Misaliged control knob - eh???
Dennis - you may have a problem with the basic calibration mechanism in the Evita. I have had this on Evita 4's and suspect that the methodology is the same for the Dura.
When the O2 cell is being calibrated there is an automatic sequence which provides air and oxygen separately, to provide a known 21% and 100% O2 calibraion point.
This is achieved by a small shuttle valve being driven across the inlet to the O2 cell and providing the requiste gas at the appropriate time. The valve seal is provided by a rubber/neoprene? disc which should eliminate any dilution of the intended calibration gas. If the disc wears then some dilution can occur and the calibration settings are assumed by the vent to have taken place as expected, however there will have been some electronic bias placed on the measuring circuit to compensate for this drift.
In your case, if oxygen is leaking round the seal when the vent assumes a 21% cal point to have been accepted, it will allow enough oxygen into the patient circuit to achieve the same O2 cell voltage, thus giving you an unexpectedly high O2 reading both on the vent and your monitor.
You can fairly easily check to see if you have this problem - assuming the Dura to be the same as the Evita - by removing the O2 cell and looking into the opening thus revealed in the calibration block. The black sealing disc in question should be visible seated neatly on a brass shaft in the centre of the opening. The wear on the seal tends to occur at the shaft when the mounting hole becomes eccentric thus allowing the seal to slip slightly to one side during calibration.
Replacement of the sealing disc is simple if a little fiddly and the results are remarkable.
Failing that I think you might be looking at replacing one of the main mixing valves - expensive!
Hope this may be of use.