I was looking at TDI's course outlines and it says "Rule of 1/3's for single tank and 1/6th for doubles" I understand the rule of 1/3's but why after cavern and during intro do they cut that back to a 1/6th when you're on SM or BM doubles? You have twice the amount of gas but they limit you to effectively the same amount as if you were on a single tank. It did say after full cave you're allowed 1/3's. Is it to keep the dives shorter?