Examples, manometer gases

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 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. 

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 )... 

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. 

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. 

A final example of the application of effusive flow is found in the so-called absolute manometer designed by Knudsen for the measurement of very low pressures. If a disk is suspended near a heated surface at a distance which is small compared to the mean free path of gas molecules, then a molecular effusion will take place between the gas molecules in the space between the disk and surface and the rest of the gas (Fig. VII.8). The rate at which molecules enter this space will be proportional to PgTg, where To refers to the temperature of the gas and Po refers to the pressure. The rate at which molecules leave will be proportional to PsTr, where 5T, refers to the mean temperature of the space between disk and surface and... 

It is obvious that n and have to be determined accurately, n is usually determined by measuring the pressure, for example with a capacitance manometer such as a baratron, taking care to allow for temperature differences between the gas cell and the measuring region (Blaauw et al, 1980). Usually one has an effusive molecular flow through the entrance and exit apertures, which leads to large density gradients in the gas, and the product mf in the exponent of (2.10) has to be replaced by... 

Example 2.14. Figure 2.14 shows a tank of gas connected to a manometer. The manometer is a U-shaped glass tube open to the atmosphere at one end and containing water. From the elevations shown, calculate the gauge pressure in the vessel. ... 

The pressure measurements should always be done by capacitance manometers (CAs) and not by thermal conductive gauges. TM depend on the gas mixture (water vapor and permanent gases) and are not reproducible enough for BTM measurements details are given on pp 327, 328 in [2]. For leak testing no special equipment specifications are required. The leak rate for the chamber, e.g., <1 x 10 mbarL/s, and for the condenser, e.g., <1 X 10 mbarL/s, should be specified since it might be helpful to measure the chamber and condenser separately. The maximum tolerable leak rate in the two examples is <1 x 10 mbarL/s, if the DR data at 0.1%/h should have an error <10%. If a helium leak tester is not available in production, it should be specified for quotation. 

An instrument to measure the dielectric permittivity of a gas adsorption system basically consists of an electric capacitor (plates, cylinders, spheres) placed within an adsorption vessel. The vessel should be placed within a thermostat (water, oil etc.) and provided with tubes for gas supply and evacuation. Also manometers and thermometers are needed to measure the gas pressure (p) and temperature (T) inside the chamber. The capacitor is filled with sorbent material (powder, pellets, continuous matter etc.) which can be considered to be homogenous as long as its characteristic length - for example the diameter of cylindrical pellets - is small compared to a characteristic length of the capacitor. 

Preparation of Concentrate—Prepare a concentrate of the impurities expected to be encountered. A certified calibration blend containing the expected impurities can be obtained and used as the concentrate. An example of a satisfactory concentrate is given in Table 1. The concentrate can be prepared using the gas blending manifold as shown in Fig. 1 or using a similar apparatus as follows Evacuate the apparatus and add the components in the order of increasing vapor pressure that is, propylene, carbon dioxide, ethane and methane. Record the increase in pressure on the manometer as each component is added. Close the reservoir and evacuate the manometer after each addition. 

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