I am just a lowly nursing student, but we actually just tested on this exact thing. In fact, our instructor told us a story about when she first got out of school (40ish years ago), they didn't know back then about the back-up mechanism and that COPD pts' breathing relied on O2 levels instead of CO2 levels and they actually had a few people stop breathing because they were given WAY too much O2...she also said that pts and their families often don't understand this and think, if a little's good then a lot must be better...which can cause a lot of trouble. Anyway, after all this babbling

I'm pretty sure you're right about the 1-2 L