The inspiration calibrates in a unique manner.. what it does is flush the head with oxygen until it sees no further change on the cells, it then sets the calibration to the pressure setting multiplied by the enetered oxygen percentage.. The cal should be verified and the enetered oxygen percentage could be adjusted +/- 1 percent or so until the correct value is obtained.. that is ambient pressure in bar multiplied by the analyzed oxygen percentage.. The Inspiration displays po2 in bar even though the standard is to express po2 in Atas.. so in reality the displayed po2 (assuming a perfect calibration) is 1.3% lower than the standard.