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Strain-Based Measurement Introduction:the bending diaphragm is well-supported and no mechanical joints ordissimilar materials are present.FIGURE 1-53. The Cup Diaphragm Structure:Diffusion ZonesLeadsSilicon Substructure(N-Type typically)The silicon clamp and diaphragm must ultimately be sealed to an outerhousing, or made into a free-floating and hermetically-sealed pressurecapsule, to ensure that the internal pressure reference is maintained(Figure 1-54). The joining method commonly used to bond the sealingplate to the cup-diaphragm structure is known as the Mallory bond andis performed at elevated temperature by applying a high voltagebetween the two highly-polished surfaces. The electrostatic forces thatresult, due to the applied voltage potential between the two parts, producea high-quality nuclear-level seal between the two parts. Since thestrain gages are a diffused, and integral component of the bending diaphragmwith no adhesive bonding material required, the cup structureoffers a stable and low-hysteresis sensor design.FIGURE 1-54. The Cup-Diaphragm Mallory-Sealed Capsule:Diffusion ZonesLeadsP refMallory BondCHAPTER 1-86 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-13:The cup-diaphragm and capsule designs result in the active strain gagesgenerally being exposed to the pressure medium with the exception of thenew Sensonor isolated cup design. In water-laden environments or in thecase of blood pressure measurement, the silicon elements are either coatedwith parylene C or isolated from the medium by the principle of exclusionusing a silicone gel isolating material of Figure 1-49.The highly-miniaturized piezoresistive flush-membrane designs (Figure 1-50) provide extremely wide- and flat-frequency (± 5%) response ranges inexcess of 400 Kilohertz in higher pressure ranges. The combination of ahigh-stiffness, small-diameter, low-mass diaphragm and low operatingstrain requirements (due to the high gage factor shown by the piezoresistivestrain gage) all contribute to this extremely high-frequency capability.The small physical size of the diffused piezoresistive strain gage allows thefabrication of sensors as small as .030Ó in diameter, and the low mass of thediaphragm yields an exceptionally shock-tolerant sensor. It is not uncommonfor this style of sensor to survive exposure to 30,000-gravity (g) shockinputs.Diaphragm Strains:The following relationships may be used to approximate the induced strain in apressure-loaded membrane that is peripherally fixed and subject to the followingassumptions:¥ Uniform diaphragm thickness¥ The material is homogeneous and isotropic¥ InÞnitely-rigid perimeter clamp (For highest performance, the side-wall thickness is at least 10times the plate thickness)¥ Small, perfectly-elastic deßections of not greater than 30% of the diaphragm thickness.¥ No diaphragm stiffening contribution due to the presence of the strain gages.¥ The diaphragm thickness is less than 20% of the diaphragm diameter.¥ The diaphragm deßection is due to bending where no tensile forces exist in the median plane ofthe diaphragm.The quality of the above assumptions is often the reason for differences betweenthe computed and actual strains present. For example, the perimeter clamp will notThe Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-87
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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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Strain-Based Measurement Introducti
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