Standard atmosphere A unit

Standard atmosphere a unit of measurement for pressure equal to 760 mm Hg or 101, 325 Pa. 

Standard atmosphere a unit of pressure equai to 760 mm Hg. (5.1) Standard enthalpy of formation the enthaipy change that accompanies the formation of one moie of a compound at 25°C from its eiements, with aii substances in their standard states at that temperature. (6.4)... 

Atmosphere (standard atmosphere) A unit of pressure latm = 1.013 x lO pascals (Pa) = 760mmHg. 

The atmosphere is made up of a number of gases and, near the earth s surface, water vapor as well. The pressure exerted by atmospheric air is referenced to sea level. Average atmospheric pressure is 1013 mbar (equivalent to the atmosphere , a unit of measure used earlier). Table VIII in Chapter 9 shows the composition of the standard atmosphere at relative humidity of 50 % and temperature of 20 °C. In terms of vacuum technology the following points should be noted in regard to the composition of the air ... 

ATMOSPHERE, STANDARD (atm). A unit of pressure. One standard atmosphere equals 101,325 newtons per square meter. 

The following demonstration shows one of the properties of a solution, called colligative properties, which can be observed when a nonvolatile compound dissolves in a solvent, such as water. The normal boiling point of water under standard atmospheric pressure (1 atmosphere [a unit of pressure]) is 100°C. At high altitudes, the atmospheric pressure is lower, and the boiling point is lower. Let us say that we perform the experiment at standard pressure (although in Jemsalem, where the demonstrator lives, the pressure is lower, and the boiling temperature is also lower). 

Here, we first discuss whether such semi-closure (which is introduced so that CO2 separation can be undertaken more easily) is likely to lead to higher or lower thermal efficiency, and in this discussion it is helpful to consider recirculation in relation to an air standard cycle (see Fig. 8.4). Fig. 8.4a shows a closed air standard cycle with unit air flow Fig. 8.4b shows an open cycle similarly with unit air flow and an air heater rather than a combustion chamber. The cycles are identical in every respect except that in the former the turbine exhaust air from the turbine is cooled before it re-enters the compressor. In the latter, the turbine exhaust air is discharged to atmosphere and a fresh charge of air is taken in by the compressor. The quantities of heat supplied and the work output are the same for each of the two cycles, so that the thermal efficiencies are identical. 

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]. 

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. 

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). 

A convenient unit for gaseous expansion is obtained by measuring p in standard atmospheres and v in litres the work done by expansion through a volume of 1 litre under a constant pressure of one atmosphere is called a litre-atmosphere (1. atm,). Its value in ergs or other units may be calculated as follows ... 

Traditionaiiy, chemists define the units of pressure in terms of the Earth s atmosphere and the mercury barometer. The standard atmosphere (atm) is the pressure that wiii support a coiumn of mercury 760 mm in height. 

A second common pressure unit, the torr, aiso is based on the mercury barometer. One torr is the pressure exerted by a coiumn of mercury 1 mm in height. Because the standard atmosphere supports a 760-mm coiumn of mercury, the reiationship between the atmosphere and the torr is 1 atm = 760 torr = 760 mm Hg. 

Some Sl-derived units with special names are included in Table 2.2. The standard atmosphere may be used temporarily with SI units it is dehned to be equal to 1.01325 X 10 Pa. The thermochemical calorie is no longer recommended as a unit of energy, but it is defined in terms of an SI unit, joules, symbol J, as 4.184 J [4]. The unit of volume, liter, symbol L, is now defined as Idm. ... 

Nernst zero of potential phys chem An electrode potential corresponding to the reversible equilibrium between hydrogen gas at a pressure of 1 standard atmosphere and hydrogen ions at unit activity. nernst zir-o av pa ten chal )... 

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. 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

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. 

For many years, atmospheric pressure was measured in millimetres of mercury (mm Hg). In the British Commonwealth and the United States, inches of mercury were used. Standard atmospheric pressure, the pressure of the atmosphere at sea level and 0°C, is 760 mm Hg. More recently, in honour of the work of Torricelli, standard atmospheric pressure has been defined as 760 torr. 1 torr represents a column of mercury 1 mm in height at 0°C. Another common unit for measuring pressure is atmospheres (atm), where 1 atm is equivalent to 760 torr. While mm Hg, torr, and atm are still used to measure pressure, especially in technological and medical applications, the SI units are pascals (Pa) or kilopascals (kPa). 

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