Pressure Expansion Thermometers

The typical pressure expansion thermometer is a volume of gas that maintains the same number of molecules throughout a test. Because the volume, number of molecules, and gas constant are all constant, any drop in temperature will subsequently cause a drop in pressure. Likewise, any rise in temperature will cause a rise in pressure. 

The reaction was effected in a suitable pressure reaction vessel having a capacity of about 6.2 liters and equipped with a thermometer, pressure gauge, expansion valve and an inlet tube by means of which the reactants could be introduced into the lower portion of said reaction vessel. 

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... 

With the same apparatus as in example (1), the absolute temperature of the gas in its initial state was Ti. The tap was then opened so that the gas rapidly expanded to atmospheric pressure, and the temperature, determined immediately after expansion by a platinum resistance thermometer, or a thermo-element, in the centre of the vessel, was T2. Show that ... 

None of these is far from = 0.003663, which is therefore commonly taken as the expansion coefficient for gases especially as ihc value for hydrogen, commonly used in the standard gas thermometer, is very near it. If the pressure as well as the volume is allowed to vary, the behavior of the ideal gas must be expressed by the Boyle-Charlcs law or the ideal gas law and the behavior of a real gas by one of the other equations of stale. See also Ideal Gas Law. 

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. 

An alternate technique to rejoin broken liquid columns is to expand the liquid into the contraction or expansion chamber by heat. Be careful to avoid filling the expansion chamber more than two-thirds full, as extra pressure may cause the top of the thermometer to burst. Never use an open flame to intentionally heat any part of a thermometer as the temperature from such a source is too great and generally uncontrollable. 

Liquid in Glass Thermometers. Mercury-in-glass thermometers (or better yet, mercury-in quartz) function well between -25°C and 360°C their typical precision is 0.1 K. They must be corrected for (1) relatively small pressure effects and (2) a relatively large "exposed-stem correction," due to the different coefficients of thermal expansion of mercury and glass in the part of the thermometer not immersed in the system being measured. 

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... 

It results from this law, as is taught in elementary classes, that the two coefficients of expansion, under constant pressure and constant volume, of a gas obeying this law have the same value, and this with whatever thermometer used besides, according to the observations of Charles and of Gay-Lussac, this value is the same for all gases which follow sensibly Mariotte s Law finally, if the thermometer chosen is one constructed with one of these gases, the value in question evidently does not depend upon the temperature. 

In a modification of the foregoing procedure (0. Lummer and E. Pringsheim, 1891), the stopcock is allowed to remain open after the adiabatic expansion, and the temperatures before (Pi) and immediately after (P2) expansion are measured by a sensitive thermometer. Since the corresponding pressures P and P% are known, y can be obtained from equation (10.6) in the form... 

The apparatus shown in Figure 2.23(b) consists of a long necked hard-glass flask to which a thermometer is fitted by means of a cork having a shallow vertical vent cut (as shown in the figure) to allow expansion of contents of the flask without pressure developing inside it. 

Figure 13-6 Apparatus for RESS A = SCF reservoir PV = high pressure volumetric pump HI, H2 = heat exchangers V, VI, V2, V3 = on-off valves S = saturation vessel EN = expansion nozzle E = expansion vessel F = flow meter G = rotameter PI = pressure indicator TI = thermometer.

During the first decade of the 20th century, the Dutch physicist Heike Kamerlingh Onnes established a cryogenics (low-temperature) laboratory in Leiden where, in 1908, he was the first to make liquid helium (4.2 K see chapter 4). By expansion of liquid helium he even achieved a temperature of 1.5 K. These extreme temperatures were measured by a helium-gas thermometer 145 mm pressure at 273 K (32°F or 0°C 3 mm at 4.25 K (-452°F or -269°C). It would take two more decades to achieve 0.1 K (see chapter 4). The Leiden laboratory attraaed world-famous physicists who visited and conducted experiments at temperatures never before experienced on the planet. 

In terms of the primary definition of temperature which we have for the time being adopted, Charles s law, which states that gas pressure is proportional to the absolute temperature, would be a tautologous statement. It is not necessarily so, and was not so in its historical setting, since other scales of temperature, notably that based upon the expansion of a mercury column by heat, are possible. The mercury thermometer and the gas thermometer provide scales... 

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