Pressure, measurement instrumentation application

As well as measurement errors due to the pressure measurement instrument itself, other errors related to pressure measurements must be considered. In ventilation applications a frequently measured quantity is the duct static pressure. This is determined by drilling in the duct a hole or holes in which a metal tube is secured. The rubber tube of the manometer is attached to the metal tube, and the pressure difference between the hole and the environment or some other pressure is measured. 

GaBmann, E Gries, A. Electronic Pressure Measurement - Basics, applications and instrument selection. Munieh Suddeutscher Verlagonpact GmbH 2010. 

On industrial plants, the instruments are therefore required not only to act as indicators but also to provide some link which can be used to help in the control of the plant. In this chapter, pressure measurement is briefly described and methods of measurement of flowrate are largely confined to those which depend on the application of the energy... 

Head flow meters operate on the principle of placing a restriction in the line to cause a differential pressure head. The differential pressure, which is caused by the head, is measured and converted to a flow measurement. Industrial applications of head flow meters incorporate a pneumatic or electrical transmitting system for remote readout of flow rate. Generally, the indicating instrument extracts the square root of the differential pressure and displays the flow rate on a linear indicator. 

This chapter will be limited to the description of CSIL therapy to ex vivo studies in adult mammalian hearts. Due to page limitations, cell culture, gene delivery and in vivo studies will not be included. Therapeutic efficacy of CSIL in preservation of myocardial viability as well as function (by left ventricular developed pressure measurements) as assessed in globally ischemic Langendorff instrumented hearts is both dose and time dependent. This approach of cell membrane lesion repair and sealing may have broader applications in other cell systems. 

Level is common in many safety instrumented applications as well. It is used in separation units to prevent high pressure Tjlow-by" and is common in tank farms. There are a number of different technologies used to measure level. 

In order to control the compressor, its purpose in terms of a process variable needs to be known. The purpose of the compressor in this application example is to control the pressure at the top of the column. A suitable measuring instrument would be a pressure transmitter located at the knock out (KO) drum. The compressor throughput is controlled by speed control on the steam turbine. Steam turbines generally have... 

Differential Pressure. The differential pressure method measures the pressure of a vertical column of the fluid as well as the height of the column to obtain the density. This method has the advantages of relatively simple equipment, small component size, and the possibility of application as a field-type instrument. But it also has several disadvantages. The method is dependent upon two separate measurements, pressure differential and fluid liquid level. Errors in the accuracy of either of these two separate measurements will affect the overall method accuracy. Because of the extreme low density of liquid hydrogen, for instance, the accuracy, sensitivity, and hysteresis of the differential pressure measurement can be adversely affected. 

A recent design of the maximum bubble pressure instrument for measurement of dynamic surface tension allows resolution in the millisecond time frame [119, 120]. This was accomplished by increasing the system volume relative to that of the bubble and by using electric and acoustic sensors to track the bubble formation frequency. Miller and co-workers also assessed the hydrodynamic effects arising at short bubble formation times with experiments on very viscous liquids [121]. They proposed a correction procedure to improve reliability at short times. This technique is applicable to the study of surfactant and polymer adsorption from solution [101, 120]. 

The measured pressure differences in ventilation applications are low or very low. The measurement range varies from a few pascals to several thousand pascals. At the lower end are typically building leakage and air movement-related measurements, where only a few pascals can cause a remarkably large air-tlow. The largest pressure differences probably occur in fan performance determination and similar applications. This wide range requires special demands on the measuring equipment and selection of the correct instrument for each application (Fig. 12.15). 

Electrical manometers have developed during the last 30 years. Modern electrical manometers are well suited for ventilation applications, both in the laboratory and in the field. The advantage of this type of instrument is fhat they are sensitive enough to measure small pressure differences with electrical output, enabling monitoring. A convenient feature, especially in the field is that the instrument is hand-held and there is no need for leveling on a bench, as for fluid manometers. The conversion of the pressure difference into an electrical signal can be based on several different phenomena. 

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a flow meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the... 

The application of NMR to the study of chemical reactions has been expanded to a wide range of experimental conditions, including high pressure and temperatures. In 1993, Funahashi et al. [16] reported the construction of a high pressure 3H NMR probe for stopped-flow measurements at pressures <200 MPa. In the last decade, commercial flow NMR instrumentation and probes have been developed. Currently there are commercially available NMR probes for pressures of 0.1-35 MPa and temperatures of 270-350 K (Bruker) and 0.1-3.0 MPa and 270-400 K (Varian). As reported recently, such probes can be used to perform quantitative studies of complicated reacting multicomponent mixtures [17]. 

S.M. Marcus and R.L. Baine, Estimation of bias in the oxidative induction time measurement by pressure DSC, Application note TA-228, TA Instruments, Inc., New Castle, DE, USA. 

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