Measurement of pressure

The unit of pressure used in the preceding paragraph is that of the International System of Units (SI), the newton per square metre, now known as the pascal. Pa. Conversion of pressures expressed in one unit to values expressed in another unit is frequently required, and the following conversion factors will be found useful  

In a later section the advantage of comparative ebulliometry for vapour-pressure measurement will be stressed because the method does not require that the pressure be directly measured. Discussion of pressure measurement is apposite, however, because the comparative method is not always applicable and, from the point of view of thermodynamic measurements in general, pressures frequently must be measured with accuracy - in this discussion we shall be concerned with techniques of the precision and accuracy which can reasonably be achieved in a laboratory engaged in this type of work, not with the ultimate limits sought in a standards laboratory. 

Besides the familiar Bourdon gauge, numerous instruments for the measurement of pressures have been described which are based on elastic deformation either of a diaphragm (of glass, of metal, or of a plastic ), 

A simple barometer, an instrument used to measure atmospheric pressure, consists of a long glass tube, closed at one end and filled with mercury. The tube is carefully inverted in a container of 

Evangelista Torricelli (1608-1674). Italian mathematician. Torricelli was supposedly the first person to recognize the existence of atmospheric pressure. 

The height of a column of fluid supported by a given pressure is inversely proportional to the density of the fluid. Mercury has been used historically because its high density made it possible to construct barometers and manometers of manageable size. For example, a btirometer filled with mercury that stands 1 m tall would have to be over 13 m tall if it were filled with water. 

Sample Problem 11.1 shows how to calculate the pressure exerted by a column of fluid. 

Think About It Make sure your units cancel properly in this type of problem. Common errors include forgetting to express height in meters and density in kg/m.  

Think About It Make sure your units cancel properly in this type of problem. Common errors include forgetting to express height in meters and density in kg/m. You can avoid these errors by becoming familiar with the value of atmospheric pressure in the various units. A column of mercury slightly less than 760 mm is equivalent to slightly less than 101,325 Pa and slightly less than 1 atm. 

Calculate the pressure exerted by a column of mercury 70.0 cm high. Express the pressure in pascals and in atmospheres. The density of mercury is 13.5951 g/cm.  

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Measurements of pressure driven fluxes may also be performed on mixtures, and here care must be exercised in interpretation, since the fluxes of the two substances are not in proportion to their mole fractions in the mixture. Consequently the fluxes and must, in principle, be measured... 

For tank work, inches water column (in. wc) or ounces per square inch (osi) are commonly used to express the value of pressure or vacuum in the vapor space of a tank. These pressures are usually very low relative to atmospheric pressure. The common measures of pressure are compared as follows ... 

Precision mass and volumetric methods use very precise measurements of pressure and/or level in the tank to detect leaks. The tank must be closed so that no Hquid enters or leaves the tank. The threshold of detection and fuimel required to perform a rehable test become greater as tank size increases. 

Vahdation of the measurements may require the simultaneous measurement of pressure and temperature. Typical sample locations do not have thermowells and pressure indicators. Consequently, modifications will be reqmred to facilitate validation. 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

For the present purposes, this includes the measurement of pressure, acceleration and strain. 

In a series of experiments on the flow of flocculated kaolin suspensions in laboratory and industrial scale pipelines(26-27-2Sl, measurements of pressure drop were made as a function of flowrate. Results were obtained using a laboratory capillary-tube viscometer, and pipelines of 42 mm and 205 mm diameter arranged in a recirculating loop. The rheology of all of the suspensions was described by the power-law model with a power law index less than unity, that is they were all shear-thinning. The behaviour in the laminar region can be described by the equation ... 

Continuous Polymerizations As previously mentioned, fifteen continuous polymerizations in the tubular reactor were performed at different flow rates (i.e. (Nj g) ) with twelve runs using identical formulations and three runs having different emulsifier and initiator concentrations. A summary of the experimental runs is presented in Table IV and the styrene conversion vs reaction time data are presented graphically in Figures 7 to 9. It is important to note that the measurements of pressure and temperature profiles, flow rate and the latex properties indicated that steady state operation was reached after a period corresponding to twice the residence time in the tubular reactor. This agrees with Ghosh s results ). 

Manometry Measurement of pressures within organs of the body. Manometry is often used to indicate muscular activity in motile tubes such as the esophagus. 

Flow control will usually be based on the measurement of pressure drop across an orifice plate. 

A special adaptation of this analysis was applied by Donachie (D8) to estimate the pipe resistances using field measurements of pressures. In that application the u s correspond to pipe resistances and the x s correspond to the nodal pressures. Using linearized sensitivity information adjustments of u s are made, one at a time, to reduce the discrepancies between calculated and measured pressures according to the least-squares criterion. The Am which causes the largest change in the sum of squares of discrepancies is given precedence over others. 

Wilke HJ et al (1999) New in vivo measurements of pressures in the intervertebral disc in daily life. Spine 24(8) 755-762... 

