Manometer example

Example 3 Venturi Flowmeter An incompressible fluid flows through the venturi flowmeter in Fig. 6-7. An equation is needed to relate the flow rate Q to the pressure drop measured by the manometer. This problem can he solved using the mechanical energy balance. In a well-made venturi, viscous losses are neghgihle, the pressure drop is entirely the result of acceleration into the throat, and the flow rate predicted neglecting losses is quite accurate. The inlet area is A and the throat area is a. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Maintenance "indicators" are available to help facility staff determine when routine maintenance is required. For example, air filters are often neglected (sometimes due to reasons such as difficult access) and fail to receive maintenance at proper intervals. Installation of an inexpensive manometer, an instrument used to monitor the pressure loss across a filter bank, can give an immediate indication of filter condition without having to open the unit to visually observe the actual filter. Computerized systems are available that can prompt staff to carry out maintenance activities at the proper intervals. Some of these programs can be connected to building equipment so that a signal is transmitted to staff if a piece of equipment malfunctions. Individual areas can be monitored for temperature, air movement, humidity, and carbon dioxide, and new sensors are constantly entering the market. 

Pressure measurement deviees sueh as a manometer are used without disturbing the system being monitored. Another type of reaeting system that ean be monitored involves one of the produets being quantitatively removed by a solid or liquid reagent that does not affeet the reaetion. An example is the removal of an aeid formed by reaetions in the gas phase using hydroxide solutions. From the reaetion stoiehiometry and measurements of the total pressure as a funetion of time, it is possible to determine the extent of the reaetion and the partial pressure or eoneentrations of the reaetant and produet speeies at the time of measurement. 

The question is often asked. How often should calibration be carried out Is it sufficient to do it once, or should it be repeated The answer to this question depends on the instrument type. A very simple instrument that is robust and stable may require calibrating only once during its lifetime. Some fundamental meters do not need calibration at all. A Pitot-static tube or a liquid U-tube manometer are examples of such simple instruments. On the other hand, complicated instruments with many components or sensitive components may need calibration at short intervals. Also fouling and wearing are reasons not only for maintenance but also calibration. Thus the proper calibration interval depends on the instrument itself and its use. The manufacturers recommendations as well as past experience are often the only guidelines. 

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. 

EXAMPLE 4.2 Sample exercise Calculating the pressure inside an apparatus by using a manometer... 

Example 4-1 Manometer. The pressure difference between two points in a fluid (flowing or static) can be measured by using a manometer. The manometer contains an incompressible liquid (density pm) that is immiscible with the fluid flowing in the pipe (density pf). The legs of the manometer are connected to taps on the pipe where the pressure difference is desired (see Fig. 4-2). By applying Eq. (4-7) to any two points within either one of the fluids within the manometer, we see that... 

A manometer is a device employing the change in liquid levels to measure gas pressure differences between a standard and an unknown system. For example, a typical mercury manometer consists of a glass tube partially filled with mercury. One arm is open to the atmosphere and the other is connected to a container of gas. When the pressure of the gas in the container is greater than atmospheric pressure, the level of the mercury in the open side will be higher and... 

This procedure is a good excercise in planning insufficient planning may give rise to undesired delay during the various operations, which may be a major cause of decreased yields. To give only one example one should control the pressure of the acetylene cylinder in advance. At least 100 liters of this gas are needed, and if the manometer on a 10 or 20-1 cylinder indicates a pressure of only a few atmospheres, one is likely to be confronted with the extremely unpleasant fact that the acetone in the cylinder wants to keep the acetylene for itself (If you are lucky, there is another acetylene cylinder in the lab )... 

The height of the mercury in the system-side column of an open-tube mer- cury manometer was 10 mm above that of the open side when the atmo- spheric pressure corresponded to 756 mm of mercury. The pressure in the system therefore corresponds to 756 mm — 10 mm = 746 mm of mercury i (746 mmHg). To express the pressure in pascals, we proceed as we did in Example 4.1 ... 

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

H. Example Calibration of a Trap and Measurement of a Gas Sample. In contrast to the previous example, the objective here is to measure the amount of an entire gas sample, such as the BF3 recovered in the trap-to-trap distillation discussed in Section 5.3.E. in this type of measurement a manometer is included in the calibrated volume, and it is necessary to account for the change of volume as the mercury level changes with changes in pressure. As described in Chapter 7 a constant-volume gas buret can be used, but this is somewhat cumbersome. So the simpler procedure described here is more frequently used. 

Suppose that we wish to calibrate the volume of trap E connected to manometer D on the vacuum line in Fig. 5.2. A known quantity of a gas, such as CO, is condensed into trap E using the calibrated bulb and the techniques just outlined in Example 5.3.G. The stopcocks are then turned so that trap E communicates with the central manometer D but is isolated from the rest of the vacuum system. At this point, the cold trap is removed from E, the trap is allowed to come to room temperature, and the pressure and room temperature are measured. The volume of the manometer-trap combination is determined from the known moles of gas, the pressure, and the temperature, using the ideal gas law. The process is repeated with successively larger samples of CO2, and a plot of volume versus pressure is constructed from the data. Since the bore of the manometer is of constant diameter, this plot should be a straight line. It also is possible to... 

Low-temperature adsorption is also useful for the quantitative transfer of gases into special apparatus. For example, a small quantity of activated molecular sieve in a freeze-out tip on an infrared cell may be used for the identification of methane or, when placed in a freeze-out tip on a manometer and calibrated volume, the molecular sieves permit the measurement of the quantity of noncon-densable gas. 

The pressure of a gas can be measured by attaching a manometer to the vessel containing the gas. A manometer is a tube (U-shaped in our examples) containing a liquid, usually mercury. The height of the liquid is read in mm Hg (i.e torr) pressure units. 

For example, suppose we measure the pressure with a simple U-tube manometer filled with mercury. Suppose the manometer is set up with a 1 cm diameter tube exposed to 1 atm nitrogen at room temperature (298K) on one end, and exposed to vacuum on the other end (which of course will be approximately 760 mm higher). The observed pressure can only change when the column of mercury has time to flow the device (and any other measuring device) will have a nonzero response time. A reasonable estimate for the response time of a manometer might be 0.1 seconds, so the amount the pressure will appear to fluctuate will depend on the number of collisions with the top of the column in that time. 

Obviously, the differential pressure may be measured by any suitable manometer arrangement and does not require an actual column of the fluid concerned. If the fluid were air, for example, this would be impossible. 

Example 10.6 A 3-in ISA flow nozzle is installed in 4-in pipe carrying water at 72°F. If a water-air manometer shows a differential of 2 in, find the flow. 

Manometers consisting of liquid columns of, commonly, mercury or a fluid such as silicone oil, have been used extensively in the past to measure gas mixtures in, for example, experimental, static investigations of the overall kinetics of gas-phase reactions. They continue to be used in many applications, including the establishment of primary pressure standards in several countries. 

Example In a mercury manometer, the level of mercury in contact with a reaction vessel is 70.0 mm lower than the level exposed to the atmosphere. 

Fig. 7.38 In this example, take the measurement from the closed side (439 mm) of the manometer, and subtract from it the measurement from the side of the manometer connected to the vacuum system (426 mm) to obtain the vacuum reading (13 mm Hg). Remember, avoid parallax problems, and with mercury, read the top of the meniscus.

Example 1.4 At 27°C a manometer filled with mercury reads 60.5 cm. The local acceleration of gravity is 9.784 m s-2. To what pressure does this height of mercury correspond ... 

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