The Standard Atmosphere

Neither Eq. (4-13) nor Eq. (4-16) would be expected to provide a very good representation of the pressure and temperature in the real atmosphere, 

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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. 

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). 

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. ... 

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 ... 

Resistivity is sensitive to temperature and humidity, and tests are usually made after conditioning in the standard atmosphere of 23 2°C and 50 5%RH. Surface resistivity is particularly sensitive to humidity and the standard humidity should be maintained during the test. Where results are to be used as design or performance data, it would be advisable to test over a range of humidities and (perhaps) temperatures. The resistance of the test piece is measured after the test voltage has been applied for a set period, usually 1 min, although this will very often not be an adequate time for the current, and hence the measured resistivity, to reach an equilibrium value. If it is suspected that this is the case, resistivity can be monitored as a function of time of electrification. 

The vesicles (bubbles) in basaltic lava flows can be used to determine paleoelevation at the time of eruption. In the repertoire of paleoelevation proxies presently available to the research community, it represents one of very few direct proxies of elevation. The technique is based on the sizes of vesicles at the tops and bottoms of lava flows. We assume that bubbles do not know a priori that they will reside in one part of the flow or another when they are erupted from a volcanic vent. As such, the mass of gas is evenly distributed throughout the flow. The volume of the bubbles will therefore depend on pressure, which at the top of the flow is just atmospheric pressure, and at the bottom is atmospheric plus hydrostatic pressure from lava overburden. Since lava thickness can be measured in the field, and bubble size distributions (most notably the modal size) can be measured in the lab, a simple relation can be solved for atmospheric pressure, and using the standard atmosphere, elevation can be determined. 

Atmospheric density must be known to convert atmospheric depth to altitude. For midlatitudes, elevation can be converted to atmospheric depth via the standard atmosphere (Lide 1999) ... 

The main advantage derivable from the project is that hydro-electric plants do not emit any of the standard atmospheric pollutants such as C02 produced by fossil fuel power plants. 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

These values for m and c are universally accepted. The value for p, 105 Pa, is the IUPAC recommendation since 1982 [1. j], and is recommended for tabulating thermodynamic data. Prior to 1982 the standard pressure was usually taken to be p = 101 325 Pa ( = 1 atm, called the standard atmosphere). In any case, the value for p should be specified. 

Because gases are compressible, they exert pressure on their surroundings. Pressure is the force that is exerted over a unit area. For example, the atmosphere exerts a pressure known as atmospheric pressure. The Earth s atmosphere is a function of the location and the weather conditions, and it decreases with a higher altitude. The unit of pressure commonly used in chemistry is the atmosphere (atm). The standard atmosphere is 1 atm or a measurement of 760 millimeters of mercury (mm Hg or torr) on a manometer. 

Because instruments used for measuring pressure, such as the manometer (see Fig. 5.2), often use columns of mercury because of its high density, the most commonly used units for pressure are based on the height of the mercury column (in millimeters) the gas pressure can support. The unit mm Hg (millimeters of mercury) is called the torr in honor of Torricelli. A related unit for pressure is the standard atmosphere ... 

I think I gathered from one of the slides that you kept the temperature equal to the standard atmosphere. 

An analytical solution to Equations 12 and 13, subject to the couphng relation Equation 14, is diflBcult to obtain since one of the boundaries, viz. the droplet surface, is continuously regressing. Hence numerical integration was used to obtain the following results (45) on the combustion of an n-octane droplet in the standard atmosphere, with To(r) = 300°K. The total burning time is 0.236rgo / i. 

The piessure of the sumdard atmosphere is equal to thc piessure produced by a column of meicury 700 inm high. The density of mercury is 18.6g/cm b What is the standard atmospheric pressure expressed in the units g/cm- What is it in units Ibs/sq. in. ... 

One atmosphere equals the pressure at the surface of the earth that is due to the weight of the air. Pressure is often reported in units equal to the standard atmosphere. The value of this unit is 14.7 pounds per square inch. This is equal to the pressure produced by the weight of a column of mercury 760 mm high. Pressure is also reported in units equal to the weight of the corresponding column of mercury. The abbreviation mm Hg is used for mm of mercury. The vapor pressure of iodine at its melting point is 90 mm Hg, which is 90/760 atm or 0.118 atm. 

