Figure 4-46.Various types of bimetallic strips.
corrosive to brass. When the temperature around the
bellows increases, the heat gasifies the liquid inside
and causes the bellows to extend. The extension closes
a set of electrical contacts, as shown in figure 4-47,
view B. When the bellows cools again, it contracts.
The contraction opens the electrical contacts.
Remote-Bulb Device.Liquid-filled devices are
not always limited to the simple bellows. There are
some remote-bulb devices that not only have a bellows
but also have a capillary tube and a liquid-filled bulb,
as shown in figure 4-48.
When the liquid in the bulb is heated, part of it
gasifies and forces its way through the capillary tube
into the bellows. This increased pressure inside the
bellows causes it to extend and open a set of electrical
contacts (or open or close a valve). When the bulb
cools, the gas liquefies and decreases pressure inside
the bell ows. This decreased pressure allows the
bellows to contract and close the electrical contacts.
Pressure-responsive devices are actuating
mechanisms installed in units, such as steam-pressure
controls, steam-pressure gauges, and pressure
actuating device uses bellows in a way similar to that of
the remote-bulb type. In this application, the bellows
extends and contracts in response to changes in steam
pressure. The action caused by movement of the
bellows opens or closes a set of electrical contacts.
B o u r d o n T u b e .Another
pres sure-responsive actuating device is found inside of
the pressure gauge shown in figure 4-49. In this
actuating device, the pressure is applied inside a
Figure 4-47.Tension devices: A. Vapor-tension device;
B. Pressure-tension device closing electrical contacts.