Manometer, closed

Manometer (D), the correct answer, shows the air pressure to be 10 mm Hg greater than that of the gas. Manometer (A) shows the gas pressure to be 10 mm Hg greater than air pressure. Manometer (B) shows the gas pressure to be equal to that of the air pressure. Manometer (C) is a closed manometer showing that the pressure of the gas is 10 mm Hg greater than the unknown pressure on the left side of the manometer. Manometer (E) is also closed and shows that the gas on the right side (770 mm Hg) is exerting a pressure 10 mm Hg greater than the gas on the left side. 

Fig. 7.37 Open manometers are used to measure pressures relative to atmospheric whereas closed manometers are used for pressures far smaller than atmospheric. From Vacuum Science and Engineering, Fig. 3-1, by C. Van Atta, 1965 by McGraw-Hill, New York, reproduced with permission.

Fig. 4.2. Closed manometer (a) and scheme of the device for its graduation (b) 1 - porous plate 2 -...

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. 

When constructing a manometer of the type shown in Fig. 12(c), it is impor tant to apply a very high vacuum (e.g., with a Hy-Vac pump) to the manometer while the mercury in the left-hand (sealed) limb is heated until it boils unless this is done, traces of air will remain in this limb and cause inaccurate readings. During a distillation, the tap I should be kept closed except when a pressure reading is being taken if it is left open indefinitely, a sudden default by the distillation apparatus or by the pump may cause the mercury in the sealed limb of G to fly back and fracture the top of the limb. 

Liquid Manometers. Liquid manometers were used extensively by scientists in the seventeenth through the early twentieth centuries to measure pressure. The practice of expressing pressure as a certain height of Hquid evolved from this usage. The typical Hquid manometer consists of a cyclindrical glass U-tube partially fiUed with Hquid. One end is coimected to the process the other can be either open or closed (Fig. 1). 

Fig. 1. U-Tube manometer where h is the height of the ( ) Hquid employed (a) open (b) closed.

The most common manometer fluids arc water, alcohol, and mercury. The density of water and alcohol arc quite close to each other, whereas the density of mercury is much higher. Many factors have to be considered when selecting a fluid for a manometer, including... 

The most frequently applied mechanical manometers in ventilation applications are fluid manometers, bur the following types are also used. The Bourdon tube is a small-voiume tube with an elliptic cross-section bent to the shape of a circular arc, the C-type. One end is open to the applied pressure while the other end is closed. The pressure inside the tube causes an elastic defonnation ot the tube and displaces the closed end, which is then converted, by means of a linkage mechanism, into the movement of a pointer. The Bourdon tube may be of a spiral or helical design as well. 

A manometer open to the atmosphere, used to measure gas pressure in a closed system. The... 

The pressure of a gas sample can be measured in a device similar to a barometer, called a manometer. Figures 4-2B and 4-2C show two types. Figure 4-2 B shows a closed-end manometer. Here the downward pressure exerted by the column of mercury is balanced by the pressure of the gas sample placed in the flask. The gas pressure is, in the example shown, 105 mm. As in the barometer, only mercury vapor is present in the right-hand tube. 

The apparatus shown in Figure 4-2C differs in that the right-hand tube is open. In this type of manometer, atmospheric pressure is exerted on the right-hand mercury column. Hence the pressure in the flask plus the height of the mercury column equals atmospheric pressure. In the example shown, the pressure is 755 — 650 = 105 mm, the same as pictured in the closed-end manometer, Figure 4-2B. 

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.

Gas is slowly added to the empty chamber of a closed-end manometer (see Figure 4-2B). Draw a picture of the manometer mercury levels, showing in millimeters the difference in heights of the two mercury levels ... 

Mittasch Stability Test for Nitrocellulose. A test based on the measurement of press developed on decompn of NC. The app may be considered a very complicated modification of devices originated by Abel (Ref 4, p 241) and Hess (Refs 1 4). Abel heated NC samples in vac and detd the press increase. Hess believed that on heating NC in a closed vessel contg air, connected to a Hg manometer, it would be possible to achieve conditions more closely resembling those in storage in closed magazines. Neither of these methods found any practical application... 

