Vapor pressure measurements thermometer

Two additional points near 17.0 and 20.3 K are required. These may be determined by using either ihe constant volume gas thermometer or by vapor pressure measurements of H . 

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). 

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

When highly accurate Pq measurements are necessary, a platinum resistance thermometer can be placed in the bath adjacent to the sample cell. Equation (13.1) can then be used to calculate the nitrogen vapor pressure. Alternatively and perhaps preferably, the vapor pressure can be measured directly by condensing nitrogen in a cell contained in the coolant and connected directly to a manometer or sensitive pressure gauge. 

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. 

Filled-System Thermometers The filled-system thermometer is designed to provide an indication of temperature some distance removed from the point of measurement. The measuring element (bulb) contains a gas or liquid that changes in volume, pressure, or vapor pressure with temperature. This change is communicated through a capillary tube to a Bourdon tube or other pressure- or volume-sensitive device. The Bourdon tube responds so as to provide a motion related to the bulb temperature. Those systems that respond to volume changes are completely filled with a liquid. Systems that respond to... 

If the vapor pressure of liquid nitrogen is used for the temperature measurement, one allows N2 gas to condense in chamber B and the N2 pressures can be read directly on a pressure gauge. If a copper-Constantan thermocouple or a platinum resistance thermometer is used, it must be well calibrated, since accurate absolute temperatures are needed. If chamber B is not used, all further instractions concerning it may be disregarded. 

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. 

Some filled-bulb thermometers take account of the pressure increase in the capillary stream. This increase is due to the vapor-pressure rise of the liquid phase plus the rise due to the compression of the gas. A relatively high pressure may hence occur. Bending the capillary tube is sometimes used, and the tube straightens to a degree as the pressure rises. The amount of straightening can be calibrated with a pointer on a dial to measure the temperature. 

A student breaks a thermometer and spills most of the mercury (Hg) onto the floor of a laboratory that measures 15.2 m long, 6.6 m wide, and 2.4 m high, (a) Calculate the mass of mercury vapor (in grams) in the room at 20°C. (b) Does the concentration of mercury vapor exceed the air quality regulation of 0.050 mg Hg/m of air (c) One way to treat small quantities of spilled mercury is to spray sulfur powder over the metal. Suggest a physical and a chemical reason for this action. The vapor pressure of mercury at 20°C is 1.7 X 10 atm. 

The best device, however, is the quartz coil manometer, the coil of which can be heatedto500°C(in special cases to 600-700°C). In all cases the null point of the instrument must be checked after each measurement. Therefore the manometer should be provided with a heating coil, which doe snot need to be at the test temperature but must nevertheless be at a sufficiently high temperature to prevent condensation in the coil and in the capillary connections (which are likewise provided with a heating coil). With compensation to zero, the pressure is read off on the Hg manometer. In those cases where it cannot be ascertained by the usual method (with a thermometer and distillation flask) the boiling point is determined more accurately by extrapolation of the vapor pressure curve. 

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). 

The vapor pressure of mercury is 0.0020 mmHg at 26°C. (a) Calculate K and Kp for the process Hg(/) V Hg( ). (b) A chemist breaks a thermometer and spills mercury onto the floor of a laboratory measuring 6.1 m long, 5.3 m wide, and 3.1 m high. Calculate the mass of mercury (in grams) vaporized at equilibrium and the concentration of mercury vapor in mg/m. Does this concentration exceed the safety limit of 0.05 mg/m (Ignore the volume of furniture and other objects in the laboratory.)... 

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