Helium gas thermometer

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). 

Between the fixed points, temperatures on the ITS-90 are obtained by interpolation using standard instruments and assigned formulae. These standard instruments are the helium gas thermometer (3 K to 24.5 K), the platinum resistance thermometer (13.8 K to 1235 K), and the optical thermometer (above 1235 K). 

The establishment of the International Temperature Scale has been accomplished largely with the aid of measurements made with the helium gas thermometer. The most precise gas thermometry method is the constant-volume method, in which a definite quantity of the gas is confined in a bulb of constant volume Eat the temperature T to be determined and the pressure p of the gas is measured. A problem is encountered however in measuring the pressure a way must be found to communicate between the bulb and the pressure gauge. This is usually accomplished by connecting the bulb to the room-temperature part of the system by a slender tube and allowing a portion of the gas to occupy a relatively small, constant dead-space volume at room temperature. Thus, it is important that the gas volume in the pressure manometer be as small as possible. 

The size of the kelvin, the SI temperature unit with symbol K, is defined by the statement that the triple point of pure water is exactly 273.16 K. The practical usefulness of the thermodynamic scale suffers from the lack of convenient instruments with which to measure absolute temperatures routinely to high precision. Absolute temperatures can be measured over a wide range with the helium-gas thermometer (appropriate corrections being made for gas imperfections), but the apparatus is much too complex and the procedure much too cumbersome to be practical for routine use. 

Range 3.0 to 24.5561 K. Over this range, Tgo is defined in terms of a constant-volume helium-gas thermometer. If " He is used above 4.2 K, the basic equation is... 

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. 

STATIC AND DYNAMIC BEHAVIOR OF HELIUM-GAS THERMOMETERS BELOW IT K ... 

Static and Dynamic Behavior of Helium-Gas Thermometers Below 77° K... 

This paper will outline a variation of the highly predictable helium gas thermometer and its practical adaptation as a simplified and predictable interface sensor. 

The fixed points in the lTS-90 are given in Tabie 11.39. Platinum resistance thermometers are recommended for use between 14 K and 1235 K (the freezing point of silver), calibrated against the fixed points. Below 14 K either the vapor pressure of helium or a constant-volume gas thermometer is to be used. Above 1235 K radiometry is to be used in conjunction with the Planck radiation law,... 

Fig. 1. Gas thermometer A, helium gas B, mercury P, plunger for adjusting mercury column and P, pressure, where in (a), bulb is surrounded by water...

Gas thermometers that employ equation (1.10) can be constructed to measure either pressure while holding the volume constant (the most common procedure) or volume while holding the pressure constant. The (pV) product can be extrapolated to zero p. but this is an involved procedure. More often, an equation of state or experimental gas imperfection data are used to correct to ideal behavior. Helium is the usual choice of gas for a gas thermometer, since gas imperfection is small, although other gases such as hydrogen have also been used. In any event, measurement of absolute temperature with a gas thermometer is a difficult procedure. Instead, temperatures are usually referred to a secondary scale known as the International Temperature Scale or ITS-90. 

The tightness of the two parts was assured by an indium gasket, and the vacuum chamber was immersed in liquid helium. A small volume (about 1 cm3) of helium gas was inserted into the vacuum chamber for the thermalization at 4.2 K of all the parts of the experiment. The exchange gas was successively pumped out, and the pressure during the measurements was kept below 10 5 torr to reduce any contribution from convection. Each end of the sample strip was tightened between two copper blocks (C, D of Fig. 11.3) by means of brass screws one copper block was anchored to the Cu top (A) and the other (the hot end ) held the heater (FI) and the thermometer (TH). 

Gas Thermometers. These are expansion thermometers that depend on the coefficient of thermal expansion. They use, for example, helium gas and have helped to establish the thermodynamic temperature scale, and also for measurements at very low temperatures. 

The gas thermometer bulb is first filled with the gas to be used, preferably helium or nitrogen (free of water contamination). With the stopcock Xopen, the entire system is evacuated and then filled to the desired pressure with the gas. If complete evacuation is not possible (as is the case when a laboratory vacuum line is used), the filling must be done... 

The lower limit to the utility of the gas thermometer is determined by the condensation of the gas. The hydrogen thermometer, however, and better still the helium thermometer, may be used down to exceedingly low temperatures. Theoretically, there is no upper limit to the applicability of the gas thermometer, but in practice the difiiculty of finding a material of which to make the containing vessel, which will not become permeable to gases at high temperatures, makes it scarcely possible to measure temperatures above 1600° t in this way. 

In this study, the heat capacities of Ti, Zr, and Hf were measured using a vacuum calorimeter. The zirconium sample was placed in a copper capsule that was fitted with an external heater winding and a re-entrant thermometer as well as a small amount of helium gas to ensure heat transfer. The temperature was measured with a calibrated platinum resistance thermometer and measurements were taken from 20 to 200 K. The purity of the Zr was 99.5%, the major impurity being Na. 

The pump, shown in Figs. 1 and 2, is made up of four basic components (1) one dense helium-cooled cold plate (2) two interconnected, liquid-nitrogen-cooled chevron panels (3) one gas thermometer for display of helium panel discharge gas temperature and (4) one feedthrough plate, an external bulkhead, providing connections to the helium and liquid nitrogen circuits as well as to the gas thermometer. 

Temperature Control. Liquid nitrogen was supplied to the chevron panels and helium transfer line jackets from self-pressurized dewars. Cold helium gas was circulated in a closed loop. Temperatures at the helium cold plate were measured at two points by means of helium-filled gas thermometers equipped with a small cold-gas bulb and a large warm-gas bulb, giving an almost linear pressure vs. temperature curve in the 4.2 to 30 K temperature zone. The thermometers were calibrated at 4.2 , 20.4°, and 77.8°K. Liquid-nitrogen temperatures were not measured, but adequate provisions were made to keep the circuits flooded. Helium temperature to the cryopump was controlled by means of in-line electric heaters. 

The fundamental method is gas thermometry, a method most commonly carried out with a constant-volume gas thermometer. This device consists of a bulb or vessel containing a thermometric gas and a means of measuring the pressure of this gas. The thermometric gas is usually helium, because it has minimal deviations from ideal-gas behavior. 

Figure 2.9. The structure of a gas thermometer shown schematically. A container (a) with helium He(g) is connected to a manometer (b). At (c) the liquid column is kept at a constant level. When in equilibrium the gas pressure is determined by the head of pressure (c)-(d).

When hydrogen H2(g) or helium He(g) is used at low pressure, the ideal gas state can be approximated with close accuracy. For a number of years the gas thermometer was the basic instrument for fundamental measurements in the thermodynamic temperature scale. 

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