I need to rethink. What is happening is that a man is encased in a bubble as punishment (in some future world) and left to slowly die of hypoxia. As punishment, they want the process to be drawn out some so that the criminal has time to reflect upon his crime. Society would not be satisfied without a drawn out period where the person knows they're going to die and are helpless to prevent it.

At some point, of course, they will start hallucinating and eventually die due to CO2 buildup.

So I could have them feed a resupply of air but not quite as much as is used so that he slowly dies and knows it's happening and is going to happen.

I'd like to be able to throw in a techie term or gauge reading or something to make it believable. I've been doing a bit of reading but haven't gleaned enough to help me out.