved and the switch is operated.
actuating device by a small tube or pipe. Temperature
changes cause a change in the volume of the sealed-in
Although the switch assembly is of rugged
gas, which causes movement of a diaphragm. The
construction, it must be checked regularly for proper
movement is transmitted by a plunger to the switch arm.
performance.
The moving contact is on the arm. A fixed contact may
be arranged so that the switch will open or close on a
temperature rise.
Maintenance
When the switch is used to control pressure, the
temperature element is replaced by a tube that leads to
You should ensure that the switch contacts are kept
the pressure tank. The pressure inside the tank then
in good electrical condition. Determine the kind of
operates the switch mechanism.
metal used for the contacts, whether copper or silver,
and apply the maintenance procedures outlined in Naval
Pressure or temperature controls may be used as a
Ship's technical Manual, chapter 300. While power is
pilot device (fig. 2-39). The circuit operation is exactly
Figure 2-38.--A representative pilot device control circuit.
2-32