A flow sheet of the process, as given by Ur banski (Ref 75, his Fig 53), is shown below According to Urbanski To start the nitration, current from the switch (18) is applied to the electromagnet (6), which closes the air inlet to the injector. Mixed acid is admitted by opening the valve under the acid rotameter (4). The injector now comes into operation. The manometer (13) must show full vacuum. The needle valve (7) is then opened and the vacuum adjusted to about 300mm Hg. The glycerine-glycol mixture is sucked in thru the rotameter (3) to the injector from (2)... 

Although a pressure gauge is more commonly used to measure the pressure inside a laboratory vessel, a manometer is sometimes used (Fig. 4.5). It consists of a U-shaped tube connected to the experimental system. The other end of the tube may be either open to the atmosphere or sealed. For an open-tube manometer (like that shown in Fig. 4.5a), the pressure in the system is equal to that of the atmosphere when the levels of the liquid in each arm of the U-tube are the same. If the level of mercury on the system side of an open manometer is above that of the atmosphere side, the pressure in the system is lower than the atmospheric pressure. In a closed-tube manometer (like that shown in Fig. 4.5b), one side is connected to a closed flask (the system) and the other side is vacuum. The difference in heights of the two columns is proportional to the pressure in the system. 

FIGURE 4.5 (a) An open-tube manometer. The pressure inside the apparatus to which the narrow horizontal tube is connected pushes against the external pressure. In this instance, the pressure inside the system is lower than the atmospheric pressure by an amount proportional to the difference in heights of the liquid in the two arms, (b) A closed-tube manometer. The pressure in the adjoining apparatus is proportional to the difference in heights of the liquid in the two arms. The space inside the closed end is a vacuum. 

Self-Test 4.2B What is the pressure in millimeters of mercury inside a system when a closed mercury manometer shows a height difference of 10. cm at 15°C ... 

A reaction is performed in a vessel attached to a closed-tube manometer. Before the reaction, the levels of mercury in the two sides of the manometer were at the same height. As the reaction proceeds, a gas is produced. At the end of the reaction, the height of the mercury column on the vacuum side of the manometer has risen 30.74 cm and the height on the side of the manometer connected to the flask has fallen by the same amount. What is the pressure in the apparatus at the end of the reaction expressed in (a) Torr (b) atm (c) Pa (d) bar ... 

An unknown liquid is used to fill a closed-tube manometer. The atmosphere is found to produce a height difference of 6.14 m in this manometer at the same time that a mercury manometer gives a displacement of 758.7 mm. What is the density of the unknown liquid ... 

If you want to measure the pressure in your vacuum distillation setup, you ll need a closed-end manometer. There are a few different types, but they all work essentially the same way. I ve chosen a stick type (Fig. 75). This particular model needs help from a short length of rubber tubing and a glass T to get connected to the vacuum distillation setup. 

Suppose, by luck of the draw, you ve had to prepare and purify 1-octanol (B.P. 195°C). You know if you simply distill 1-octanol, you run the risk of having it decompose, so you set up a vacuum distillation. You hook your setup to a water aspirator and water trap and attach a closed-end stick manometer. You turn the water for the aspirator on full-blast and open the stick manometer. After a few minutes, nothing seems to be happening. You pinch the tubing going to the vacuum distillation setup, (but not to the manometer) closing the setup off from the source of vacuum. Suddenly, the mercury in the manometer starts to drop. You release the tube going to the vacuum distillation setup, and the mercury jumps to the upper limit. You have air leaks in your vacuum distillation setup. 

The flow of fluids is most commonly measured using head flowmeters. The operation of these flowmeters is based on the Bernoulli equation. A constriction in the flow path is used to increase the flow velocity. This is accompanied by a decrease in pressure head and since the resultant pressure drop is a function of the flow rate of fluid, the latter can be evaluated. The flowmeters for closed conduits can be used for both gases and liquids. The flowmeters for open conduits can only be used for liquids. Head flowmeters include orifice and venturi meters, flow nozzles, Pitot tubes and weirs. They consist of a primary element which causes the pressure or head loss and a secondary element which measures it. The primary element does not contain any moving parts. The most common secondary elements for closed conduit flowmeters are U-tube manometers and differential pressure transducers. 

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