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Strain-Based Measurement Introduction:ity internal vacuum sealed by hermetic methods, with the exception ofthe Mallory bonded capsule. As a result, most miniature sensors arefabricated using various epoxy materials that would outgas and deterioratean internal vacuum, even if one were initially provided.FIGURE 1-42. The Electrically-Referenced Absolute Pressure Sensor:AppliedPressureSeal (usually epoxy)Lead wiresAtmospheric Pressure Sealed InsideOutput14.7 mV0@ 1mV/PsiaPressure14.7 Psia( 0 Psig)Sensor Zero BeforeTrimmingThe zero level of the Electrically-Referencedpressure sensor is trimmed in the positivedirection.The use of the electrically-referenced method implies that, if the sensoris pressure-cycled from a high vacuum to a greater than 14.7 psia pressure,the bending diaphragm will progress through the stress-strainÒinflection pointÓ where the membrane transitions from negative topositive bending. Due to the asymmetry of the flush-diaphragm supportingclamp structure, the sensitivity of the diaphragm can be mismatchedby as much as ± 3% between positive and negative bending.This characteristic will not create error provided that the sensor hasbeen calibrated from a close-to-zero psia pressure to the maximumrange of the device. Properly calibrated, the inflection error becomes anintegral component of the nonlinearity error of the sensor.Another consideration in the Òtrue-psiaÓ versus the Òelectrically-referencedÓissue is that if no pressure difference exists between the insidereference cavity of the sensor and the outside world, then no tendencyCHAPTER 1-74 The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999
Applications Note 1-12:for leakage exists either. If the transducer is destined for terrestrial use atsea level, the differences between the true-psia and electrically-referenceddevice become academic. If, on the other hand, one erroneously assumesthat all psia sensors are of the Òtrue-psiaÓ sealed type, and the device purchasedis actually electrically-referenced, in a high-vacuum space environmentthe internal reference cavity will eventually leak out to the vacuum ofspace and this would be perceived as zero drift.The classical argument against the use of the electrical reference method isthat the internal gas will expand with increasing temperature, thus modifyingthe reference pressure. The statement is correct; the internal gas willthermally expand with increasing temperature in accordance with the relationship:P 1 /T 1 = P 2 /T 2Where pressure P 1 and pressure P 2 occur at temperature T 1 and temperatureT 2 respectively.Provided that the sensor is sealed prior to the thermal compensation process,the internal gas expansion with increasing temperature becomes a neteffect combined with the TCR mismatch shown by the bridge and is compensatedout as a net effect during the process of thermal-zero compensation.The end result is that there exists no way of determining whether a deviceis electrically-referenced or true-psia-referenced from the output terminalpoint of view. However, the difference can become very apparent dependingupon the pressure existing at the sensor lead exit.PSIS (Sealed-Gage Reference):The sealed-gage reference pressure is identical to the electrically-referencedabsolute method described above where the internal cavity is sealedat one atmosphere (14.7 psia) and the sensor zero output is trimmed to providea nominal null-bias output at 14.7 psia. This method is basically iden-The Art of Practical and Precise Strain Based Measurement 2nd Edition © 1999 CHAPTER 1-75
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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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