Fig. 1 Schematic representation of the Langmuir film balance used for the measurement of pressure-area monolayer film properties. Reprinted with permission from Arnett et al., 1989. Copyright 1989 American Chemical Society.

The accuracy in the measurement of pressure p depends on the lowest temperature down to which the CVGT is used. For example, with an allowed pressure variation Ap — 0.133 Pa, the minimum allowed temperature is 2K for 4He and 1K for 3He respectively. With a fixed number of moles N, the CVGT gives Tcarrying out two pressure measurements at the temperature T to be determined and at a known temperature ref. The requirements for the constancy (practically the knowledge) of N in a CVGT are the following ... 

Pressure drop and dispersion were the focus of work by Magnico (2003) who simulated flow at lower Re by direct numerical simulation (DNS) in beds of spheres with an in-house code. Tobis (2000) simulated a small cluster of four spheres with inserts between them to compare to his experimental measurements of pressure drop. Gunjal et al. (2005) also focused on flow and pressure drop through a small cell of spheres, in order to validate the CFD approach by comparison to the MRI measurements in the same geometry made by Suekane... 

Measurements of pressure-area (jc-A) isotherms and transfers of monolayers on a substrate were carried out by using a computer-controlled film balance system (San-Esu Keisoku, Co., Fukuoka, FSD-20). Maximum surface area on the trough was 475 X 150 mm2. The trough surface and the moving barrier were coated with Teflon, and the subphase was temperature-controlled with a thermostat (20 0.5 °C). The concentration of lipid solutions was 1 mg/ml and the spreading amount of lipid solutions was 50 - 150 pi. After solvent evaporation, the monolayer was compressed at the speed of 0.60 cm2 s-i. Measurements of n-A isotherms and transfers of monolayer on a QCM substrate were performed automatically with the usual manner [26,27]. 

At about the same time, the pyrolysis of diborane was studied by Bragg et al.88 in the temperature range 90-130 °C. These workers again used a static system (reaction vessel volume 212 cm3) and followed the conversion both by measurement of pressure increase and by determination of the amount of hydrogen formed. The system was also examined by mass spectrometric analysis. The empirical rate law was found to be... 

The thermal decomposition and photolysis of this alkyl have been studied by Buchanan and Creutzberg112. The pyrolysis mechanism is not fully understood. The overall process is first-order and is unaffected by an 8.5-fold increase in surface-to-volume ratio. Based on measurements of pressure increase, the reaction exhibits an induction period ranging from 2-3 minutes at 513 °C to 40 minutes at 466 °C. Short chains are apparently involved. A polymer initially of empirical formula (BCH2) but slowly losing hydrogen to form (BCH) is deposited on the surface. The mechanism probably involves the reactions... 

Figure 10. Experimental unit for measurement of pressure drop over a catalyst bed.

Like other measures of pressure, c has units of MPa. In theory, a liquid will break all solvent-solvent interactions on vaporization, and so c is a measure of the sum of all the attractive intermolecular forces acting in that liquid. Hydrogen-bonding and dipolar solvents therefore have high c values. Water has a large value for c, and fluorocarbons very low values (Table 1.5). 

Because of the complex flow pattern, it is not generally possible to determine visually the velocity at which the system ceases, in effect, to be a fluidised bed. From measurements of pressure fluctuations, the following approximate relation may be used, although with considerable care ... 

Several methods have been developed for calculating fugacities from measurements of pressures and molar volumes of real gases. 

For reactions in which one or more reactants or products is a gas, manometry (the measurement of pressure differences) can provide a convenient means for monitoring the course and kinetics of the reaction Thus, enzymes that can be assayed with this method include oxidases, urease, carbonic anhydrase, hydrogenase, and decarboxylases. For example, bacterial glutamate decarboxylase is readily assayed by utilizing a Warburg flask and measuring the volume of gas evolved at different times using a constant-pressure respirometer. ... 

Measurement of pressures in the rough vacuum range can be carried out relatively precisely by means of vacuum gauges with direct pressure measurement. Measurement of lower pressures, on the other hand, is almost always subject to a number of fundamental errors that limit the measuring accuracy right from the start so that it is not comparable at all to... 

This subsection summarizes selection and installation of flowmeters, including the measurement of pressure and velocities of fluids when the flow measurement technique requires it. 

A later paper presented the results of a study of the mechanism of the sympathomimetic cardiovascular actions of I.H Cats and dogs anesthetized with allobarbital-urethane were used for measurements of pressure near the bifurcation of the descending aorta and of blood flow with electromagnetic probes. Intravenous Injection of I at 20 mg/kg was found to produce an immediate, sharp increase in blood pressure lasting for about 25 s and followed after a lag of about 6 s by an increase in blood flow. A slow drift downward of the peak systolic pressure followed. Repetition of the dose of I after an hour yielded responses similar to those after the first dose, but... 

High-pressure research requires suitable calibration and measurement of pressures. 

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