Various reviews of spare air suggest it is marginal at best for shallow dives, 30 ft +_. You need to know your breathing rate to evaluate what works best for you. I use a 13 cf pony which gives me time from over 100 ft to make a safe assent with a safety stop and still have a little air left at the surface if I have bad surface conditions. The pony provides a full back-up to my primary reg. I use this configuration when solo diving or spear fishing with friends because I cannot trust them to stay close. I remove it when diving with my wife, students, etc.