Liquid thermometers physical properties

Determination of the wet-bulb temperature. Equation 13.8 gives the humidity of a gas in terms of its temperature, its wet-bulb temperature, and various physical properties of the gas and vapour. The wet-bulb temperature is normally determined as the temperature attained by the bulb of a thermometer which is covered with a piece of material which is maintained saturated with the liquid. The gas should be passed over the surface of the wet bulb at a high enough velocity (>5 m/s) (a) for the condition of the gas stream not to be affected appreciably by the evaporation of liquid, (b) for the heat transfer by convection to be large compared with that by radiation and conduction from the surroundings, and... 

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. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

In principle, any device that has one or more physical properties uniquely related to temperature in a reproducible way can be used as a thermometer. Such a device is usually classified as either a primary or secondary thermometer. If the relation between the temperature and the measured physical quantity is described by an exact physical law, the thermometer is referred to as a primary thermometer otherwise, it is called a secondary thermometer. Examples of primary thermometers include special low-pressure gas thermometers that behave according to the ideal gas law and some radiation-sensitive thermometers that are based upon the Planck radiation law. Resistance thermometers, thermocouples, and liquid-in-glass thermometers all belong to the category of secondary thermometers. Ideally, a primary thermometer is capable of measuring the thermodynamic temperature directly, whereas a secondary thermometer requires a calibration prior to use. Furthermore, even with an exact calibration at fixed points, temperatures measured by a secondary thermometer still do not quite match the thermodynamic temperature these readings are calculated from interpolation formulae, so there are differences between these readings and the true thermodynamic temperatures. Of course, the better the thermometer and its calibration, the smaller the deviation would be. 

TABLE 5.3 Physical Properties—Liquid-in-Class Thermometers (Holman, 2001)... 

In the field of cryogenics, as in many other phases of science and industry, the accurate measurement of temperature is a very critical matter. The measurement of temperature, however, is more difficult to accomplish than the measurement of many of the other physical properties of a substance. Unlike properties such as volume or length, temperature cannot be measured directly. It must be measured in terms of another property. Some of the physical properties that have been utilized include pressure of a gas, equilibrium pressure of a liquid with its vapor, electrical resistance, thermoelectric emf, magnetic susceptibility, volume of a liquid, length of a solid, refractive index, and velocity of sound in a gas. In addition, there are thermometers that respond to a temperature-dependent phenomenon rather than to a physical property. Included in this category are the optical pyrometer and the electrical noise thermometer. 

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... 

Measurement of temperature (or any energy property) has one major difference from measurement of physical materials It is not cumulative. To measure the length of a room, you can lay several meter sticks end to end. The sum of the number of meter sticks will be the length of the room. Temperature, however, is not cumulative. If you have a liquid that is hotter than the range of temperatures measurable on one thermometer, you cannot use a second thermometer to obtain the remaining temperature. 

The boiling point serves as an indicator of a chemical s physical state at ambient conditions. Mostly it is not used as an autonomous parameter in environmental hazard assessments, but for estimating further properties such as the vapour pressure. The boiling point is defined as the temperature at which the vapour pressure of a liquid is equal to the pressure of the atmosphere on the liquid (Rechsteiner, 1990). The measurement of boiling points is in principal simple, requiring only a calibrated thermometer, but the results may be severely affected by ... 

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