Torricelli manometer

There are two types of U-tube manometer the open-ended manometer is obviously inconvenient for low pressure measurements if an organic liquid of low vapour pressure is used instead of mercury. The small U-tube with one sealed end is the most common and convenient form. The sealed end is closed whilst the tube is under a good vacuum (p < 10 Torr) and, provided that precautions are taken to preserve the vacuum, the pressure in this reference space (the Torricelli vacuum) can be neglected when calculating the measured pressure. 

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. 

Because instruments used for measuring pressure, such as the manometer (see Fig. 5.2), often use columns of mercury because of its high density, the most commonly used units for pressure are based on the height of the mercury column (in millimeters) the gas pressure can support. The unit mm Hg (millimeters of mercury) is called the torr in honor of Torricelli. A related unit for pressure is the standard atmosphere ... 

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. 

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. 

Suppose that when Torricelli had his great idea for con-CQ structing a mercury manometer, he had rushed into the laboratory and found the following items of glass ... 

The torr, which honors the work of Torricelli, and the millimeter of mercury are essentially identical pressure units. Both terms are widely used the choice between them is one of personal preference. The advantage of the millimeter of mercury is that it has physical meaning. You can read it by observing an open-end manometer, the instrument most commonly used to measure pressure in the laboratory (Fig. 4.7). Torr, however, is easier to say and write. We will use torr hereafter in this text. 

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