The accuracy of the Hewlett-Packard 47201A ear oximeter below 50% saturation

The Hewlett-Packard 47201A ear oximeter has been shown to measure arterial oxygen saturation (SaO2) above 50% SaO2 with an accuracy of +/- 5%. Below 50% SaO2, the oximeter underestimates arterial saturation in a predictable way, thus allowing a correction factor to be used: true SaO2 = (oximeter reading + 50)/2.