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Strain-Based Measurement Introduction:FIGURE 1-50. Flush-Diaphragm Discrete-Bonded and Diffused Gages:MediaStrain GagesFlush Diaphragm (Discrete-Bonded Gages)Flush Diaphragm (Diffused Gages) Leads Diffused Silicon DieDiaphragm Support StructureThe direct exposure of the diaphragm to the measurand results inincreased thermal-transient sensitivities as the heat-dissipating straingages are thermally well-coupled to the measurand. The high heat densitiesof the miniaturized discrete-bonded piezoresistive strain gagedevices results in the warming of the measurand that is in direct contactwith the device in no-flow situations. When flow commences, thiswarmed layer is swept away and replaced with cooler medium, introducingsignificant thermal shock, particularly in the low-pressure andtherefore thin-diaphragm designs. The different coefficients of heatconduction between the sensor and the measurand, in the no-flow versusflow conditions, can cause deteriorated static data quality. In casessuch as this, the use of an essentially incompressible thermal barriermaterial, such as vacuum-degassed room-temperature-vulcanized rubber(RTV) or grease, will reach thermal equilibrium (equilibrate) withthe sensor diaphragm, reducing the thermal transient sensitivity of thesensor significantly (Figure 1-52). The vacuum-degassing of suchmaterials is essential in the removal of entrained gas bubbles that willCHAPTER 1-84 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-13:result in performance degradation due to the compressibility of theentrained gas within the pressure-communicating medium.FIGURE 1-51. Asymmetric Output Response:Outputm 1 m 2P ref > P measurandm 1P measurandm 2P measurand > P refP refFIGURE 1-52. Reduction of Thermal Transient Sensitivity:Vacuum-Degassed Thermal-Mitigating LayerMeasurand Diffused Silicon Diaphragms, Recessed and Flush Styles:Diffused silicon sensors are defined as single-crystal silicon structures intowhich piezoresistive strain gage elements are directly diffused as an integralpart of the spring member or diaphragm. Since the strain gages are anintegral part of the diaphragm, no strain gage bonding adhesive is presentor required. The diffused silicon piezoresistive sensor class are commonlyavailable in both the recessed- and flush-diaphragm styles. Silicon diaphragmsare commonly available in flat-plate and cup-diaphragm designs.The flat-plate design is as shown in Figure 1-50. The cup-diaphragmdevice of Figure 1-53 results in a superior mechanical clamp system whereThe Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-85
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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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Applications Note 1-6:ical size of
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Applications Note 1-6:and where:j i
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Applications Note 1-6:5. millivolts
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Applications Note 1-6:.Electrodynam
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Strain-Based Measurement Introducti
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