Elastic Pressure Transducers - the engineering resource

2010Elastic PressureTransducershttp://images.brighthub.com/5E/5/5E57FFD7F92397E13A98427699869325FESubmitted to:C6A811_small.jpgSubmitted By:Dr. ing Naveed RamzanWaseem Zia (2007-CHEM-15)www.engineering-resource.comCHEMICAL ENGINEERING DEPARTMENTUET Lahore, Pakistan10/31/2010

AbstractThis report describes the flexible pressure transducer (EPT). It involves the function principles,types of EPT and their use and applications. Merits and demerits of EPT are discussed along withtheir conditions of failures.But……………Instruments are always incorrect.www.engineering-resource.com

Contents1.Elastic Pressure Transducer ............................................................................................................... 11.1 Bellows ...................................................................................................................................... 11.2 Bourdon Tube Gauges ………………………………………...……………………………..3Common types of Bourdon Tube Gauges ……………………………………………...41.3 Diaphragm ................................................................................................................................ 52. Pressure Sensors Summary ............................................................................................................... 63. Functional Uses .................................................................................................................................. 74. Merits & Demerits……..………………………………………………………..……….…………. 75. Reference ……………………………………………………………………………………………..8www.engineering-resource.com

1. Elastic Pressure TransducersElastic Deformation 1 :This pressure sensing system is based on the idea that bending of flexible material is directly relative to thepressure being considered. There are mainly three sensor types that are used in this system of sensingpressure: Bourdon-tubes, diaphragms and bellows.Flexible Sensors:Most fluid pressure sensors are of the flexible type, where the fluid is enclosed in a small compartment with atleast one flexible wall. The pressure reading is thereby calculated by sensing the movement of this flexiblewall, resulting in either a direct readout through suitable connections, or a transducer electrical signal.Flexible pressure sensors are sensitive; they are commonly delicate and liable to throb, however. In addition,they tend to be much more costly than manometers, and are therefore preferentially used for transmittingdetected data and sensing pressure differences. A broad diversity of flexible elements could possibly be usedfor flexible pressure sensors; the majority of devices use some form of a pressure transducers as:‣ Bellows‣ Bourdon tube‣ Diaphragm1.1 Bellows 2 :Bellows elements are cylindrical in form and contain many folds. They deform in the axial direction(compression or expansion) with changes in pressure. The pressure that needs to be detected is applied to oneside of the bellows (either inside or outside) while atmospheric pressure is on the contrary side. Absolutepressure can be detected by evacuating either the exterior or interior space of the bellows and then sensing thepressure at the contrary side. Bellows can only be associated to an on/off switch or potentiometer and are usedat low pressures,

Figure 2 1: Basic Metallic BellowsThe bellows is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thinwalledtubing. The diameter of the bellows ranges from 0.5 to 12 in. and may have as many as 24 folds.System pressure is applied to the internal volume of the bellows. As the inlet pressure to the instrumentvaries, the bellows will expand or contract. The moving end of the bellows is associated to a mechanicallinkage assembly. As the bellows and linkage assembly moves, either an electrical signal is generated or adirect pressure indication is provided. The flexibility of a metallic bellows is similar in character to that of ahelical, coiled compression spring. Up to the flexible limit of the bellows, the relation between increments ofload and movement is linear. However, this relationship exists only when the bellows is under compression. Itis necessary to construct the bellows such that all of the travel occurs on the compression side of the point ofequilibrium. Therefore, in practice, the bellows must always be opposed by a spring, and the movementcharacteristics will be the resulting force of the spring and bellows.Figure 2: Common Metallic Bellows2007-CHEM-15 Page 2www.engineering-resource.com

3. Functional Uses 2Pressure sensors perform the following basic functions:‣ Indication‣ Alarm‣ ControlIf a pressure sensor becomes inoperative:‣ A spare sensor element may be used (if installed).‣ A local mechanical pressure gauge can be used (if available).‣ A precision pressure gauge may be installed in the system.Environmental concerns:‣ Atmospheric pressure‣ Ambient temperature‣ Humidity4. Merits & Demerits 3Merits of Flexible Pressure transducer‣ Flexible Pressure transducer cost is low.‣ Flexible Pressure transducer has easy structure‣ Flexible Pressure transducers have been time tested in application.‣ Flexible Pressure transducer allows high accuracy, spatially in relation to cost.Demerits of Flexible Pressure transducer‣ Flexible Pressure transducer has a low spring pitch.‣ Flexible Pressure transducer is inclined to shock and throb.‣ Flexible Pressure transducer is inclined to hysteresis.2007-CHEM-15 Page 7www.engineering-resource.com