Measuring pressure barometer

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.

Figure 1.2 A barometer is a device for measuring pressures. A vacuum-filled glass tube (sealed at one end) is placed in a trough of mercury with its open end beneath the surface of the liquid metal. When the tube is erected, the pressure of the external air presses on the surface and forces mercury up the tube. The height of the mercury column li is directly proportional to the external pressure p...

Meter (jisxp-qXT]g) meter a meter, to meter gas meter, metrical barometer (i.e. measures pressure)... 

Although mercury is known from early times and was used hy alchemists, its first modern scientific applications date hack to 1643 when Torricelli used it in the barometer to measure pressure and about eight decades later Fahrenheit used it in the thermometer to measure temperature. Before this, mercury s use was confined to decorative work, gold extraction and medicines. The element was named after the planet mercury and its symbol Hg is taken from the Latin word hydrargyrum, which means hquid silver. 

Mercury Manometer. An open-end U-tube mercury manometer, known historically as the Torricelli barometer, can typically be read to 0.05 mmHg, but should be corrected for the capillary depression of mercury in glass and for residual gases in the "vacuum" above the column these two small effects are usually corrected for by the manufacturer s scale next to the column. The isoteniscope is just a fancy term for a U-tube of the Torricelli type, containing a liquid, which measures pressure differences by different heights of the liquid in the two arms, one open to the system under study, the other open to air. 

Gases exert a pressure on other objects as they collide. This pressure exerted by a gas can be defined as the amount of force exerted on an area. Anyone who has watched or listened to a weather report can recall hearing about the barometric pressure or atmospheric pressure. These pressures can differ as high and low pressure systems move across a particular region. There are two devices used to measure pressure exerted by gases, the mercury barometer and the manometer. Both devices can be useful depending upon the situation. 

The difference in level of the mercury surfaces in the limbs, as measured with a metre stick, gives the pressure in the limb attached to the apparatus. The open limb is exposed to atmospheric pressure, which must be measured on the laboratory barometer and allowed for. A good standard of accuracy in measuring pressures is possible with this type of gauge if the mercury and the inside surface of the glass U-tube are clean and the levels are measured with a cathetometer. 

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. 

The atmospheric pressure is measured with an instrument called a barometer, which is also called a closed manometer. When you watch your local weather report, you will often hear the meteorologist discuss the barometric pressure, which is a measure of the force exerted by the atmospheric gases per unit of area. The news often reports pressure in inches of mercury, because a barometer, as are many thermometers, is filled with mercury. The SI unit for pressure is the Pascal, which is derived from Newtons and meters. Other units that are commonly used to measure pressure include the atmosphere (atm), torr, and millimeters of mercury (mm of Hg). You should make sure that you know how to convert between the various units of pressure. The necessary conversion factors are shown here. 

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...

Pressure is defined as force per unit area. Fluids (liquids and gases) exert pressure in all directions. The pressure of a gas is equal to the pressure on the gas. A way of measuring pressure is by means of a barometer. The standard atmosphere (atm) is defined as the pressure that will support a column of mercury to a vertical height of 760 mm at a temperature of 0 °C. It is convenient to express the measured gas pressure in terms of the vertical height of a mercury column that the gas is capable of supporting. Thus, if the gas supports a column of mercury to a height of only 76 mm, the gas is exerting a pressure of 0.10 atm ... 

One unit used to measure pressure is defined by using Torricelli s barometer. The standard atmosphere (atm) is defined as the pressure that supports a 760-mm column of mercury. This definition can be represented by the following equation. 

When you measure tire pressure, you are measuring pressure above atmospheric pressure. The recommended tire inflation pressures listed by manufacturers are gauge pressures that is, pressures read from a gauge. A barometer measures absolute pressure that is, the total pressures exerted by all gases, including the atmosphere. To determine the absolute pressure of an inflated tire, you must add the barometric pressure to the gauge pressure. 

Pressure is the force per unit area acting perpendicular to a surface. The unit of pressure in the SI system of units is Newtons per square meter. This unit is also called the Pascal and abbreviated as Pa. Another unit frequently encountered in practice is the torr. This unit corresponds to a millimeter of mercury in a standard barometer. The standard barometer is a glass tube filled with mercury connected to vacuum on one side and to the measured pressure on the other. The mercury is at 0 °C in a location having gravity corresponding to the standard gravitational acceleration, g = 9.807 m s-2. One atmosphere (1 atm) is 760 torr exactly, which corresponds to 101325 Pa. 

A manometer is a device used to measure the pressure of gases other than the atmosphere. The principle of operation of a manometer is similar to that of a barometer. There are two types of manometers, shown in Figure 5.4. The closed-tube manometer is normally used to measure pressures below atmospheric pressure [Figure 5.4(a)] while the open-tube manometer is better suited for measuring pressures equal to or greater than atmospheric pressure [Figure 5.4(b)]. 

Describe how a barometer and a manometer are used to measure pressure. 

Gases exert pressure (force/area) on all surfaces with which they make contact. A barometer measures atmospheric pressure in terms of the height of the mercury column that the atmosphere can support (760 mmHg at sea level and 0°C). Chemists measure pressure in units of atmospheres (atm), torrs (equivalent to mmHg), and pascals (Pa, the SI unit). 

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. 

Determination method is mainly divided into two point method and synchronization method. Using the method of base point to determine the mine ventilation resistance, it has the characteristics of time saving, labor saving, fast speed, the more commonly used method is the method of base point. Point method is that a barometer is placed in well or at a point well, at a certain time measured pressure reading and record the time of determination and the changes of surface and atmospheric pressure, which is to correct of pressure data of downhole measurement Another barometer meter air pressure numerical along pre-selected route by point and records the time measurement. 

The height of water in an evacuated tube can be used to measure pressure. However, a 33-ft tube is inconvenient. Consequently, a more dense liquid, usually mercuiy (Hg), is used in barometers, devices that measure pressure. At sea level on an average day, the height of mercuiy in a perfectly evacuated tube is 760 mm, about 2.5 ft. The barometer, therefore, gives us the unit of pressure, mm Hg. The pressure at sea level averages 760 mm Hg, while the pressure on top of Mt. Everest averages 240 mm Hg. Two other commonly used units of pressure are inches of mercuiy (in. Hg) and atmospheres (atm), both summarized in Table 11-1. 

A number of different units are used to measure pressure. Because atmospheric pressure is often measured by a mercury barometer, pressure can be expressed in terms of the height of a mercury column. Thus, the common unit of pressure is miiiimeters of mercury, symboiized mm Hg. 

The reader should not confuse head of slurry in meters or feet of slurry with meters or feet of water. This is not a barometer or some instrument measuring pressure for this reason everything is kept consistent by using meters or feet of water. By itself, the term / relates only to clear water having the same velocity as the slurry flow. It is convenient to use water as a reference benchmark. (See Figure 4-9.)... 

Pressure is measured in several different units. A common unit of pressure, the millimeter of mercury (mmHg), originates from how pressure is measured with a barometer (Figure 5.4 ). A barometer is an evacuated glass tube, the tip of which is submerged in a pool of mercury. The liquid mercury is forced upward into the evacuated tube by atmospheric pressure on the hquid s surface. Because mercury is so dense (13.5 times more dense than water), attnospheric pressure can support a column of Hg that is only about 0.760 m or 760 mm (about 30 in) tall. (By contrast, atmospheric pressure can support a column of water that is about 10.3 m taU.) This makes a column of mercury a convenient way to measure pressure. 

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