Pressure measurement barometers

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... 

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. 

Fig. 4-2. Pressure measurement. A. Barometer pressure — 755 mm. B. Closed-end manometer pressure = 105 mm. C. Open-end manometer pressure = 755 — 650 - 105 mm.

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. 

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

Pressure Measurements The term mm Hg used with respect to pressure within an apparatus, or atmospheric pressure, refers to the use of a suitable manometer or barometer calibrated in terms of the pressure exerted by a column of mercury of the stated height. 

The agreement is usually to 0-1° and the divergence never exceeds 0-3°. Schumann s vapour pressure measurements were made with the static barometer-tube method for the lower pressures, and by the dynamic method of determining the b.p. under different pressures for the higher pressures. 

Pressure-measuring devices may be divided into two groups those which measure the pressure directly and can be calibrated without reference to another intrument and those which involve some other physical measurement which will be related to the pressure. Of the former type, the barometer, manometer, and McLeod gauge are the most common. 

Units of pressure The SI unit of pressure is the pascal (Pa). It is named for Blaise Pascal, a French mathematician and philosopher. The pascal is derived from the SI unit of force, the newton (N), which is derived from three SI base units the kilogram, the meter, and the second. One pascal is equal to a force of one newton per square meter 1 Pa = 1 N/m. Many fields of science still use more traditional units of pressure. For example, engineers often report pressure as pounds per square inch (psi). The pressures measured by barometers and manometers can be reported in milhmeters of mercury (mm Hg). There also is a unit called the torr, which is named to honor Torricelli. One torr is equal to one mm Hg. 

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... 

Metallic mercury is used in a variety of household products and industrial items, including thermostats, fluorescent light bulbs, barometers, glass thermometers, and some blood pressure measuring devices. You must be careful when you handle and dispose of all items in the home that contain metallic mercury. 

Figure 12-1 Some laboratory devices for measuring pressure, (a) Schematic diagram of a closed-end barometer. At the level of the lower mercury surface, the pressure both inside and outside the tube must be equal to that of the atmosphere. There is no air inside the tube, so the pressure is exerted only by the mercury column h mm high. Hence, the atmospheric pressure must equal the pressure exerted by h mm Hg, or h torr. (b) The two-arm mercury barometer is called a manometer. In this sample, the pressure of the gas inside the flask is greater than the external atmospheric pressure. At the level of the lower mercury surface, the total pressure on the mercury in the left arm must equal the total pressure on the mercury in the right arm. The pressure exerted by the gas is equal to the external pressure plus the pressure exerted by the mercury column of height h mm, or (in torr) = (in torr) + h torr. (c) When the gas pressure measured by the manometer is less than the external atmospheric pressure, the pressure exerted by the atmosphere is equal to the gas pressure plus the pressure exerted by the mercury column, or...

Figure 12-2 Some commercial pressure-measuring devices, (a) A commercial mercury barometer. 

Figure 12-3 A representation of Boyle s experiment. A sample of air is trapped in a tube in such a way that the pressure on the air can be changed and the volume of the air measured, is the atmospheric pressure, measured with a barometer. P = + P, Pj 2 atm-...

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

Modern methods of pressure measurement began as early as 1643 with the development of the mercury-column manometer by Torricelli, which are still used in some antique domestic barometers. The unit 1 Torr = 1 mmHg = 133.322 Pa (by definition exact) is a reminder of this almost outdated technique. 

Measuring Pressure A device that measures atmospheric pressure, the barometer, was invented in 1643 by an Italian scientist named Evangelista Torricelli (1608-1647), who had been a student of the famous astronomer Galileo. Torricelli s barometer is constructed by filling a glass tube with liquid mercury and inverting it in a dish of mercury. 

ABSOLUTE PRESSURE - Total pressure measured from an absolute vacuum. It equals the sum of the gauge pressure and the atmospheric pressure corresponding to the barometer. 

Pressure Measurement and the Flow System.—For moderately precise g.l.c.-determinations of activity coefficients [as obtained from equation (3)], the outlet pressure is usually considered as being atmospheric pressure and can be measured on a normal Fortin barometer. The inlet pressure is usually determined by difference using a manometer placed in the carrier-gas flow line. An excellent review by Ambrose gives details of such pressure measurements. 

Both the barometer and the capacitance manometer respond fairly directly to the force exerted by gas molecules. But not all pressure sensors work that way. A useful pressure-measuring device can be based on any gas property that varies predictably with pressure. 

A variety of manometers and other devices is available to measure the pressure of a fluid, each type useful in a particular pressure range. Some devices measure the pressure of the fluid directly. Others measure the differential pressure between the fluid and the atmosphere the fluid pressure is obtained by combining this measurement with the atmospheric pressure measured with a barometer. 

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