Vapor pressure thermometer

The international temperature scale is based upon the assignment of temperatures to a relatively small number of fixed points , conditions where three phases, or two phases at a specified pressure, are in equilibrium, and thus are required by the Gibbs phase rule to be at constant temperature. Different types of thermometers (for example, He vapor pressure thermometers, platinum resistance thermometers, platinum/rhodium thermocouples, blackbody radiators) and interpolation equations have been developed to reproduce temperatures between the fixed points and to generate temperature scales that are continuous through the intersections at the fixed points. 

Cryostats described elsewhere (14) were used between 123° and 273°K. Temperature control was within 0.1°C and temperature was read using appropriate vapor-pressure thermometers. For still lower temperatures liquid nitrogen and liquid oxygen were used. 

Fig. 9.11. The vapor-pressure thermometer. The manometer is filled as described in the caption to Fig. 7.1. The manometer is then evacuated and outgassed through a joint attached at point A. A pure compound is condensed into the probe, and the apparatus is sealed off under vacuum at point A. For thermometric compounds which boil below room temperature, care must be taken not to overfill the apparatus.

G. Temperature Determinations. In vacuum line work it is frequently necessary to measure low temperatures. While the mercury-in-glass thermometer is convenient, it does not extend below the freezing point of mercury, — 38.9°. Pentane-in-glass thermometers have a similar convenience and may be used down to about — 150°C, but they are only useful for rough work (an error of 5°C is common). For more precise determinations, a calibrated thermocouple or thermistor, or a vapor pressure thermometer, is useful. 

The vapor pressure thermometer consists of a manometer with one arm ex-... 

Apparatus. The gas-handling and vacuum systems used were standard volumetric equipment (5). Liquid nitrogen was used as a constant temperature bath around the calorimeter and the pilot sample its temperature was checked by means of an oxygen vapor pressure thermometer. 

Other Thermometric Devices. The vapor pressure of a pure liquid or solid is a physical property sensitive to temperature and thus suitable for use as a thermometer. The use of a liquid-nitrogen vapor-pressure thermometer is suggested for the range 64 to 78 K in Exp. 47. At very low temperatures (1 to 4.2 K), the vapor pressure of liquid helium can be used. 

From outward appearance vapor-pressure thermometers may be distinguished from liquid- or gas-filled instruments by the form of the tempeT + T cnoUc pressure thermometers have an the length of the intervals for ci, ... 

The action of the vapor-pressure thermometer depends upon the fact that the pressure inside the thermometer is determined solely by the temperature of the free surface of the liquid. It follows therefore that the thermometer must be so constructed that one free surface is always in the bulb. If this condition is fulfilled the reading of the instrument will not be sensibly affected by changes in the temperature of the gage and capillary. 

Fig. 20. Cryostat with electrical control h) bath liquid d) copper vaporizer f) down pipe h) siphon h) condenser 1) liquid nitrogen r) relay Sj) power supply of about 2 volts Sa) power supply of about 18 volts t) vapor pressure thermometer u) pressure head regulator (manostat) v) electromagnetic gas valve the valve plunger must be sufficiently heavy not to stick in its seat.

Nitrogen trifluoride is stored in glass flasks, in a gasometer over water, or under pressure in steel cylinders. It may be used as filling material for vapor pressure thermometers [W. Menzel a.nd F. Mohry, Z. anorg. allg. Chem. 210, 257 (1933)]. 

It may be noted that the above equation represents the singular design point of a full-range vapor-pressure thermometer. 

Control of the system is primarily manual with the exception of the pressure regulating valves and the temperature controller. Local or remote operation is possible. The operating temperature of the system is automatically controlled by an electric heater in the load return line. The controller supplies sufficient heat to maintain the helium temperature at the entrance to the heat exchanger at a given set point, which is generally 20 K. Temperature sensing is accomplished with a hydrogen vapor pressure thermometer. The amount of heat... 

Readout instruments to monitor operation of the system include pressure gauges, located at various points, an indicating pyrometer for the purifier, and temperature indication by vapor pressure thermometers at the inlet and exhaust of the expander. 

Vapor pressure thermometer (pressure measurement above a liquid in contact with the unknown system—particularly useful at low temperatures where other types of thermometers may not be applicable, see also Appendix 8 for the application of vapor pressure thermometers in e maintenance of the ITS 90). 

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. 

Table I. Probing of a Helium Bath at 4.2°K with a Vapor-Pressure Thermometer...

The difference between the pressure measured in the vapor-pressure bulb and the controlled surface vapor pressure is AP and 0.030 in. Hg corresponds to a temperature difference of one millidegree. The immersion depth of the vapor-pressure bulb in liquid helium is /i. At corresponding depths in the liquid helium, there is little difference in the pressures indicated by the vacuum-jacketed and non- jacketed vapor-pressure thermometers. These results indicate that the effect of a cold spot on the vapor pressure thermometer at 4.2 K is negligible. Just above the point, however, effects of cold spots have been observed. This type of bulb eliminates the effect. 

Vapor pressure thermometers constitute a special measuring device used mostly in low-temperature calorimetry. These instruments measure the vapor pressure of a known liquid substance - usually a liquefied gas - which is always associated with a given temperature. Owing to the close and definite relationship between temperature and the vapor pressure of a given substance (Figure 2.1), this method permits accurate measurements of temperature. 

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