Fluid-expansion thermometers

Changes in dimension. With such devices, a change in physical dimension occurs with a change in temperature. In this category are liquid-in-glass or other fluid-expansion thermometers, bimetallic strips, and others. 

Fahrenheit could not make a thermometer of the same sort, despite repeated attempts. The type of thermometer that he was trying to make used water, open to the atmosphere, as the fluid of expansion. His original thermometers were thus sensitive to air pressure, and acted as both barometers and thermometers at the same time. 

The most common method of temperature measurement is with a liquid-in-glass thermometer. This method depends on the expansion of fluids when they are... 

All thermometers, regardless of fluid, read the same at zero and 100 if they are calibrated by the method described, but at other points the readings do not usually correspond, because fluids vary in their expansion characteristics. An arbitrary choice could be made, and for many purposes this would be entirely satisfactory. However, as will be shown, the temperature scale of the SI system, with its kelvin unit, symbol K, is based on the ideal gas as thermometric fluid. Since the definition of this scale depends on the properties of gases, detailed discussion of it is delayed until Chap. 3. We note, however, that this is an absolute scale, and depends on the concept of a lower limit of temperature. 

These are mostly used to heat round-bottom flasks at temperatures above 100°C. The oil bath, containing the heating fluid, is usually a non-ferrous metal or Pyrex dish and heated on a stirrer hot plate, and the temperature of the bath is measured with a thermometer. The oil bath should never be more than half-full, to allow a margin of safety for thermal expansion of the oil, and stirred with a magnetic flea to ensure even heating. The equipment used in a typical oil bath is shown in Fig. 5.12b. 

The three most important types of thermometers are expansion-type thermometers (pressure thermometers), electrical thermometers, and radiation thermometers. In expansion-type thermometers the primary sensing element is a bulb containing an expansible fluid. The bulb is connected to a pressure spring through capillary tubing. Expansion of the thermometric fluid with rising temperature causes expansion of the pressure spring, which in turn is converted to a mechanical displacement as the final measure of temperature. The response of these thermometers... 

Solution 5.1b. In the case of placing an organic fluid in the cylindrical body of the thermometer immersed in the water, two factors affect the pressure gauge reading the expansion of the air as well as the increased vapor pressure of the organic fluid. The vapor pressure of the fluid is calculated according to Antoine s equation, while the overall pressure is determined from the ideal gas law. Amontons law is inapplicable because the total number of moles of gas in the vapor phase of the cylinder will increase. (We shall also assume that the change in volume of the liquid is inconsequential.)... 

In 1755 Cullen read to the Edinburgh Philosophical Society an essay Of the Cold produced by evaporating Fluids, and of some other means of producing Cold . In this he describes the lowering of temperature produced by the evaporation of a volatile liquid under the receiver of an air pump, and by the sudden (adiabatic) expansion of air. He froze water in a vessel in which was immersed another vessel containing nitrous ether evaporating rapidly in an exhausted receiver, using an air thermometer in these experiments. In this work, Cullen was assisted by Matthew Dobson of Liverpool and afterwards of Bathi ... 

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