The "must be back on the boat with 50bar or 500psi" convention is probably to ensure that novice or inexperienced divers don't push the limits and run out of gas. As we all know, there's various reasons this could happen:
OK, idiots aside.
- "You didn't tell me that I needed a reserve"
- "You didn't tell me that my gauge reads from 30 bar / 300psi"
- "You didn't listen to me when I told you I was low on gas"
- "I was distracted whilst following all the others"
- "What do you mean there's a surcharge for returning with an empty tank"
- etc...
OK, idiots aside.