Steam point

Any instrument which can be used for measuring temperatures is called a thermometer. Thermometers may be, and are, constructed which utilise any property of a body such as those mentioned above. To evade the difficulty of comparison of scales, they are usually all referred to a gas thermometer, with Centigrade scale as standard. The ice and steam-points on the latter are taken as 0° and 100° respectively. 

The values of the constants in the Uiermometric function are determined with reference to fixed thermometnc points whose temperatures are arbitrarily assumed. The fixed thermometric points most frequently employed are the ice point, steam point and triple point of water. 

A Centigrade (or Celsius) temperature scale is obtained by choosing the thennoraetric function, Equation (3), and assigning die following arbitrary values of temperature, 0, to die ice point [0, i and steam point (ft ) respectively... 

Different empirical temperature scales will naturally differ from each other except at the respective fixed thermometnc points, Even different scales of the same type (say different Centigrade scales) will differ at all temperatures, except the steam point and ice point, depending on the fortuitous properties of the system chosen as a thermometer. It is, therefore, necessary to remove these differences and to obtain a more universal scale. This has been achieved in two ways. The practical way of achieving uniformity is to lay down detailed rules concerning the thermometer (actually different thermometers depending on the range of temperatures to be measured). Such rules have been agreed on internationally and... 

If the exhaust steam (Point 2, Fig. 7.4) is "dry," i.e., saturated vapor, then isentropicexpansion to the same pressure (Point 2, Fig. 7.4) must produce "wet" steam, withentropy ... 

Steam bath calibration for the liquid-steam point of H20 is not easy for many labs to use because of its expense, and the equipment is difficult to use. In addition, the user needs to account for atmospheric pressure variations and the effects of local variations of gravity on the barometer. Fortunately, the ice-point calibration is easy to set up and use. 

Example 1.2 Table 1.3 lists the specific volumes of water, mercury, hydrogen at l(atm), and hydrogen at lOO(atm) for a number of temperatures on the International Practical Temperature Scale. Assume that each substance is the fluid in a thermometer, calibrated at the ice and steam points as suggested at the beginning of this section. To determine how good these thermometers are, calculate what each reads at the true temperatures for which data are given. 

The object of the experiment described below is to set up a gas thermometer, calibrate it at the ice point (in lieu of the experimentally more difficult triple point), and use it to determine the temperatures of one or more other fixed points. These may include the steam point, the boiling point of liquid nitrogen, the sublimation temperature of solid carbon dioxide (Dry Ice), the transition temperature of sodium sulfate decahydrate to the monohydrate and saturated solution, etc. The experiment will be performed with an apparatus that... 

A quantity of 2.500 g. of a metal of mean heat capacity 0.0591 (defined) cal. deg." g." is cooled from the steam point to the ice point. Determine the total heat evolved in (i) ergs, (ii) abs. joules. 

On the Celsius scale, the experimentally measured temperature of the steam point, which is the boiling point of water at a pressure of one atmosphere, is lOOWC. 

When the two equilibrium states are the freezing and boiling points of water, this ratio is about 1.3661, a perfectly fixed, if imperfectly known number. If the temperature of the ice point is called 1000° A (degrees Anderson), the steam point is then 1336.1°A, and this is a valid thermodynamic temperature scale. However not... 

If it was decided by an international body that the Crerar Scale should be changed such that it had a value of 100°C at the ice point, how many degrees would it have between the ice and steam points ... 

The procedure is easily reduced to a formula by which the temperature can be calculated from the measured value of the thermometric property y. Let y be the value at the ice point and y be the value at the steam point. These points are separated by 100 degrees. Then... 

Originally the ice point on the Kelvin scale was determined by using a constant-volume gas thermometer to measure the pressure and assigning 100 degrees between the ice point and the steam point. The temperature on this centigrade gas scale is given by... 

This point is very close to the ice point t = +0.0002 °C. Similarly, 100 °C is very close to the steam point but is not exactly at the steam point. The difference is much too small to cause any concern. 

Equation (1-4) is used for the mercury or alcohol thermometer, as well as for gas thermometers containing or He. It is found experimentally that the niimerical value of the temperature, except the values at the ice and steam points which are fixed, depends upon the particular system and thermometric property employed. Because there is less variation among gas thermometers, a gas is usually chosen as the standard thermometric substance. 

Celsius scale A temperature scale in which the fixed points are the temperatures at standard pressure of ice in equilibrium with water (0°C) and water in equiUbrIum with steam (100°C). The scale, between these two temperatures, is divided in 100 degrees. The degree Celsius (°C) is equal in magnitude to the kelvin. This scale was formerly known as the centigrade scale the name was offlciaUy changed in 1948 to avoid confusion with a hundredth part of a grade. It is named after the Swedish astronomer Anders Celsius (1701-44), who devised the inverted form of this scale (ice point 100°, steam point 0°) in 1742. 

Another scale was developed in France by R6aumur in 1730. It was based on experiments that showed the even expansion of an 80/20 wt-% mixture of water and ethyl alcohol. R6aumur found that this mixture expands exacdy 80 parts per thousand between the ice and steam points. Thus, he invented the scale which showed the freezing of water at zero and boiling 80°. 

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