Abstract
Vapor pressures have been used for a long time for temperature measurements or for calibrating thermometers against a physical property, since the saturated vapor pressure of a pure substance above its liquid phase depends only on temperature. The physical basis of vapor-pressure thermometry has already been discussed in Sect. 2.1 (see Fig. 2.1). Vapor pressures are very commonly used as well for the realization of the fixed points called “boiling points”. These fixed points are simply specific points on the vapor pressure line, generally those at 101 325 Pa (normal boiling point). They do not deserve special attention, as they differ in no way from any other point of the vapor pressure line, and are often simply the highest point attained by the experimenter.
The pressure fixed points based on triple points, treated in Chapter 9, are another application of a specific point of the vapor pressure line.
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Notes
- 1.
At least for experiments in the earth’s gravitational field.
- 2.
This always occurs when working, as usual, with a fixed total amount of substance in the thermometer : n = n L + n V = const, so that when pressure increases—in the same volume —n L decreases and n V increases.
- 3.
When solid nitrogen is used also as a refrigerant , allowing a minimum temperature of 45–50 K.
- 4.
It can include only a dew point , which is only a slope discontinuity (see Sect. 3.3).
- 5.
Most of the studies having been carried out using 4He , this phenomenon will be discussed in the next subsection.
- 6.
On the contrary, the 1929, 1932, and 1937 scales were empirical scales.
- 7.
Below the first 4 mm from the surface , the hydrostatic pressure gradient in 4He is of 0.14 mK cm−1 at 4.2 K, of 0.27 mK cm−1 at 4.2 K, and of 1 mK cm−1 at 2.2 K.
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Pavese, F., Molinar Min Beciet, G. (2013). Vapor-Pressure Thermometry. In: Modern Gas-Based Temperature and Pressure Measurements. International Cryogenics Monograph Series. Springer, Boston, MA. https://doi.org/10.1007/978-1-4419-8282-7_4
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DOI: https://doi.org/10.1007/978-1-4419-8282-7_4
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