Barometer, atmospheric

Barometer atmospheric pressure in the fire service, this is used at wildland fires to detect pressure changes that could show local wind shifts and is a basic component of the pressure gauges on the pump inches or millimeters of mercury... 

These regions are characterized by the mean free path of the residual gas molecules and the type of gas streaming. The units for expressing the amount of pressure remaining in a vacuum system are based on the force of atmospheric air under standard conditions. This force amounts to 1.03 kg cm 2. Gas pressure is stated in terms of the height of a column of mercury supported by that pressure in a barometer. Atmospheric pressure of 1.03 kg cm 2 will support a column of mercury 760 mm. One torr is the equivalent of one millimeter or of 1/760 of atmospheric pressure. In modern vacuum technology the common unit is the bar. 1 mbar corresponds to 0.75 torr and this is equal to 102 pascal. 

Figure 5.4 I A schematic diagram of the forces at work in a mercuiy barometer. Atmospheric pressure exerts a force pushing down on the surfiice of the mercuiy and tends to push the column of mercuiy in the barometer higher up the mbe. The force due to the weight of the mercury in the column tends to push the column of mercury lower in the tube. When the two forces are equal, the column height remains constant.

The mercury barometer (Fig. 10-11) indicates directly the absolute pressure of the atmosphere in terms of height of the mercuiy column. Normal (standard) barometric pressure is 101.325 kPa by definition. Equivalents of this pressure in other units are 760 mm mercury (at 0°C), 29.921 iuHg (at 0°C), 14.696 IbFin, and 1 atm. For cases in which barometer readings, when expressed by the height of a mercuiy column, must be corrected to standard temperature (usually 0°C), appropriate temperature correction factors are given in ASME PTC, op. cit., pp. 23-26 and Weast, Handbook of Chemistty and Physics, 59th ed., Chemical Rubber, Cleveland, 1978-1979, pp. E39-E41. 

Barometer An instrument used to measure the barometric pressure at a given location in the earth s atmosphere. 

Pressure, atmospheric The p>ressure of the atmosphere as indicated by a barometer, in kPa. 

Any Pressure Leve) Above Atmospheric (gauge or absolute = (gauge) + (barometer))... 

Vacuum in process systems refers to an absolute pressure that is less than or below the local barometric pressure at the location. It is a measure of the degree of removal of atmospheric pressure to some level between atmospheric-barometer and absolute vacuum (which cannot be attained in an absolute value in the real world), but is used for a reference of measurement. In most situations, a vacuum is created by pumping air out of the container (pipe, vessels) and thereby lowering the pressure. See Figure 2-1 to distinguish between vacuum gauge and vacuum absolute. 

An air compressor is required to raise 4,600 scfm of atmospheric air to 100 psig. The ambient summer temperature is 95°F dry bulb for two months and lower for the balance of the operating time. The air usually has a relative humidity of 65%, but during the wet season, the humidity may be 100% while the temperature is 95°F. The elevation is sea level the barometer 14.7 psia. The continuity of air supply is very critical. 

Atmospheric pressure can be measured by any of several methods. The common laboratory method uses a mercury column barometer. The height of the mercury column serves as an indicator of atmospheric pressure. At sea level and at a temperature of 0° Celsius (C), the height of the mercury column is approximately 30 inches, or 76 centimeters. This represents a pressure of approximately 14.7 psia. The 30-inch column is used as a reference standard. 

Psychrometry has to do with the properties of the air-water vapor mixtures found in the atmosphere. Psychrometry tables, published by the US Weather Bureau, give detailed data about vapor pressure, relative humidity and dew point at the sea-level barometer of 30 in Hg, and at certain other barometric pressures. These tables are based on relative readings of dry bulb and wet bulb atmospheric temperatures as determined simultaneously by a sling psychrometer. The dry bulb reads ambient temperature while the wet bulb reads a lower temperature influenced by evaporation from a wetted wick surrounding the bulb of a parallel thermometer. 

The weight of the earth s atmosphere pushing down on each unit of surface constitutes atmospheric pressure, which is 14.7 psi at sea level. This amount of pressure is called one atmosphere. Because the atmosphere is not evenly distributed about earth, atmospheric pressure can vary, depending upon geographic location. Also, obviously, atmospheric pressure decreases with higher altitude. A barometer using the height of a column of mercury or other suitable liquid measures atmospheric pressure. 

A mercury barometer. This is the type of barometer first constructed by Torricelli. The pressure of the atmosphere pushes the mercury in the dish to rise into the glass tube. The height of the column of mercury is a measure of the atmospheric pressure. 

A device commonly used to measure atmospheric pressure is the mercury barometer (Figure 5.1), first constructed by Evangelista Torricelli in the seventeenth century. This consists of a closed gas tube filled with mercury inverted over a pool of mercury. The pressure exerted by the mercury column exactly equals that of the atmosphere. Hence the height of the column is a measure of the atmospheric pressure. At or near sea level, it typically varies from 740 to 760 mm, depending on weather conditions. 

