Vapor pressure thermometry

Any substance which may be obtained pure and for which accurate vapor pressure data are available may be used for vapor pressure thermometry. Stock recommended a convenient series of compounds and, in collaboration with Hen-nig and others, he obtained accurate vapor pressure data for these compounds.15 These data were obtained on a temperature scale for which 0°C = 273.1K however, more recent data on several of the compounds show it would be difficult to improve on Stock s vapor pressure tables. When high accuracy is desired, the usual precautions involved in precise manometry (Chapter 7) must be observed. 

A descriptive flowchart has been prepared by Sparks (Materials at Low Temperatures, ASM, Metal Park, OH, 1983) to show the temperature range of cryogenic thermometers in general use today. Parese and Molinar (Modem Gas-Based Temperature and Pressure Measurements, Plenum, New York, 1992) provide details on gas- and vapor-pressure thermometry at these temperatures. 

The ITS-90 is defined over a very wide temperature range, from 0.65 K up to at least 1358 K. There is a specified procedure for each measurement of T90, depending on the range in which T falls vapor-pressure thermometry (0.65-5.0 K), gas thermometry (3.0-24.5561 K), platinum-resistance thermometry (13.8033-1234.93K), or optical pyrometry (above 1234.93 K). For vapor-pressure thermometry, the ITS-90 provides formulas for T90 in terms of the vapor pressure of the helium isotopes He and He. For the other methods, it assigns values of several fixed calibration temperatures. The fixed temperatures are achieved with the reproducible equilibrium systems listed in Table 2.3 on the next page. 

Vapor Pressure Thermometry. The pressure exerted by a saturated vapor in equilibrium with its liquid is a very definite function of temperature, and can be used to measure the temperature of the liquid. A number of useful fixed points for several cryogenic fluids are given in Table 8.3. With a good pressure-measuring device, the vapor pressure thermometer is an excellent secondary standard since its temperature response depends upon a physical property of a pure compound or element. Many expressions have been... 

Sample purity is very important in vapor pressure thermometry. With hydrogen, the ortho-para composition is the most important composition parameter. No common impurities are sufficiently soluble in liquid hydrogen or helium to affect the vapor pressure. Of the possible impurities in the vapors... 

Most thermometry using the KTTS direcdy requites a thermodynamic instmment for interpolation. The vapor pressure of an ideal gas is a thermodynamic function, and a common device for reali2ing the KTTS is the helium gas thermometer. The transfer function of this thermometer may be chosen as the change in pressure with change in temperature at constant volume, or the change in volume with change in temperature at constant pressure. It is easier to measure pressure accurately than volume thus, constant volume gas thermometry is the usual choice (see Pressure measurement). 

The ITS-90 has its lowest point at 0.65 K and extends upward without specified limit. A number of values assigned to fixed points differ from those of the immediately previous scale, IPTS-68. In addition, the standard platinum resistance thermometer (SPRC) is specified as the interpolation standard from 13.8033 K to 961.78°C, and the interpolation standard above 961.78°C is a radiation thermometer based on Planck s radiation law. Between 0.65 and 13.8033 K interpolation of the scale rehes upon vapor pressure and constant-volume gas thermometry. The standard thermocouple, which in previous scales had a range between the upper end of the SPRT range and the lower end of the radiation thermometer range, has been deleted. 

V = vapor pressure, G = gas thermometry, T = triple point (gas, liquid, and solid in equilibrium), MIF = melting/freezing at 1 atm. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

A special case of constant-volume gas thermometry is that based on the vapor pressure of a liquid. As the temperature of a solvent is increased, its vapor pressure increases proportionately over a wide range of conditions. The temperature range is Umited to temperatures between the boiling point and freezing point of the solvent and it must also be lower than the critical point of the fluid. These thermometers are easy to adapt to common experimental equipment. 

In the last few years a program of precision secondary thermometry in the 2-4 K range has been greatly facilitated by the use of metallic storage dewars which contain a few liters of liquid helium. The temperature distribution in these nearly "constant temperature" baths have been explored with both resistance and vapor-pressure thermometers. The results of these investigations, and also the reproducibilities of resistance thermometers, are presented. 

Vapor-presstue thermometry can only be based on He or " He, being the only non-sohd materials below 4 K. The pressure may be meastrred relatively easily by means of a manometer or McLeod gauge and then compared with international vapor pressme tables in order to find the... 

Vapor pressure and gas thermometry offer sensitive methods of temperature measurement with the advantage that no calibration is necessary. Further advantages are that these transducers are not sensitive to magnetic fields or electric fields. In the case of vapor pressure thermometers, the time response may be made comparable to the resistance thermometers. 

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