Atmosphere, standard

The vapour pressure of a liquid increases with rising temperature. A few typical vapour pressure curves are collected in Fig. 7,1, 1. When the vapour pressure becomes equal to the total pressure exerted on the surface of a liquid, the liquid boils, i.e., the liquid is vaporised by bubbles formed within the liquid. When the vapour pressure of the liquid is the same as the external pressure to which the liquid is subjected, the temperature does not, as a rale, rise further. If the supply of heat is increased, the rate at which bubbles are formed is increased and the heat of vaporisation is absorbed. The boiling point of a liquid may be defined as the temperature at which the vapour pressure of the liquid is equal to the external pressure dxerted at any point upon the liquid surface. This external pressure may be exerted by atmospheric air, by other gases, by vapour and air, etc. The boiling point at a pressure of 760 mm. of mercury, or one standard atmosphere, may be termed the normal boiling point. 

Defining fixed points of the International Temperature Scale of 1990 (ITS-90). Except for the triple points, the assigned values of temperature are for equilibrium states at a pressure of one standard atmosphere (101 325 Pa). 

Pressure. Standard atmospheric pressure is defined to be the force exerted by a column of mercury 760-mm high at 0°C. This corresponds to 0.101325 MPa (14.695 psi). Reference or fixed points for pressure caUbration exist and are analogous to the temperature standards cited (23). These points are based on phase changes or resistance jumps in selected materials. For the highest pressures, the most rehable technique is the correlation of the wavelength shift, /SX with pressure of the mby, R, fluorescence line and is determined by simultaneous specific volume measurements on cubic metals... 

Fig. 3. Standard system of pressure measurement (3), where the bold line represents standard atmospheric pressure at sea level.

TABLE 2-236 Thermodynamic Properties of the International Standard Atmosphere ... 

Extracted from U.S. Standard Atmosphere, 1976, National Oceanic and Atmospheric Administration, National Aeronautics and Space Administration and tte U.S. Air Force, Washington, 1976. Z = geometric altitude, T = temperature, P = pressure, g = acceleration of gravity, M = molecular weight, a = velocity of sound, i = viscosity, k = thermal conductivity, X = mean free path, p = density, and H = geopotential altitude. The notation 1.79.—5 signifies 1.79 X 10 . ... 

TABLE 12-1 Thermodynamic Properties of Moist Air (Standard Atmospheric Pressure, 29 921 inHg)... 

The boiling point of a liquid varies with the atmospheric pressure to which it is exposed. A liquid boils when its vapour pressure is the same as the external pressure on its surface, its normal boiling point being the temperature at which its vapour pressure is equal to that of a standard atmosphere (760mm Hg). Lowering the external pressure lowers the boiling point. For most substances, boiling point and vapour pressure are related by an equation of the form. 

National Oceanic and Atmospheric Administration, "United States Standard Atmosphere." U.S. Government Printing Office, Washington, DC, 1976. 

ASME Power Test Code, 417 ASME Pressure Vessel Code, 315, 316 ASME, standard atmospheric conditions, 21 Aspect ratio, 227 Atlas Copco, 96 Automatic control systems, 356, 357... 

The mechanical properties for Trogamid T are for dry material at 20 C those for Grilamid TR-55 at standard atmosphere at 23 C. This will account, in part, for the differences in the figures for mechanical properties of the two polymers. 

Boiling Point (BP) — the temperature at which a liquid changes to gas under standard atmospheric pressure (760 mm mercury). The BP of water is 100°C, while the BPs of ethyl alcohol and n-hexane are 78.4°C and 68.7°C, respectively. Lowering the atmospheric pressure (e.g., by applying a vacuum) will lower the BP conversely, higher pressures result in elevated boiling points. 

Hyperbaria Pressures greater than standard atmospheric pressure (760 mm... 

Tensile strength of the fibers is also determined by the refinement of the fiber [14] (Fig. 4). Hydrophilic properties are a major problem for all cellulose fibers. The moisture content of the fibers amounts to 10 wt% at standard atmosphere. Their hydrophilic behavior influences the properties of the fiber itself (Table 3) as well as the properties of the composite at production [15]. 

There are other organizations within the United States and regions around the world that have established different standards. The ASME standard atmosphere is at a temperature of 68°F, a pressure of 14.7 psia, and a relative humidity of 36%. The British use a standard atmosphere with a temperature of 60°F and a pressure of 30.00 in.Hg. The Europeans use a standard atmosphere with a temperature of 15°C (59°F) and pressure of 750 mmHg (14.5 psia) [24,25]. 

When selecting and sizing compressors, care should be taken in determining which standard has been used to rate a compressor under consideration, particularly if the compressor has been produced abroad. All further discussions in this section will utilize only the API Mechanical Equipment Standards standard atmosphere. 

The temperature at which a liquid hoils is not constant, hut varies with the pressure. Thus, while the hoiling point of water is commonly taken as 100°C, this is only true at a pressure of one standard atmosphere (1.013 har) and, hy varying the pressure, the hoiling point can he changed (Table 1.1). This pressure-temperature property can he shown graphically (see Figure 1.2). 

The atmosphere consists of a mixture of dry air and water vapour. Air is itself a mixture of several elemental gases, mainly oxygen and nitrogen, hut the proportions of these are consistent throughout the atmosphere and it is convenient to consider air as one gas. This has a molecular mass of 28.97 and the standard atmospheric pressure is 1013.25 mhar or 101 325 Pa. 

Another unit commonly used to express gas pressure is the standard atmosphere, or simply atmosphere (atm). This is the pressure exerted by a column of mercury 760 mm high with the mercury at 0°C. If we say that a gas has a pressure of 0.98 atm, we mean that the pressure is 98% of that exerted by a mercury column 760 mm high. 

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