2 Pressure Measurement

1 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 1 MEASUREMENT TECHNOLOGY PRESSURE MEASUREMENT BUI Dang Thanh, NGUYEN Thi Lan Huong School of Electrical Engineering, Hanoi University of Science and Technology 1 Dai Co Viet road, Hà Nôi, Viêt Nam Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 2 Presentation outline 1. Introduction 2. Sensing Principle 3. Detection Method 4. Bourdon Tubes 5. Bellows 6. Connect to control system 7. Industrial Products 8. Industrial Symbols 9. References Introduction  What is pressure?  Pressure is defined as the normal force per unit area exerted by a fluid (liquid or gas) on any surface.  Finally, it should be noted that pressure is not defined as a vector quantity and is therefore nondirectional. 3 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Introduction  Everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other adhoc reference. When distinguishing between these zero references, the following terms are used:  Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.  Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.  Differential pressure is the difference in pressure between two points. 4 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 2 Introduction  Pressure sensors usually consist of two parts:  The first converts pressure to a force or displacement,  and the second converts the force or displacement to an electrical signal. Pressure measurements are made only for gases and liquids.  The simplest pressure measurement yields a gauge pressure, which is the difference between the measured pressure and ambient pressure. 5 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Introduction  Pressure Unit Conversion Table 6 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Sensing Principle  Since pressure is defined as the force per unit area, the most direct way of measuring pressure is to isolate an area on an elastic mechanical element for the force to act on.  The deformation of the sensing element produces displacements and strains that can be precisely sensed to give a calibrated measurement of the pressure.  This forms the basis for essentially all commercially available pressure sensors today. * 7 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Sensing Principle  Many types of pressure-sensing elements are currently in use. These can be grouped as diaphragms, capsules, bellows, and tubes, as illustrated in Figure 8 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 3 Sensing Principle (Cont.) 9 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Sensing Principle (Cont.) 10 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Detection Method 11  A detection means is required to convert the deformation of the sensing element into a pressure readout. In the simplest approach, the displacements of a sensing element can be amplified mechanically by lever and flexure linkages to drive a pointer over a graduated scale,*  Some of the earliest pressure sensors employed a Bourdon tube to drive the wiper arm over a potentiometric resistance element.  In linear-variable differential-transformer (LVDT) pressure sensors, the displacement of a Bourdon tube or capsule is used to move a magnetic core inside a coil assembly to vary its inductance. ** Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Bourdon Tubes Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 12 4 Bourdon Tubes 13  A Bourdon tube is a short bent tube, closed at one end.  When the tube is pressurized, it tends to straighten out. This motion is proportional to the applied pressure.* Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Bourdon Tubes 14  Notice that the displacement can be either linear or angular. A position sensor such as a pot or LVDT can convert the displacement into an electrical signal.  Bourdon-tube sensors are available in pressure ranges from 30 to 100,000 psi.  Typical uses include steam- and water-pressure gauges. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Bellows Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 15 Bellows 16  This sensor uses a small metal bellows to convert pressure into linear motion [Figure]. As the pressure inside increases, the bellows expand against the resistance of a spring (the spring is often the bellows itself). Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 5 Bellows 17  This motion is detected with a position sensor such as a pot. Figure 6.41(b) illustrates a differential pressure sensor, which can be made by enclosing the bellows in a canister. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Bellows 18  Bellows are capable of more sensitivity than the Bourdon tube in the lower-pressure range of 0-30 psi. Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Semiconductor Pressure Sensors Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 19 Semiconductor Pressure Sensors 20  Some commercially available pressure sensors use the piezoresistive property of silicon (Figure). * Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 6 21  These sensors have the advantage of “no moving parts” and are available in pressure ranges from 0-1.5 psi to 0-5000 psi.  An example of a commercial semiconductor pressure sensor is the ST2000 series from Sen Sym Inc. (Figure). * Semiconductor Pressure Sensors Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Connect to control system 22 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Industrial Products 23 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 Industrial Symbols 24 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 7 References 1. http://www.yokogawa.com 2. http://www.abb.com 3. http://www.wikipedia.org 4. http://www.automation.siemens.com 5. Moderm control techonlogy – components & system Buidangthanh-3i@mail.hut.edu.vn 25 Ha Noi, March 2012 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 2012 26 Thank you for your attention! . system 22 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 20 12 Industrial Products 23 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 20 12 Industrial Symbols 24 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 20 12 7 References 1. http://www.yokogawa.com 2. . signal. Pressure measurements are made only for gases and liquids.  The simplest pressure measurement yields a gauge pressure, which is the difference between the measured pressure and ambient pressure. 5 Buidangthanh-3i@mail.hut.edu.vn. therefore nondirectional. 3 Buidangthanh-3i@mail.hut.edu.vn Ha Noi, March 20 12 Introduction  Everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative