Manometers open-tube

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 closed-tube manometer, Fig. 5-1(a), is originally filled all the way to the closed end so that the difference in mercury levels is the absolute value of the pressure of the gas. An open-tube manometer shows the difference between the pressure of the gas and the barometric pressure—Fig. 5-l( ) shows less than barometric pressure and Fig. 5-l(c) shows a pressure higher than barometric pressure. 

The apparatus should be assembled as shown in Fig. 2. If desired, the carboy may be mounted in a thermostat bath if so it must be clamped securely to overcome buoyancy. The manometer is an open tube manometer, one side of which is open to the atmosphere the pressure that it measures is therefore the difference of pressure from atmospheric pressure. A suitable liquid for the manometer is dibutyl phthalate, which has a density of 1.046 g cm at room temperature (20°C). To convert manometer readings (millimeters of dibutyl phthalate) to equivalent readings in millimeters of mercury (Torr), multiply by the ratio of this density to the density of mercury, which is 13.55 g cm at 20°C. To find the total pressure in the carboy, the converted manometer readings should be added to atmospheric pressure as given by a barometer. It is unnecessary to correct all readings to 0°C, as all pressures enter the calculations as ratios. 

A diagram for an open-tube manometer is shown below. 

Apparatus The test-tube is placed in the heating bath. An older, and probably more accurate, apparatus is the tension tube, in which the liquid is heated in a bulb with a small manometer open at one end to the air, until the levels are equal. Special types of boiling-point apparatus for high accuracy are described by Swietoslawski. ... 

A variety of other gages are used to measure pressure. Those most commonly encountered are dial gages, which read relative pressure, that is the pressure in excess of barometric, analogous to the open tube manometer in Fig. 5-1 (c). Absolute gages (analogous to Fig. 5-1(a)) are usually so marked on the dial. Most gages used by motorists to check their tires read the relative pressure. 

A chemist wished to determine the molar mass of a liquid compound but feared it would decompose if heated. So he injected (via a syringe) a 0.436 g sample of the liquid into a 5.00 L flask of argon at 17°C to which an open tube manometer was attached. The liquid vaporized completely in the large flask, causing the difference in Hg levels to change from 16.7 mm to 52.4 mm. Calculate the molar mass of the liquid. 

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)]. 

A U-tube manometer, shown in Figure 6-8, is quite easy to operate. The pressure is measured in the open tube type by measuring the differences in height between the two columns of Hg. In this case, the height difference increases as the pressure decreases. The closed end type requires that the Hg be boiled under reduced pressure until the dissolved gases are removed before you fill the tube. If this is not done, then an air bubble eventually forms at the top of the sealed end and ruins the measurements. In this type, the differences in mercury height decreases with a decrease in pressure. This type of manometer can be used to measure pressures from atmospheric down to about 1 to 2 torr (0.13 - 0.26 kPa), but the accuracy decreases below 5 to 10 torr (0.67-1.3 kPa). 

Figure 6-8. U-Tube Manometers. Open end (left) and closed end (right).

Slack tube manometer. The tubes on each side of the manometer are connected to pressrtre taps (or one is open to atmosphere). The difference in liquid columns is measured with a ruler positioned between them. 

Figure 5.2 Two types of manometers used to measure gas pressures (a) Closed tube gas pressure is less than atmospheric pressure, (b) Open tube gas pressure is greater than atmospheric pressure.

Figure 11.6 (a) Closed-tube manometer, (b) Open-tube manometer. 

FIG. M-11 Manometer types, (a) U-tube manometer, open to the atmosphere, (b) Differential U-tube manometer, (c) Cistern manometer. (Source Demag Deiavai.)... 

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