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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