Atmospheric pressure standard

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

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... 

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. 

The roentgen is the international unit of quantity for both x-rays and y-rays. It is the quantity of this radiation which will produce, as a, result of ionization, one electrostatic unit of charge, of either sign, in one cubic centimeter of dry air (as measured at 0° C and standard atmospheric pressure). 

Standard Atmosphere = pressure of a column of 76 cm. pure mercury at 0° C. in latitude 45° (variation of gravitation constant affects the result per cent, per degree) ... 

NOTE The standard atmospheric pressure of mercury (Hg) is 760 mm Hg (29.92 in) at 0 °C. Thus, ignoring barometric or temperature differences, it can be seen that the condenser back-pressure is usually in the range of 29 to 29.92 inches down to 26 inches, which equals 1 to 4 in mercury absolute (3.4-13.6 kPa). 

C05-0002. In the eye of a severe hurricane, the height of a mercuiy barometer may fall to 710 mm. Express this pressure in torr and atmospheres. What percentage change from standard atmospheric pressure is this ... 

C05-0080. California s automobile emission standards require that exhaust gases contain less than 220 parts per million hydrocarbons and less than 1.2% CO (both of these values are in moles per mole of air). At standard atmospheric pressure, what are the partial pressures, in torr and in atmospheres, that correspond to these values ... 

Boiling point The temperature at which the vapor pressure of a liquid is equal to the external pressure. Normal boiling point is the temperature at which the vapor pressure of a liquid is equal to standard atmospheric pressure. 

The evaporative recovery is a very energy-intensive process. Approximately 538 chu (970 Btu) are required to evaporate lib of water at standard atmospheric pressure. Additional energy is required to raise the temperature of the solution to its boiling point. 

Burettes calibrated at 20°C and 25°C deliver different weights of water for each 10 ml, when weighed with standard brass weights in air at 50% relative humidity (RH) at standard atmospheric pressure, as given below ... 

AH = -16.426 kJ mol and K = 0.385 MPa (at 298 K) equation (2) fits H for steam + n-alkane mixtures at low densities to within experimental error. Figure 4 shows the fit to the results at standard atmospheric pressure. To fit the results at higher pressures requires further virial coefficients, and the method runs into difficulties. 

Figure 4. Excess enthalpy H of steam + n-heptane at x — 0.5 and standard atmospheric pressure ((---------------) calculated using Equation 2)...

The liter is defined as the volume of a kilogram of water at 4° C and at standard atmospheric pressure. One liter equals 1000.028 cubic centimeters. 

Pressure Units and Values for Standard Atmospheric Pressure... 

As the unit operates in a residual vacuum of 30-40 mmHg, the distillation process is carried ont at a temperature of only 30 - 35° C. Thus, energy costs are 80% less than at standard atmospheric pressure. 

The most common thermometer in the world is the Celsius thermometer, named in honor of the Swedish astronomer Anders Celsius (1701—1744), who first suggested the scale of 100 degrees between the freezing point and boiling point of fresh water. In a Celsius thermometer, the number 0 is assigned to the temperature at which pure water freezes and the number 100 is assigned to the temperature at which it boils (at standard atmospheric pressure), with 100 equal parts called degrees between these two points. 

Subatmospheric pressure usually is expressed in reference to perfect vacuum or absolute zero pressure, lake absolute zero temperature (the concept is analogous), absolute zero pressure cannot be achieved, but it does provide a convenient reference datum. Standard atmospheric pressure is 14.695 psi absolute, 30 inches of mercury absolute, or 760 mmHg of density 13.595 g/cm3 where acceleration due to gravity is g = 980.665 emir. ] mmHg. which equals 1 torr. is the most commonly used unit of absolute pressure. Derived units, the million or micrometer, representing 1/1000 of 1 mmHg or 1 torr, are also used for subtorr pressures. 

In the MKS system of units, standard atmospheric pressure is 750 torr and is expressed as 100,000 Pa (N/m2) or 100 kPa. This means that 1 Pa is equivalent to 7.5 millitorr (1 torr = 133.3 pascal). Vacuum, usually expressed in inches of mercury, is the depression of pressure below the atmospheric level, with absolute zero pressure corresponding to a vacuum of 30 inches of mercuiy. 

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