BIMETALLIC SPRINGS
Bimetallic thermometers make use of two fundamental principles: (1) metals change volume with temperature, and (2) this coefficient of change is not the same for all metals (see Figure 4.2a). If two different straight metal strips are bonded together and heated, the resultant strip will bend toward the side of the metal with the lower expansion rate. Deflection is proportional to the square of the length and the
temperature change and inversely proportional to the thickness. A bimetallic spring (Figure 4.2b) can be calibrated to produce a predictable deflection at a preset temperature. This is the basis of operation of the many bimetallic temperature switches in household appliances. The motion produced by a bimetallic spring is small; to amplify it in a reasonably sized space the bimetal strip may be wound in the form of a spiral or a helix. A classic example of an ambient air temperature thermometer is shown in Figure 4.2c. The outside edge of the spiral is pinned to the frame and a pointer is connected to the center. As the temperature increases, the spiral winds up deflecting the pointer clockwise. Knowing the coefficients of expansion of the two metals, their thickness, and the desired scale length and range, the total length of the spiral can be computed. A favorite combination of metals is low-expanding Invar (64% Fe, 36% Ni) against high-expanding nickel-iron alloy with chromium or manganese added. Thermometers Most industrial or residential bimetal thermometers use a helical coil which can be designed to fit into a stem more easily
than the spiral. The element is surrounded by a protecting tube or thermowell (see Section 4.14). The device can be mounted to measure the temperature of the gas or liquid inside a duct. The design is frequently used on domestic furnaces and over the years has replaced most of the glass stem thermometers
used earlier. Figure 4.2d illustrates a typical bimetallic thermometer using a helical coil. Also shown is a protective thermowell, which allows the removal or replacement of the thermometer without opening up the process tank or piping. When only periodic checks are needed on a particular temperature point, a thermowell only is installed and is protected from dirt accumulation by a cap. A single helix moves axially as it winds or unwinds with heat and cold.