By the way, not all dive operations have the entire group surface when one diver gets low on air. Many will have buddy teams surface separately as they reach the specified tank pressure. Make sure you understand how your divemaster is going to conduct the dive. It can be some comfort to ascend and do your safety stops together, but it's not the greatest thing in the world if you have an air-sucking-hog in your group who hits 750 psi thirty minutes into the dive when you still have plenty of air and time.