Sensors and Signal Conditioning, 2nd Edition

1.7   PRIMARY SENSORS

Primary sensors convert measurands from physical quantities to other forms. We classify primary sensors here according to the measurand. Devices that have direct electric output are plain sensors and are discussed in Chapters 2, 4, and 6. Radiation-based measurement methods are described in Chapter 9. Khazan [11] and Fraden [12] describe additional primary sensors.

Figure 1.13   A bimetal consists of two metals with dissimilar thermal expansion coefficients, which deforms when temperature changes. Dimensions and curvature have been exaggerated to better illustrate the working principle. (From E. O. Doebelin, Measurement Systems Application and Design, 4th ed., copyright 1990. Reprinted by permission of McGraw-Hill, New York.)

1.7.1 Temperature Sensors: Bimetals

A bimetal consists of two welded metal strips having different thermal expansion coefficients that are exposed to the same temperature. As temperature changes, the strip warps according to a uniform circular arc (Figure 1.13). If the metals have similar moduli of elasticity and thicknesses, the radius of curvature r, when changing from temperature T₁ to T₂, is [2]

where t is the total thickness of the piece and where α_A and α_B are the respective thermal expansion coefficients. Therefore the radius of ...