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Figure 4-43.—Typical forced warm-air control system | Up
Utilitiesman Basic Volume 02 - Manual for electric, plumbing, water and other utilities | Next
Figure 4-46.—Various types of bimetallic strips |
bimetallic strip to open and close electrical contacts.
This actuating device is made by welding together two
pieces of dissimilar metals, such as brass and Invar, as
shown in figure 4-45, view A. Below a certain
predetermined temperature, this strip does not deflect
or bend. However, when the strip is heated, it bends in
the direction of the metal that expands the least, as
shown in figure 4-45, view B.
Actually, this electrical switch is constructed, as
shown in figure 4-45, view C, by welding two electrical
connections and contacts to the strip. A switch of this type
can then be used to control electrical circuits, because the
bimetallic strip responds to temperature changes. This is
a basic example of how this principle of bimetallic strip
operation is used in many temperature-responsive
automatic units. Other control switches contain
bimetallic strips that are spiral, U-shaped, Q-shaped, or
even in the shape of a helix, as shown in figure 4-46.
Vapor-Tension Device.—The
vapor-tension
principle is also used to actuate some types of
automatic control units. This is a common type of
temperature-measuring device in which the effects of
temperature changes are transmitted into motion by a
highly volatile liquid. The most used vapor-tension
device is the simple compressible bellows, as shown in
figure 4-47, view A.
The bellows is made of brass. It is partially-filled
with alcohol, ether, or other volatile liquid not
Figure 4-45.—Bimetallic strips: A. Typical strip; B. Expansion of the strip; C. With electrical switch.
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