Page 19 - Instrumentation and Measurement
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Figure 2.9. Filled bulb thermometer.
The filling or transmitting medium is a vapor, a gas, mercury, or another liquid. The liquid-filled
system is the most common because it requires a bulb with the smallest volume or permits a smaller
instrument to be used.
2.3.1.3 Bimetallic Thermometers
The principle of operation of bi-metallic devices is that of differential expansion of two different
materials rigidly joined together. Ivar (36%Ni, 64%Fe) has a low coefficient of expansion and when
welded to a Ni-Mo alloy gives a good bimetallic strip. Figure 2.10 illustrates a typical design of the
Bimetallic Thermometers, the helix coils or uncoils with temperature variation and as one end is
fixed the movement rotates shaft and pointer. The range of the instrument is fixed by the material
used.
Figure 2.10. Bimetallic dial thermometer.
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