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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