Atmospheric pressure relationship between units

For gases, this force can be a factor of their motion or their weight. Atmospheric pressure is caused by the weight of air particles that are attracted toward earth. Inside a sealed container, pressure is exerted by the collisions of particles on the sides of the container. By determining the force of those collisions on a given area, you can determine the pressure exerted by the particles. There are many units that describe pressure. The SI unit of pressure is the pascal, Pa. The kilopascal is a bit more practical as a unit, however, since a pascal is quite small. Other units of pressure include millimeters of Hg (mm Hg), torr, bar, and atmospheres. The relationship between the units is as follows ... 

Pressure. Pressure, defined as force per unit area, can be expressed as an absolute or relative value. Although atmospheric pressure constantly fluctuates, a standard value of 101.3 kPa (14.7 psia) has been assigned as the accepted value at sea level. The MaM in the psia stands for absolute, ie, the pressure is 14.7 psi (101.3 kPa) above zero pressure or a vacuum. Most ordinary pressure-measuring instruments do not measure true pressure, but rather a pressure relative to the barometric or atmospheric pressure. This relative pressure is called gauge pressure. The atmospheric pressure is defined to be 1 psig, in which the "g" indicates that it is relative to atmospheric pressure. Vacuum is the pressure below atmospheric pressure and is, therefore, a relative pressure measurement as well. The relationship between absolute and relative pressure is shown in Figure 3 (see Pressure measurement, Vacuum technology). 

The pascal represents a very small pressure, and therefore the most common applications, such as tire pressure, will use kilopascals (kPa) instead of Pa. Other units of pressure include the torr (or millimeter of mercury, mmHg), inches of mercury, the atmosphere (atm), and the bar. A torr is an amount of pressure necessary to support a column of mercury 1 mm in height. One atmosphere of pressure is loosely defined by the atmospheric pressure at sea level, but is more precisely defined as the pressure necessary to support a column of mercury 760 mm in height. One bar of pressure is equal to 100 kPa. The relationships between the various units of pressure are given below ... 

The thermodynamic criterion for the equilibria CaCO, ) = Ca0(,) + C02 (,) is AG ° = -RT n Kp, where AG° is the change in Gibbs free energy of the reactants and products in their standard state, R is the gas constant, and Kp is the equilibrium constant. For this equilibria, A p = pco, for pressure in units of atmospheres. Values for AG are tabulated in the form AG° = a+ bT combining these expressions yields an exponential relationship between the partial pressure of CO2 and temperature for the above equilibria. Complete derivations and discussion of these equations may be found in physical chemistry textbooks such as references [13] and [14]. 

By detinition, one calorie (International Table) is exactly 4.186 8 absolute joules which converts to 1.055 056 X 10 joules for one Btu (International Table). Also, by definition, one calorie (thermochemical) is exactly 4.184 absolute joules which converts to 1.054350 X 103 joules for one Btu (thermochemical). A mean calorie is TSOth of the heat required to raise the temperature of one gram of water at one atmosphere pressure from 0°C to 100°C and equals 4.19002 absolute joules. In all cases, the relationship between calorie and British thermal unit is established by 1 cal/(g. °C) = 1 Btu/(lb. 6SF). A mean Btu, therefore is 7So h of the heat required to raise the temperature of one pound of water at one atmosphere pressure from 32°F to 212°F and equals 1.055 87 X 103 joules. When values are given as Btu or calories, the type of unit (International Table, thermochemical, mean, or temperature of determination) should be given. In all cases for this table, conversions involving jou S are based on the absolute joule. 

The relationship between partial pressure and gas-phase concentration explains why concentrations in the atmosphere are frequently expressed in parts per million (ppm) or parts per billion (ppb) (see Table 3.1). This is done on a volume basis so that 1 ppm means 1 cm3 of a substance is present in 106cm3 of air. It also requires that there is one molecule of the substance present for every million molecules of air, or one mole of the substance present for every million moles of air. This ppm unit is thus a kind of mole ratio. It can be directly related to pressure through the law of partial pressure, so at one atmosphere (1 atm) pressure a gas present at a concentration of 1 ppm will have a pressure of 10 6atm. 

The accepted SI unit for gas pressure is the pascal. Pa. A pascal is a very small amount of pressure, so the kilopascal, kPa, is more commonly used. Other units used to describe gas pressure are the atmosphere (atm), torr, millimeter of mercury (mmfig), and bar. The relationships between these pressure units are... 

Although pressure is formally defined as force unit area, it is more commonly expressed in milli-meters of mercury, mmHg, or atmospheres, atm. The mmHg is also called the torr in honor of Torricelli, who built the first barometer in 1643. The SI unit of pressure is the Pascal, although it is not widely used in the United States. One atmosphere is 1.013 x 105 Pascal (Pa) or 101.3 kilopascal (kPa). The relationships between the units of pressure are... 

An older unit of pressure, still in use, is the Torr, or mm Hg, representing the hydrostatic pressure exerted by a column of mercury i mm high. In the American Engineering system of units, pressure is measured in pounds of force per square inch, or psi. The relationship between the various units can be expressed through their relationship to the standard atmospheric pressure ... 

It may be noted that the older unit of pressure of gases was the atmosphere, which was abbreviated as atm. The relationship between these units is as given below ... 

MIP measurements were carried out with a low-pressure (Pascal 140, Theromo Fisher Scientific, Milano, Italy) and a high-pressure unit (Porosimeter 2000, Carlo Elba, Milano, Italy). The low-pressure unit was used with an evaluable pressure range of 0.13 bar to detect pore radii between 58 and 1.84 pm. The same dilatometer is then placed to the high-pressure unit, which operates from atmospheric pressure up to 2000 bar to evaluate pore radii from 7355 to 3.7 nm. The mathematical relationship of pressure to pore radius distribution is calculated with the Washburn equation [37], assuming that all pores are cylindrical. Each experiment was performed in triplicate [32]. 

Henry s law constant is dimensionless when Ug and ai have units of moles per cubic meter, but published values for Kh sometimes have units of atmospheres or torr per mole fraction. Thus the gas phase concentration is often expressed in terms of its partial pressure and the liquid phase concentration is expressed as a mole fraction. The asterisks in Equation 11.1 remind us that Henry s law is an equilibrium relationship. Equation 11.1 is not satisfied merely because gas and liquid phases are brought in brief contact. Instead, the difference between Ug and its liquid phase equivalent, KhOi, provides the driving force for mass transfer that could ultimately lead to equilibrium and the satisfaction of Equation 11.1 ... 

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