Most barometers contain mercury rather than some other liquid. Its high density allows the barometer to be a convenient size. A water barometer needs to be 10,340 mm (34 ft) high to register atmospheric pressure ... 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

The pressure of the atmosphere can be measured with a barometer, an instrument invented in the seventeenth century by Evangelista Torricelli, a student of Galileo. Torricelli (whose name coincidentally means little tower in Italian) formed a little tower of liquid mercury. He sealed a long glass tube at one end, filled it with mercury, and inverted it into a beaker (Fig. 4.4). The column of mercury fell until the pressure that it exerted at its base matched the pressure exerted by the atmosphere. To interpret measurements with a barometer, we need to find how the height of the column of mercury depends on the atmospheric pressure. 

We want to find the relation between the height, h, of the column of mercury in a barometer and the atmospheric pressure, P. Suppose the cross-sectional area of the column is A. The volume of mercury in the column is the height of the cylinder times this area, V = bA. The mass, ttt, of this volume of mercury is the product of mercury s density, d, and the volume so m = dV = dhA. The mercury is pulled down by the force of gravity and the total force that its mass exerts at its base is the product of the mass and the acceleration of free fall (the acceleration due to gravity), g F = mg. Therefore, the pressure at the base of the column, the force divided by the area, is... 

FIGURE 4.4 A barometer is used to measure the pressure of the atmosphere. The pressure ot the atmosphere is balanced by the pressure exerted bv the column of mercury. The height of the column is proportional to the atmospheric pressure. Therefore, by measuring the height of the column, we can monitor the pressure of the atmosphere. 

Self-Test 4.1A What is the atmospheric pressure when the height of the mercury column in a barometer is 756 mm ... 

Self-Test 4.1B The density of water at 20°C is 0.998 g-cm-3. What height would the column of liquid be in a water barometer when the atmospheric pressure corresponds to 760. mm of mercury ... 

Changes in the composition of the atmosphere are perhaps our most valid indicators of the influence of human activities on global systems. The next best barometer may very well be loss of biodiversity due to habitat destruction. 

The air around us is a huge reservoir of gas that exerts pressure on the Earth s surface. This pressure of the atmosphere can be measured with an instmment called a barometer. Figure 5 shows a schematic view of a simple mercury barometer. A long glass tube, closed at one end, is filled with liquid mercury. The filled tube is inverted carefully into a dish that is partially filled with more mercuiy. The force of gravity pulls downward on the mercury in the tube. With no opposing force, the mercury would all ran out of the tube and mix with the mercury in the dish. 

At sea level, atmospheric pressure supports a mercury column approximately 760 mm in height. Changes in altitude and weather cause fluctuations in atmospheric pressure. Nevertheless, at sea level the height of the mercury column seldom varies by more than 10 mm, except under extreme conditions, such as in the eye of a hurricane, when the mercury in a barometer may fall below 740 mm. 

A manometer is similar to a barometer, but in a manometer gases exert pressure on both liquid surfaces. Consequently, a manometer measures the difference in pressures exerted by two gases. A simple manometer, shown in Figure 5 3, is a U-shaped glass tube containing mercuiy. One side of the tube is exposed to the atmosphere and the other to a gas whose pressure we want to measure, hi Figure 5-3. the pressure exerted by the atmosphere is less than the pressure exerted by the gas in the bulb. The difference in heights of mercury (Ah, in mm) between the two sides of the manometer depends on the difference in the pressures. 

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. 

The difference in mercury levels in the manometer is 19 mm. This is the pressure difference in torr between the gas sample and atmospheric pressure. The latter, as measured with a barometer, is 752 torr. Shall we add or subtract the 19 torr pressure difference Notice that the mercury level in the manometer is lower on the side exposed to the atmosphere. Thus, the atmosphere pushes on the mercury harder than does the gas sample, meaning that the pressure of the gas sample is lower than the pressure of the atmosphere. Subtract the pressure difference / gas = A P = 752 toiT - 19 toiT = 733 toiT... 

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 ... 

The blanket of air that cloaks our planet behaves as an ideal gas, but the atmosphere is bound to the Earth by gravitational attraction, not by confining walls. The pres-sure exerted by the atmosphere can be thought of as the pressure of a column of air. Just as the pressure exerted by mercuiy in a barometer is the pressure of the column of mercury. The higher we rise into the atmosphere, the less air there is above us. Less air above us means that the pressure exerted by the column of air is lower. Lower pressure, in turn, means lower molecular density, as indicated... 

The gravitation of the earth attracts the gaseous components of the air, which exert a force, known as atmospheric pressure, on the surface of the planet. The pressure on any particular place on the earth s surface depends on the amount of air above the place. It follows that the atmospheric pressure decreases at high altitudes, increases at low altitudes and below sea level, and is also affected by changes in weather. Measuring the atmospheric pressure is usually done with physical instruments known as barometers (see Fig. 83). 

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