Mercury thermal expansion

The corrections to be made on the reading are as follows (1) Temperature, to correct for the difference in thermal expansion of the mercury and the brass (or glass) to which the scale is attached. 

From Mercury—Density and Thermal Expansion at Atmospheric Pressure and Temperatures from 0 to. 350 C, Tables of Standard Handbook Data, Standartov, Moscow, 1978. The density values obtainable from those cited for the specific volume of the saturated liquid in the Thermodynamic Properties subsection show minor differences. No attempt was made to adjust either set. 

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

Air or water cooled mercury discharge lamps find many uses, one of the more obvious of which is the study of photochemical reactions. These lamps are usually made of vitreous silica because of its low thermal expansion, high melting point and its transparency to ultraviolet radiation. Their operating pressure has a profound effect on the spectral distribution of the radiation produced and therefore it is important to consider the requirements in the design of such lamps. 

Modern temperature scale proposed by G. Fahrenheit, defined by a thermometer, a law and three fixed points. Fahrenheit s thermometer was a mercury-in-glass one. Thermal expansion versus temperature was assumed linear. Three fixed points were defined 0°F temperature of a mixture of water, ice and ammonium chloride 32°F temperature of melting ice 96°F temperature of human body... 

Certain alloys of iron, nickel, and cobalt (Kovar, Fernico, etc.) have thermal expansion curves which nearly match those of borosilicate glasses, and a good bond may be formed between the two. Kovar is similar to carbon steel in its chemical properties. For example, it oxidizes when heated in air and is not wet by mercury. It may be machined, welded, copper brazed, and soft soldered. Silver solders should not be used with Kovar since they may cause embrittlement. At low temperatures Kovar undergoes a phase transformation, and the change in expansion coefficient below this temperature may be sufficient to cause failure of a glass-to-Kovar seal. The transformation temperature usually is below... 

A problem which arises when a read-out chip of for example silicon is attached to a detector chip of mercury cadmium telluride is the mechanical damage which may occur when the array is cooled to cryogenic temperatures for operation. The stress is due to a mismatch in the coefficients of thermal expansion between the two materials. 

We found the latter factor-voids to be important. Experimental results showed that when green coke was calcined under the new methods, and the derived calcined coke was observed by scanning electron microscopy (Figure 2) and its pore size distribution was measured by mercury porosimetry (Figure 3), microcracks of significant sizes (1 to 60 microns) were developed. This was an important contribution to the reduction of the thermal expansion coefficients of the calcined coke processed under the new method. 

Liquid in Glass Thermometers. Mercury-in-glass thermometers (or better yet, mercury-in quartz) function well between -25°C and 360°C their typical precision is 0.1 K. They must be corrected for (1) relatively small pressure effects and (2) a relatively large "exposed-stem correction," due to the different coefficients of thermal expansion of mercury and glass in the part of the thermometer not immersed in the system being measured. 

Bimetallic Thermometers. These use two metals of different coefficients of thermal expansion that are bonded together their heating bends the device, and an electrical contact can then be made or broken. Thermometer coils that expand/contract with temperature and make or break contact with a mercury electrode are used in household thermostats and industrial control apparatus. 

A mercury thermometer is based on the differential volume thermal expansion of liquid mercury (about 1.8 X 10 and glass (about 0.2 X 10 " K ). In the manu-... 

Since the mercury in the thread, as well as that in the bulb, is susceptible to thermal expansion, it is important in precise work to take account of the temperature of the thermometer stem. Most thermometer calibrations, especially those for enclosed-stem types, are for total immersion—it is assumed that the thread is at the same temperature as the bulb. Other thermometers are meant to be used with partial inunersion, often to a ring engraved on the stem, and the remainder of the stem is assumed to be at room temperature (say, 25°C). For precise work, stem corrections should be made if the stem temperatures differ significantly from those assumed in the calibration. The correction that should be added to the thermometer reading is given by the equation... 

As stated earlier, temperature and pressure do not have large influences on the densities of solids and liquids. Nevertheless, the fact that mercury in a thermometer rises or falls with changing temperature shows that the effect of temperature on liquid density is measurable. Coefficients of linear and cubic (volume) thermal expansion of selected liquids and soiids are given as empirical polynomial functions of temperature on pp. 2-128 to 2-131 of Perry s Chemical Engineers Handbook. For example, the Handbook gives the dependence of the volume of mercury on temperature as... 

A temperature scale can be set up by reference to an observable effect of temperature on an object, for example, thermal expansion, thermoelectric effect, or intensity of radiation of energy. Thus the thermal expansion effect can be observed by partially filling a glass capillary with mercury. The height of the meniscus of the mercury rises or falls as the hotness rises or falls. The capillary tube can be marked with numbers as a linear progression of distance to indicate temperature. Many devices can be devised for the measurement of temperature. The fundamental problem is to find a temperature scale that is independent of any arbitrarily selected measuring device employing an arbitrarily selected medium. 

These thermometers contain either mercury, alcohol, etc., as liquids. The thermal expansion of these liquids is greater than the glass, so the height of liquid in the glass capillary rises as the temperature increases. A major problem is that the glass can be easily broken. Furthermore, mercury causes toxicity problems if the thermometer breaks. Visual observations are usually required to read the thermometers. Often these instruments are restricted to temperatures from about 0°C to 400°C. Their advantages are low costs, long life if properly protected, and reasonable accuracy. They still are widely used in experimental setups and for various home uses. 

The low melting point (234 K) of Hg results in its being a unique metal. Its high thermal expansion coefficient makes it a suitable liquid for use in thermometers, and it has widespread application in barometers, diffusion pumps and in Hg switches in electrical apparatus. An older use was in mirrors. Some other metals dissolve in mercury to give amalgams, their uses are varied, for example ... 

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