The measurement of pressure

Pressure is measured extensively in the chemical processing industries and a wide variety of pressure measuring methods has been developed. Some of these have already been discussed in Volume 1, Section 6.2.2, viz. the manometer (which is an example of a gravity-balance type of meter), the Bourdon gauge (an example of an elastic transducer) and mention is made of the common first element in most pressure signal transmission systems—the differential pressure (DP) cell (Volume 1, Section 6.2.3). The latter also frequently forms part of a pneumatic transmission system and further discussion of this can be found in Section 6.3.4. 

Higham 17) has listed three categories of pressure measurement, viz.  

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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. 

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 ). 

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

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 ... 

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. ... 

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

In order for chemists to prepare and handle gases under a variety of conditions, they must understand the relationships between the weight, volume, temperature, and pressure of a gas sample. Measurement of the first three of these quantities is relatively straightforward. Because the measurement of pressure can present some complications, we discuss it before we consider the interrelationship of all four variables. 

Elastic elements such as diaphragms and capsules are used as sensing elements in combination with different displacement transducers (e.g. inductive, potentiometric, etc.) for low/medium vacuum measurements down to about 1 torr. Elastic elements have been employed in conjunction with capacitive transducers for the measurement of pressures as low as 10 5 torr. 

When working with apparatus such as catalytic flow reactors, the measurement and control of gas flow becomes as important as the measurement of pressure just described for vacuum line work. Some of the more common methods for measuring and controlling gas flow in gas-handling systems will now be discussed. 

The measurement of pressure from a few torr to 104 torr in an all-metal system can be accomplished by means of a metal Bourdon or diaphragm gauge with a dial readout. Although a bronze chamber is the most common, these gauges are available with more corrosion-resistant metals, and some are available which are accurate to 0.5 torr.7... 

A frequently used simple mercury manometer which is employed for the measurement of pressure in the range 0.5-17 cm (6.6 to 226 mbar) is the U-tube design illustrated in Fig. 2.124 (Anschutz manometer, Gallenkamp). It consists of a U-tube charged with mercury and mounted in a wooden stand. The scale B, graduated in millimetres and sometimes made of mirror glass in order to eliminate errors due to parallax, is sited between the two arms and is movable. This... 

The ease with which a fluid can flow through a porous medium, permeability, can be determined through the measurement of pressure drop (Ap) across the porous medium under steady flow. The intrinsic permeability (k) is defined by Darcy s law and is given by k=(Q/A)( /L/Ap) where Q is the discharge flow rate, A is the... 

Two exceptions are noteworthy. In the case of homogeneous pure fluids, density can usually be determined more accurately by an indirect measurement, namely, the measurement of pressure and temperature which is then coupled with the analytical relationship between these in-... 

Examples of the use of models for the design of large-scale systems include the measurement of pressure drop and heat transfer in model heat exchangers, the mixing and rate of reaction in a bench-top batch reactor and the prediction of pressure drops in pipelines. 

Methods based upon the measurement of pressures at various points, at... 

The most recent work on the measurements of pressure drop in a three-phase fluidized-bed column is by Javdani et al.48 They measured the pressure drop in the flow of a nitrogen-Blandol oil-coal slurry in an 0.7-cm-i.d. tube. Three coal concentrations (of 11, 20.4, and 27.1 weight percent) were examined. The data showed that the pressure drop increases with the gas velocity and the coal concentration in the slurry. 

Pressure build-up and boil-off of the main tank will be analyzed on the basis of the measurement of pressure and filling level over time, see Figure 1.9. From the... 

Another experimental source for interaction dipoles is the measurement of pressure induced absorption . Strictly speaking this effect does not belong to the realm of Van der Waals complexes, because one measures here infrared radiation absorbed by unbound complexes. But since much can be learned about Van der Waals interactions from an interpretation of the data, we briefly review the work in this area. 

Amongst the main parameters of interest in rapid compression studies are the temperature and pressure that are reached at the end of compression. Pressure measurements are made by fast response pressure transducers (>10kHz), and ignition delay times are measured from the pressure-time profiles. The measurements of pressure may be supplemented by the detection of light output through windows, and by chemical analysis at intermediate stages of reaction by rapid expansion and quenching methods [22, 99-101]. 

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