case that has a removable cover, an electrical
connection, and a pressure-sensing connection.
The switch contains a seamless metallic bellows
located in its housing. Changes in the measured
pressure causes the bellows to work against an
adjustable spring. This spring determines the
pressure required to actuate the switch. Through
suitable linkage, the spring causes the contacts to
open or close the electrical circuit automatically
when the operating pressure falls below or rises
above a specified value. A permanent magnet in
the switch mechanism provides a positive snap on
both the opening and closing of the contacts. The
switch is constantly energized. However, it is the
closing of the contacts that energizes the entire
electrical circuit.
Another pressure switch is an electric-
hydraulic assembly that is used for shutting off
the pumps motor whenever the system pressure
exceeds a pre-determined maximum value (fig.
8-10). The switch is mounted on the pump housing
so that the formers low pressure ports drain
directly into the pump housing.
This pressure switch principally consists of a
flange-mounted hydraulic valve to which is fixed
a normally closed electrical limit switch.
The valve consists of two hydraulically
interconnected components, the pilot valve sub-
assembly, which bolts on the bottom of the
body (l), functions to sense system pressure
continuously and initiates pressure switch action
whenever this pressure exceeds the adjusted setting
of the pilot adjustment. System pressure is
directed into the bottom port and is applied
against the exposed tip of the pilot piston (5). This
piston is held on its seat by compression from the
piston spring (6) which is dependent on the
position of the adjusting screw (8). Whenever the
pressure causes a force sufficiently large enough
to raise the pilot piston from its seat, fluid
flows through an interconnecting passage to the
actuating piston (2) chamber. The accompanying
fluid force raises the actuating piston against the
force of spring 3 and causes depression of
the extended switch plunger. This, in turn,
disconnects the contained electrical switch, which
may be connected into the pump motors electric
supply system.
Pressure switches come in many sizes and
configurations depending on how they will be
used.
Figure 8-10.Electric-hydraulic pressure switch.
TEMPERATURE-MEASURING
INSTRUMENTS
Temperature is the degree of hotness or
coldness of a substance measured on a definite
scale. Temperature is measured when a measuring
instrument, such as a thermometer, is brought
into contact with the medium being measured.
All temperature-measuring instruments use
some change in a material to indicate temperature.
Some of the effects that are used to indicate
temperature are changes in physical properties and
altered physical dimensions. One of the more
important physical properties used in temperature-
measuring instruments is the change in the length
of a material in the form of expansion and
contraction.
Consider the uniform homogeneous bar
illustrated in figure 8-11. If the bar has a given
8-6