emttim
Contributor
Because different schools/scuba systems teach different tables. I have both PADI and SSI tables, and those two are completely different (SSI is more conservative in time allowed, and has fewer pressure groups). I don't know what other systems teach, but I presume there's quite a lot of different tables and pressure group values out there...
Also, if you already use a computer the whole point of the pressure group/tables is to give you a conservative limit that you can deal with when your computer breaks. So it's almost useless to have your computer give you the pressure group information when it's working... and it can't do it when it's not.
Finally, as was mentioned previously, even if everyone used the PADI table, and computers gave you the pressure group info according to the table, you will discover that a large chunk of the time the computer would just say something like 'pressure group INVALID', since computer dives almost always go beyond the limits of the tables.
Hmm, so then what's the generally accepted rule with dive computers so you avoid chancing a hit? Stay within the "yellow" bar or below on your nitrogen uptake according to your computer?
I like the idea of just assuming pressure group Z according to PADI tables (if a PADI diver) if you really want to use one since as Charlie said, if you're doing no deco diving and your dive computer says you still have bottom time left, then Z is a conservative assumption so you should be ok by doing that.