Linear Expansion Thermometers

Linear expansion is most commonly used in bimetal spiral thermometers, which use two metals with different coefficients of expansion (see Fig. 2.29). The two metals can be welded, soldered, or even riveted together. As the metals are heated, the metal with the greater expansion will cause the spiral to flex open or close depending on which side the metal with the greater coefficient of expansion is on. A reverse in temperature will cause a commensurate reversal in the flexing. 

FIGURE 2.12 Test unit for determination of coefficient linear expansion of RubCon (1) temperature chamber, (2) RubCon sample, (3, 4) steel plates, (5) pile, (6) indicator, (7) leg of indicator, (8) thermometer. (Reprinted from Yu. Potapov, O. Figovsky, Yu. Borisov, S. Pinaev, and D. Beilin, Joint Work of Reinforcement and Polymer Concrete Matrix, J. Scientific Israel Technological Advantages 4, nos. 3-4 (2002) 14-19. With permission.)... 

Thermostats can be controlled to 300°C by contactthermometers expansion regtilators are used up to 500°C. The latter are based on the difference in linear expansion between a quartz rod and an aluminum or iron tube. Alternatively, a contact thermometer may be located in a cooler part of the furnace, provided a fairly constant temperature difference can be maintained between the hot reaction area and the cooler measurement section. The frequently recommended arrangement consisting of an auxiliary furnace in series with the main furnace, whereby the contact thermometer is placed inside the smaller furnace, can only work if both furnaces are very well insulated and have equal heat losses. This is not easily achieved in practice. 

Bimetal thermometer A thermometer that uses two dissimilar bars of metals (with different rates of linear thermal expansion) riveted together. A variation in temperature produces a bending moment on the bar, which is magnified by a lever to record temperature on a dial. 

Modern temperature scale proposed by G. Fahrenheit, defined by a thermometer, a law and three fixed points. Fahrenheit s thermometer was a mercury-in-glass one. Thermal expansion versus temperature was assumed linear. Three fixed points were defined 0°F temperature of a mixture of water, ice and ammonium chloride 32°F temperature of melting ice 96°F temperature of human body... 

Liquid-in-glass thermometers used mercury or alcohol as the liquid that expands as it gets warmer. Most countries mandate the removal of any mercury-filled devices due to its extreme toxicity, but alcohol and other fillings are still used. The expansion rate is linear with temperature and can be accurately calibrated. Bimetallic thermometers bond two dissimilar metals with different coefficients of expansion to produce the bimetallic element in thermometers, temperature switches, and thermostats. Filled System Thermometers can be filled with either liquid or vapor. Liquid-filled units are the most popular although they require compensation for the weight of the liquid head and for capillary length. Vapor-filled elements cannot be used if the operating temperature crosses the vapor/liquid point. 

As stated earlier, temperature and pressure do not have large influences on the densities of solids and liquids. Nevertheless, the fact that mercury in a thermometer rises or falls with changing temperature shows that the effect of temperature on liquid density is measurable. Coefficients of linear and cubic (volume) thermal expansion of selected liquids and soiids are given as empirical polynomial functions of temperature on pp. 2-128 to 2-131 of Perry s Chemical Engineers Handbook. For example, the Handbook gives the dependence of the volume of mercury on temperature as... 

In 1848 William Thomson (1824-1907), later Lord Kelvin, proposed that gas thermometers (in which the expansion of gas is measured as the temperature is increased) be used as the ultimate standards against which other thermometers should be calibrated. This proposal was adopted in 1887, and in consequence any modem experiment to verify Charles s law by determining the volume of a certain quantity of gas at different temperatures is bound to reveal a linear relationship, as that is the basis of the calibration of the thermometer. It is interesting to note that Gay-Lussac himself used no thermometer he simply surrounded his vessels with melting ice and boiling water. 

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