Barometer Using

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 barometer is an instrument that measures air pressure. Traditional barometers used a long, enclosed vertical glass tube containing mercury. 

Many modern barometers use different materials (mercury is known to be highly toxic to humans) but work in basically the same way. These instruments are often used in weather prediction to distinguish high-and-low pressure weather systems. A high-air pressure system means good weather is coming while a low-air pressure system means storms are more likely. 

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

Torricelli makes the first barometer using mercury in a sealed glass tube The Dutch scientist Anton van Leeuwenhoek develops a microscope Isaac Newton invents a reflecting telescope... 

Prelab Exercise Compare the operation of a barometer, used to measure atmospheric pressure, and a mercury manometer, used to measure the pressure in an evacuated system. 

Describe a simple mercury barometer. How is such a barometer used to measure the pressure of the atmosphere ... 

Suppose you make a mercury barometer using a glass tube about 50 cm in length, closed at one end. What would you expect to see if the tube is filled with mercury and inverted in a mercury dish, as in Figure 10.2 Explain. 

A mercury manometer or barometer— used for pressures in the range 15,000-110,000 Pa. 

If you prepared a barometer using water instead of mercury, how high would the column of water be at one atmosphere pressure (Neglect the vapor pressure of water.)... 

In 1643, Italian physicist and mathematician Evangelista Torricelli proved that atmospheric pressure would support the weight of a thirty-five-foot water column leaving a vacuum above that in a closed tube and that this height would change with the weather. Later Torricelli barometers used liquid mercury to reduce the size of the column and make such instruments more practical. 

The simplest instrument for measurement of pressure with moderate accuracy by determination of the height of a mercury column is the fixed cistern (Kew type) barometer models are available on which heights from a nominal zero up to 1000 mm can be read. The error associated with the reading, however (and with that on the normal Fortin barometer used in the laboratory), is not less than 0.15 mm because of the effect of capillarity in the relatively narrow tube, and because of the uncertainties in determining the height of the mercury meniscus above its level in the cistern, and in the temperature. Reduction of the error requires refinement... 

Mercury is extensively used in various pieces of scientific apparatus, such as thermometers, barometers, high vacuum pumps, mercury lamps, standard cells (for example the Weston cell), and so on. The metal is used as the cathode in the Kellner-Solvay cell (p. 130). 

The metal is widely used in laboratory work for making thermometers, barometers, diffusion pumps, and many other instruments. It is used in making mercury-vapor lamps and advertising... 

For records on weather maps, meteoroiogists customariiy correct barometer readings to sea ievei, and some barometers may be caiibrated accordingiy. Such instruments are not suitabie for iaboratory use where true pressure under standard conditions is required. Scaie corrections shouid be specified in the maker s instructions with the instrument, and are aiso indicated by the iack of correspondence between a gauge mark usuaiiy piaced exactiy 76.2 cm from the zero point and the 76.2-cm scaie graduation. 

In the late fifteenth century AD mercury was successfully used as a treatment for syphilis. In the late sixteenth century the development of the Patio process for the recovery of silver by amalgamation (see Silverand silveralloys) greatiy increased the consumption of mercury. Usage of mercury increased in 1643 when Torricelli invented the barometer, and again in 1720 when Fahrenheit invented the mercury thermometer. Other scientific and medical appHcations foUowed. Industrial usage after 1900, particularly in electrical appHcations, expanded rapidly, offsetting the sharp decline in its use in amalgamation. 

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

Mercury is used in the manufacture of thermometers, barometers and switchgear, and in the production of amalgams with copper, tin, silver and gold, and of solders. A major use in the chemical industry is in the production of a host of mercury compounds and in mercury cells for the generation of chlorine. Mercury has a significant vapour pressure at ambient temperature and is a cumulative poison. 

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

A number of units are used to express a pressure measurement. Some are based on a force per unit area for e.xample, pound (force) per square inch (psi) or dyne per square centimeter (dyne/enr). Otliers are based on a fluid height, such as inches of water (in H O) or millimeters of mercury (iimiHg) units such as these are convenient when tlie pressure is indicated by a difference between two levels of a liquid, as in a imuiometer or barometer. Barometric pressure is a measure of the ambient air pressure. Standard barometric pressure is 1 atm and is equivalent to 14.696 psi and 29.921 in Hg. 

The use of mercury for extracting precious metals by amalgamation has a long history and was extensively used by Spain in the sixteenth century when her fleet carried mercury from Almaden to Mexico and returned with silver. However, environmental concerns have resulted in falling demand and excess production capacity. It is still used in the extraction of gold and in the Castner-Kellner process for manufacturing chlorine and NaOH (p. 72), and a further major use is in the manufacture of batteries. It is also used in street lamps and AC rectifiers, while its small-scale use in thermometers, barometers and gauges of different kinds, are familiar in many laboratories. 

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. 

The sucdon pressure of an ejector is expressed in absolute units. If it is given as inches of vacuum it must be converted to absolute units by using the local or reference barometer. The suction pressure follows the ejector capacity curve, varying with the non-condensable and vapor load to the unit. 

Figure 6-37. Typical capacity performance curve for a process liquid ring vacuum pump. Note that the vacuum is expressed here as gauge, referenced to a 30" Hg barometer, when 60°F seal water is used. For higher temperature water, the vacuum will not be as great. By permission, Nash Engineenng Co.

The hot well is the sump where the barometric leg is sealed. It must be designed to give adequate cross-section below the seal leg and for upward and horizontal flow over a seal dam or weir. At sea level the hot well must be a minimum of 34.0 ft below the base of the barometric condenser. For safety to avoid air in-leakage, a value of 35-36 ft is used. For an altitude corresponding to a 26-in. Hg. barometer, the theoretical seal height is 29.5 ft actual practice still uses about 34 ft. 

B = minimum barometric pressure (mbar) (use 0.94 of mean barometer reading),... 

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. 

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. 

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. 

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. 

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