Pressure Measuring Devices

Pressure Measuring Devices - 2.7 Mechanical and Electronic...

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Unformatted text preview: 2.7 Mechanical and Electronic Pressure Measuring Devices 55 ' , Mechanical and Electronic Pressure Measuring Devrces .. A Bourdon tube ' pressure gage uses a: hollow, elastic, and curveditub'e to measure pressure. Although manometers are widely used, they are not well suited for measuring very high pressures, or pressures that are changing rapidly with time. In addition, they require the measurement of one or more column heights, which, although not particularly difficult, can be time consuming. To over— come some of these problems numerous other types of pressure measuring instruments have been developed. Most of these make use of the idea that when a pressure acts on an elastic structure the structure will deform, and this deformation can be related to the magnitude of the pressure. Prob— ably the most familiar device of this kind is the Bourdon pressure gage, which is shown in Fig. 2.13a. The essential mechanical element in this gage is the hollow, elastic curved tube (Bour- don tube) which is connected to the pressure source as shown in Fig. 2.131). As the pressure within the tube increases the tube tends to straighten, and although the deformation is small, it can be translated into the motion of a pointer on a dial as illustrated. Since it is the difference in pressure between the outside of the tube (atmospheric pressure) and the inside of the tube that causes the movement of the tube, the indicated pressure is gage pressure. The Bourdon gage must be cali— brated so that the dial reading can directly indicate the pressure in suitable units such as psi, psf, or pascals. A zero reading on the gage indicates that the measured pressureLis equal to the local atmospheric pressure. This type of gage can be used to measure a negative gage pressure (vacuum) as well as positive pressures. The aneroid barometer is another type of mechanical gage that is used for measuring atmos— pheric pressure. Since atmospheric pressure is specified as an absolute pressure, the conventional Bourdon gage is not suitable for this measurement. The common aneroid barometer contains a hol- low, closed, elastic element which is evacuated so that the pressure inside the element is near absolute zero. As the external atmospheric pressure changes, the element deflects, and this motion can be translated into the movement of an attached dial. As with the Bourdon' gage, the dial can be calibrated to give atmospheric pressure directly, with the usual units being millimeters or inches of mercury. ' ' For many applications in which pressure measurements are required, the pressure must be measured with a device that converts the pressure into an electrical output. For example, it may be desirable to continuously monitor a pressure that is changing with time. This type of pressure mea— suring device is called a pressure transducer and many different designs are used. One possible type of transducer is one in which a Bourdon tube is connected to a linear variable differential transformer (LVDT), as is illustrated in Fig. 2.14. The core of the LVDT is connected to the free end of the Bourdon tube so that as a pressure is applied the resulting motion of the end of the tube moves the core through the coil and an output voltage develops. This voltage is a linear function of the pressure and could be recorded on an oscillograph or digitized for storage or processing on a computer. (a) (b) F l G U R E 2.13 (a) Liquid-filled Bourdon pressure gages for various pressure ranges. (b) Internal elements of Bourdon gages. The “C-shaped” Bourdon tube is shown on the left, and the “coiled spring” Bourdon tube for high pressures of 1000 psi and above is shown on the right. 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Plastic domes are filled with fluid and connected to blood vessels through a needle or catheter. (Photograph courtesy of Spectramed, Inc.) ([1) Schematic diagram of the P23XL transducer with the dome removed. Deflection of the diaphragm due to pressure is measured with a silicon beam on which strain gages and an associated bridge circuit have been deposited. .. Hydrostatic Foce on a Plan Surface m :. urn-~14: nema- » ‘."n‘o'.«'~r>‘n'.b..\§‘:l 2...... 41,1» - . ». :I‘ALr-Jrk. we.» rc m- a: When a surface is submerged in a fluid, forces develop on the surface due to the fluid. The deter— mination of these forces is important in the design of storage tanks, ships, dams, and other hy- draulic structures. For fluids at rest we know that the force must be perpendicular to the surface since there are no shearing stresses present. We also know that the pressure will vary linearly with depth as shown in Fig. 2.16 if the fluid is incompressible. For a horizontal surface, such as the bot- tom of a liquid—filled tank (Fig. 216(2), the magnitude of the resultant force is simply F R = pA, Where p is the uniform pressure on the bottom and A is the area of the bottom. For the open tank shown, p = 3%. Note that if atmospheric pressure acts on both sides of the bottom, as is illustrated, the resultant force on the bottom is simply due to the liquid in the tank. Since the pressure is con— stant and uniformly distributed over the bottom, the resultant force acts through the centroid of the area as shown in Fig. 2.166;. As shown in Fig. 2.1617, the pressure on the ends of the tank is not uniformly distributed. Determination of the resultant force for situations such as this is presented below. ...
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