Consider an electrically healed house that has a floor space of 200 m and an average height of 3 m at 1000 m elevation, where the standard atmospheric pressure is 89.6 kPa, The house is maintained at a temperature of 22°C, and the infiltration losses are estimated to amount to 0.7 ACH. Assuming the pressure and the temperature in the house remain constant, determine the amount of energy loss from the house due to infiltration fora day during svhich Ihe average ouidoor temperature is 5°C. Also, determine the cost of this energy loss for that day if the unit cost of electricity in that area is 0,082/kWh. 

Pressure is expressed in various units. The SI unit for pressnre is the pascal (Pa), which is 1 kg m s. One standard atmosphere (1 atm) is defined as exactly 1.01325 X 10 Pa. The standard atmosphere is a nsefnl nnit becanse the pascal is inconveniently small and because atmospheric pressure is important as a standard of reference. We must express pressures in pascals when we perform calculations entirely in SI units. 

For historical reasons, a number of different pressure units are commonly used in different fields of science and engineering. Although we will work primarily with the standard atmosphere, it is important that you recognize other units and be able to convert among them. For example, the atmospheric pressure (often called the barometric pressure) recorded in weather reports and forecasts is typically expressed as the height (in millimeters or inches) of the column of mercury it supports. One standard atmosphere supports a 760-mm column of mercury at... 

Figure 1.12 Pressure terminology. The standard atmosphere is shown by the heavy horizontal line. The dashed line illustrates the atmospheric (barometric) pressure, which changes from time to time. Point Q in the figure is a pressure of 19.3 psi referred to a complete vacuum or 5 psi referred to the barometric pressure (2) is the complete vacuum, O represents the standard atmosphere, and 0 illustrates a negative relative pressure or a pressure less than atmospheric. This type of measurement is described in the text as a vacuum type of measurement. Point (D also indicates a vacuum measurement, but one that is equivalent to an absolute pressure above the standard atmosphere.

You definitely must not confuse the standard atmosphere with atmospheric pressure. The standard atmosphere is defined as the pressure (in a standard gravitational field) equivalent to 1 atm or 760 mm Hg at 0 C or other equivalent value, whereas atmospheric pressure is a variable and must be obtained from a barometer each time you need it. The standard atmosphere may not equal the bara-metric pressure in any part of the world except perhaps at sea level on certain days, but it is extremely useful in converting from one system of pressure measurement to another (as well as being useful in several other ways to be considered later). Expressed in various units, the standard atmosphere is equal to... 

List the values and units of the standard atmosphere for six different methods of expressing pressure. 

The boundaries between atmospheric layers are not rigidly fixed for example, the boundary between the troposphere and the stratosphere varies from an average of about 7,500 m (25,000 ft) near the poles to 17,000 m (55,000 ft) near the equator, and fluctuates seasonally. A reference profile, the Standard Atmosphere, is defined by the International Civil Aviation Organization to represent typical atmospheric conditions at midlatitudes (Table 4-1). At sea level, the Standard Atmosphere exerts a pressure of 760 mm of mercury (1 atm) and has a temperature of 15°C (59°F). (Note that English units are still in widespread use in the meteorology and aviation communities in the United States.) Pressure decreases approximately exponentially with increasing altitude at 5500 m (18,000 ft), pressure is half that at sea level. 

In actuality, temperature cannot be considered constant if the preceding calculation is performed for large altitude differences. In the Standard Atmosphere, temperature decreases at the rate of approximately 6.5°C per 1000 m (3.5°F per 1000 ft) up to an altitude of about 11,300 m (37,000 ft), the lower bound of the stratosphere. At that height, the temperature becomes constant at — 55°C (— 67°F). The constant temperature of the stratosphere greatly inhibits vertical mixing, thus leading to its name (think of stratification). Vertical temperature changes are discussed further in Section 4.2. 

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