Gas Pressure and Its Measurement

Pressure is defined as the force exerted per unit area of surface. A coin resting on a table exerts a force, and therefore a pressure, downward on the table due to gravity. The air above the table exerts an additional pressure on the table, because the air is also being pulled downward by gravity. 

To obtain the SI unit of pressure and a feeling for its size, let us calculate the pressure on a table from a perfectly flat coin with a radius and mass equal to that of a new penny (9.3 mm in radius and 2.5 g). The force exated by the coin from gravity equals the mass of the coin times the constant acceleration of gravity. Acceleration is the change of speed per unit time, so the SI unit of acceleration is meters per second per second, abbreviated m/s. The constant accelCTation of gravity is 9.81 m/s, and the force on the coin due to gravity is 

Gases such as oxygen and nitrogen can be transported as compressed gases in steel cylinders. Large volumes of gas at normal pressures can be compressed into a small volume. Note the pressure gauges. 

Carbon monoxide CO Colorless Odorless Very toxic 

Chlorine CI2 Pale green Irritating Very toxic 

Blowing up a balloon provides clear evidence that a gas exerts pressure on the walls of its container. Pressure (F) is defined as the force exerted per unit of surface area  

Earth s gravitational attraction pulls the atmospheric gases toward its surface, where they exert a force on all objects. The force, or weight, of these gases creates a pressure of about 14.7 pounds per square inch (lb/in psi) of surface. 

The molecules in a gas are moving in every direction, so the pressure of the atmosphere is exerted uniformly on the floor, walls, ceiling, and every object in a room. The pressure on the outside of your body is equalized by the pressure on 

Notice that we did not specify the diameter of the barometer tube. If the mercury in a 1-cm diameter tube rises to a height of 760 mm, the mercury in a 2-cm diameter tube will rise to that height also. The weight of mercury is greater in the wider tube, but the area is larger also thus the pressure, the ratio of weight to area, is the same. 

Since the pressure of the mercury column is directly proportional to its height, a unit commonly used for pressure is mmHg, the height of the column in millimeters (mm). At sea level and 0 C, normal atmospheric pressure is 760 mmHg at the top of Mt. Everest (29,028 ft, or 8848 m), the atmospheric pressure is only about 270 mmHg. Thus, pressure decreases with altitude the column of air above the sea is taller and weighs more than the column of air above Mt. Everest 

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An Overview of the Physical States of Matter Gas Pressure and Its Measurement... 

Because analytes plus residual mobile phase (and often also collision gas in the case of tandem mass spectrometry) are being injected continuously into a mass spectrometer, the vacuum pumps must be capable of continuously evacuating significant input gas flows in order to maintain a sufficiently low pressure within the analyzer and detector. Before describing the types of pump commonly employed, it is necessary to discuss the concepts of gas flow and its measurement. In this regard two helpful websites are recommended for both introductory and more detailed discussions www.newequation.net and www.vacuumlab.com. 

The volumetric method is mainly used for the purpose of determining specific surface areas of solids from gas (particularly nitrogen) adsorption measurements (see page 134). The gas is contained in a gas burette, and its pressure is measured with a manometer (see Figure 5.4). All of the volumes in the apparatus are calibrated so that when the gas is admitted to the adsorbent sample the amount adsorbed can be calculated from the equilibrium pressure reading. The adsorption isotherm is obtained from a series of measurements at different pressures. 

Schmidt et al. (102) carried out a detailed experimental study of PET blow molding with a well-instrumented machine and compared the results with theoretical predictions using FEM and an Oldroyd B constitutive equation. They measured and calculated internal gas pressure, coupled it with the thermomechanical inflation and performed experiments and computations with free parison inflation. 

The attractive forces between molecules in the bulk liquid are uniform in all directions (zero net force). However, the molecules at the liquid surface cannot form uniform interaction because the molecules on the gas side are widely spaced and the molecular interactions are mainly between surface molecules and the subsurface liquid molecules (non-zero net force). As a result, the molecules at the liquid surface have greater free potential energies than the molecules in the bulk liquid. This excess free energy per unit area that exists in the surface molecules is defined as surface tension (y). Surface tension is a thermodynamic property and can be measured under constant temperature and pressure and its value represents... 

Equation (6.4) is the general one for vapor pressure, and it shows that the undetermined constant in In P, in Eq. (5.9), is just the chemical constant that we have already determined in Eq. (3.16) of Chap. VIII. The simplest experimental method of finding the chemical constants is based on Eq. (6.4) one measures the vapor pressure as a function of the temperature, finds the specific heats of solid and gas, so that one can calculate the term in the specific heats, and computes the quantity... 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

The gravimetric method has in its favour the fact that all variables adsorbed mass, gas pressure and temperature are measured independently. Apparatus are more complicated and more expensive on account of the additional vacuum microbalance. Balance operating requires some skills. The sample is not in direct contact with the thermostat and must be shielded against false heat radiation. Measures to avoid the influence of eddy gas flow may be required. At low pressure the Knudsen pressure difference causes thermal gas flow which seriously interferes in measurements in the Henry region. 

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... 

It is possible, however, to determine a relationship exists between the fluid pressure and its density, thus allowing indirect measurement and control of the density. For an ideal gas, the relationship is simply, PV/RT= 1, where Fis the molar volume (reciprocal of the molar density). From the molecular weight of the gas (M), the mass density, p, can be calculated as M/V. The simple equation breaks down at the high densities characteristic of supercritical fluids, but, the work of Pitzer et allows the ideal gas law to be extended... 

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