Manometers, atmospheric pressure

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. 

Figure 4 7 Open-end manometers. Open-end manometers are governed by the same principles as mercury barometers (Fig. 4.6). The pressure of the gas, Pg, is exerted on the mercury surface in the closed (left) leg of the manometer. Atmospheric pressure, P, is exerted on the mercury surface in the open (right) leg. With a meter stick, the difference between these two pressures, P[jg, may be measured directly in millimeters of mercury (torr). Gas pressure is determined by equating the total pressures at the lower liquid mercury level, indicated with a dashed line. 

The torr is a remnant from the days when pressure was routinely measured by a mercury manometer. Atmospheric pressure can support a 760 mm high column of mercury and 1 Torr was defined as 1/760 of atmospheric pressure. More usefully, 1 Torr = 1.333 mbar. 

The pressure of a confined gas can be measured by a manometer of the type shown in Figure 5.2. Here again the fluid used is mercury. If the level in the inner tube (A) is lower than that in the outer tube (B), the pressure of the gas is greater than that of the atmosphere. If the reverse is true, the gas pressure is less than atmospheric pressure. 

Repeat Problem 9 but with an open-end manometer (see Figure 4-2C). Atmospheric pressure is 760 mm. 

Draft is usually measured in inches or centimeters of water using a U-tube manometer, with one side connected to the sample point (such as the furnace section, convective-pass section or the boiler stack) and the other side open to the atmosphere. The difference in the manometer column height indicates boiler draft pressure, which may be either higher (overpressure) or lower (underpressure) than atmospheric pressure. 

A liquid hydrocarbon is fed at 295 K to a heat exchanger consisting of a 25 mm diameter tube heated on the outside by condensing steam at atmospheric pressure. The flow rate of the hydrocarbon is measured by means of a 19 mm orifice fitted to the 25 nnn feed pipe. The reading on a differential manometer containing the hydrocarbon-over-water is 450 mm and the coefficient of discharge of the meter is 0.6. 

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. 

The height of the mercury in the system-side column of an open-tube mercury manometer was 10. mm above that of the open side when the atmospheric pressure corresponded to 756 mm of mercury and the temperature was 15°C. What is the pressure inside the apparatus in millimeters of mercury and in pascals ... 

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. 

Sfi.F-TeST 4.2A What is the pressure in a system when the mercury level in the system-side column in an open-tube mercury manometer is 25 mm lower than the mercury level in the atmosphere-side column and the atmospheric pressure corresponds to 760. mmHg at 15°C ... 

A student attaches a glass bulb containing neon gas to an open-tube manometer (refer to Fig. 4.5) and calculates the pressure of the gas to be 0.890 atm. (a) If the atmospheric pressure is 762 Torr, what height difference between the two sides of the mercury in the manometer did the student find ... 

The difference in mercury levels in the manometer is 19 mm. This is the pressure difference in torr between the gas sample and atmospheric pressure. The latter, as measured with a barometer, is 752 torr. Shall we add or subtract the 19 torr pressure difference Notice that the mercury level in the manometer is lower on the side exposed to the atmosphere. Thus, the atmosphere pushes on the mercury harder than does the gas sample, meaning that the pressure of the gas sample is lower than the pressure of the atmosphere. Subtract the pressure difference / gas = A P = 752 toiT - 19 toiT = 733 toiT... 

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... 

AP can be measured by placing a sensitive pressure gauge or manometer at the inlet to the powder bed while venting to atmospheric pressure. 

The pressure within the chamber was balanced at atmospheric pressure, as indicated with a manometer, through the use of an AADC0 vacuum pump. All components exposed to the acids were fabricated from plastic or glass. 

U-Tube Manometers. These are generally made from a glass tube bent in the shape of the letter U and partly filled with a working liquid, most often with mercury. The operation of the manometer is based on the displacement of the levels of the working liquid in both arms of the tube depending on the difference in the pressures over these levels. One arm of a manometer is connected to the vacuum setup in which the pressure is to be measured, while the other arm is either closed (soldered) or remains open, i.e. is constantly at atmospheric pressure. 

