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Strain-Based Measurement Introduction:eæb 2 -- + t öè øe i = -------------------b--2(EQ 1-37)Where: e i = The indicated strain which will be greater than the beam surfacestrain.The above correction is of particular importance when thin beams are beingconsidered.FIGURE 1-40. Adhesive Thickness Strain Correction:b / 2b / 2Strain Gage Backing /Adhesivee surfacee indicated (> e surface )BeamStrain GagesttApplications Note 1-12:Pressure ReferencesThe diaphragm-based pressure sensor bends in proportion to the pressure differencethat exists across the diaphragm. The pressure that exists ÒinsideÓ thepressure sensor may be fixed at some known value or it may be variable, as inthe case of the differential pressure sensor. Several different types of pressurereferences are used and will be discussed in turn:CHAPTER 1-72 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-12:PSIA (Absolute Reference):The psia-referenced sensor may be created in one of two ways. The moststraightforward method is to create an internal high-quality vacuum and tothen seal the reference cavity hermetically by welding a fused glass-tometalÒheaderÓ lead-wire feed-through onto the back of the sensor. Normallysuch headers are electron-beam welded in place, which is convenientsince the weld process occurs in a vacuum environment. The end result isthe creation of a Òclose-to-zero psiaÓ internal reference volume where thediaphragm will be prestressed due to normal atmospheric pressure toapproximately 14.7 psia. An absolute-referenced pressure sensor that producesan output of 1mV/psia and having a range of 15 psia will show 14.7mV ± zero-bias level output while sitting on your desk. Reference Figure1-39.FIGURE 1-41. True Absolute Pressure:WeldedFused Glass-to-Metal HeaderOutput14.7 mV@ 1mV/psia14.7 PsiaVacuum reference (Approximately 0 Psia)0Pressure14.7 psia(Zeroed Sensor)The second method of creating a psia-referenced pressure sensor is commonlyemployed in the miniature sensor fabrication industry and is termedthe ÒElectrically-ReferencedÓ method. Figure 1-42. In the electrically-referencedmethod, the internal cavity of the sensor is simply sealed with noattempt made to create an internal vacuum environment. Since the sensoroutput is idealized as a Y = mx+ b linear line, the zero-bias level of the sensoris electrically offset by means of the bridge-balance resistor to elevatethe output of the sensor to simulate the sea level output (nominally 14.7psia). Most miniature sensors are too small for the creation of a high-qual-The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-73
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The Art of Practical andPrecise Str
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Special Credits:Mrs. Helen Pierson,
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Table of ContentsBy James G. Pierso
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ContentsApplications Note 4-10:Lead
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ContentsCHAPTER 8Electrical Conside
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CHAPTER 1Strain-BasedMeasurementInt
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Applications Note 1-1: Introduction
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Applications Note 1-1: Introduction
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Applications Note 1-2:The Art of Ap
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Applications Note 1-31. The presenc
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Applications Note 1-3result in a pe
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Applications Note 1-4:The measureme
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Applications Note 1-4:uncertainty i
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Applications Note 1-5:All strain-ba
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Applications Note 1-6:Applications
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