Barometer mercury

The values in the table below are to be subtracted from the readings taken on a mercurial barometer to correct for the decrease in gravity with increase in altitude. 

A barometer located at an elevation above sea level will show a reading lower than a barometer at sea level by an amount approximately 2.5 mm (0.1 in) for each 30.5 m (100 ft) of elevation. A closer approximation can be made by reference to the following tables, which take into account (1) the effect of altitude of the station at which the barometer is read, (2) the mean temperature of the air column extending from the station down to sea level, (3) the latitude of the station at which the barometer is read, and (4) the reading of the barometer corrected for its temperature, a correction which is applied only to mercurial barometers since the aneroid barometers are compensated for temperature effects. 

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. 

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. 

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. 

Suppose you were marooned on a tropical island and had to use seawater (density 1.10 g-cm 3) to make a primitive barometer. What height would the water reach in your barometer when a mercury barometer would reach 73.5 cm The density of mercury is 13.6 g-cm 3. 

FIGURE 8.1 The apparatus is a mercury barometer. The closeup on the left shows the vacuum above the mercury column. In the right-hand closeup, we see the effect of the addition of a small amount of water. At equilibrium, some of the water has evaporated and the vapor pressure exerted by the water on the mercury has lowered the height of the mercury column. The vapor pressure is the same however much water is present in the column. 

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. 

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 corrections above do not apply to aneroid barometers. These instruments should be calibrated at regular intervals by checking them against a corrected mercurial barometer. 

Boyle easily confirmed the widely accepted hypothesis that it was the weight of the atmosphere that held up the column of mercury in the Torricellian barometer. This theory dated from an experiment by Blaise Pascal, who had sent his brother-in-law to carry a mercury barometer up a mountain to observe what happened. Indeed, the mercury stood at a lower level in the tube when measured on top of the mountain than it did at the... 

Note If the atm press is observed on a mercurial barometer having a brass scale, calc the corrected atm press(P) as follows ... 

If a barometer were filled with a liquid of density 1.60 g/ml, what would be the reading when the mercury barometer read 730 torr The density of mercury is 13.56 g/ml. 

A mercury barometer reading of 728.3 mm is obtained at 23°C with a brass scale. What is the barometer reading "corrected to 0°C"—that is, in torr ... 

CAPILLARY. I Hair-like, especially in application to fine tubes. 2, A minute thin-walled blood vessel intervening belween the arteries and veins. 3. A cylindrical space of small radius, or a tube containing such a space. The numerous uses of such tubes has given rise to a number of derived terms. Thus, the capillary correction is a correction applied to mercury barometers, widebore thermometers, etc., for the effect of capillarity an the height of the column. Capillary pressure is a pressure due to capillary force. See also Capillarity. 

PRESSURE. If a body of fluid is at rest, the forces are in equilibrium or the fluid is in static equilibrium. The types of force that may aci on a body are shear or tangential force, tensile force, and compressive force. Fluids move continuously under the action of shear or tangential forces. Thus, a fluid at rest is free in each part from shear forces one fluid layer does not slide relative to an adjacent layer. Fluids can be subjected to a compressive stress, which is commonly called pressure. The term may be defined as force per unit area. The pressure units may be dynes per square centimeter, pounds per square foot, torr. mega-Pascals, etc. Atmospheric pressure is the force acting upon a unit area due to the weight of the atmosphere. Gage pressure is the difference between the pressure of the fluid measured (at some point) and atmospheric pressure. Absolute pressure, which can be measured by a mercury barometer, is the sum of gage pressure plus atmospheric pressure. 

FIGURE 9.3 A mercury barometer is used to measure atmospheric pressure by determining the height of a mercury column supported in a sealed glass tube. The downward pressure of the mercury in the column is exactly balanced by the outside atmospheric pressure that presses down on the mercury in the dish and pushes it up the column. 

Gas pressure inside a container is often measured using an open-end manometer, a simple instrument similar in principle to the mercury barometer. As shown in Figure 9.4, an open-end manometer consists of a U-tube filled with mercury, with one end connected to a gas-filled container and the other end open to the... 

Air pressure is measured by a barograph, aneroid barometer, mercury barometer... 

Mercury barometer gives the most accurate measurement of air pressure. But it is difficult to carry around. 

Figure 6.1 A modern barometer uses the same principles as mercury barometers to measure air pressure.

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