Thermocouple manometers are used to measure the pressure in the tubing of a preliminary vacuum where no high accuracy is needed. These manometers are not damaged if the vacuum system gets under atmospheric pressure during their operation. 

Mercury manometer (a) Atmospheric pressure in both tubes (b) Gas pressure in flask, and vacuum in righthand tube. 

Figure 6. Block drawing of the pilot installation for the production of trichloromethyl chloroformate by exhaustive photochlorination [39] 1 Dryer for gaseous Cl2 (H2S04 cone.). 2 Safety tank. 3 Thermoregulated immersion-type photochemical reactor. 4 Raschig column. 5 Cl2 detection system (1,2,4-trichlorobenzene). 6 Neutralization tank (20% NaOH). 7 Reservoir of 20% NaOH. 8 Buffer to atmospheric pressure (20% NaOH). 9 Active carbon filter. 10 Reservoir of crude trichloromethyl chloroformate. 11 Buffer to normal atmosphere via CaCl2 filter and direct entry for trichloromethyl chloroformate to be distilled. 12 Distillation flask with Vigreux column. 13 Exit to vacuum pump. 14 Solid NaOH filter before pump. 15 Cooling water alarm linked to power supply of the light source. 16 Medium pressure mercury arc. 17 Heater for distillation apparatus. 18 Magnetic stirrers. /T thermometer /P manometer.

We studied combustion of the mixture in a vertical tube with an inner diameter of 4.4 cm and length of 65 cm equipped with electrodes and closed with polished caps at both ends. We first introduced the carbon tetrachloride into the evacuated tube the pressure was determined with an oil manometer. Next a mixture of CO, 02, N2 was introduced from a gasometer until atmospheric pressure was reached. After mixing, the mixture was ignited first with a charge (a high tension spark which was transformed into an arc... 

Fig. 7.2. A versatile bubbler manometer. The bubbler manometer Is securely mounted by the reservoir and attached to the vacuum system. It is then easily filled by the following process. The level of the bottom end of the vertical tube dipping into the reservoir is marked on the outside of the reservoir. Next, a calculated amount of mercury is filtered into the reservoir. With the valve between the two arms open, a vacuum is slowly drawn on the manometer. The mercury level must not drop below the mark on the reservoir, or else bubbles will enter the vertical tube and shoot mercury through the vacuum system. If the mercury level in the reservoir comes close to the mark, the manometer is brought up to atmospheric pressure and more mercury is added. When the proper amount of mercury is present in the fully evacuated manometer, the mercury level should be about 10 mm above the mark on the reservoir, and the upper meniscus should be in a region of the manometer suitable for measurement, as illustrated. Once the manometer is properly filled and evacuated, the valve is closed to isolate the reference arm at high vacuum.

Assume that you are using an open-end manometer (Figure 9.4) filled with mineral oil rather than mercury. What is the gas pressure in the bulb (in millimeters of mercury) if the level of mineral oil in the arm connected to the bulb is 237 mm higher than the level in the arm connected to the atmosphere and atmospheric pressure is 746 mm Hg The density of mercury is 13.6 g/mL, and the density of mineral oil is 0.822 g/mL. 

PROBLEM 9.3 What is the pressure (in atmospheres) in a container of gas connected to a mercury-filled, open-end manometer if the level in the arm connected to the container is 24.7 cm higher than in the arm open to the atmosphere and atmospheric pressure is 0.975 atm ... 

Assume that you have an open-end manometer filled with chloroform (density = 1.4832 g/ mL at 20°C) rather than mercury (density = 13.546 g/mL at 20°C). What is the difference in height between the liquid in the two arms if the pressure in the arm connected to the gas sample is 0.788 atm and the atmospheric pressure reading is 0.849 atm In which arm is the chloroform level higher ... 

Close tap F and fill the apparatus with hydrogen to atmospheric pressure, as indicated by the manometer D, by slowly opening tap M close tap M. 

Notice that a positive AP means the system has a pressure higher than atmospheric pressure, whereas a negative AP means the object has a pressure lower than atmospheric pressure. Manometers become inconvenient for measuring gauge pressures of greater than about 1 atm because of the large column of mercury that must be contained. Less dense liquids, such as water, can be used in a manometer to measure smaller pressure differences. 

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