A new way to look at gas consumption using rock bottom: Why simply planning to surface with 500psi isn’t the best way to manage air

This article was provided to me by James Mott of Unified Team Diving and Sea The World Scuba Center.  I met James at the Great Lakes Shipwreck Festival in which he spoke on the topic of gas management. James shared the concept of “rock bottom” in which divers determine when is the best time to begin an ascent from the deepest part of the dive to allow for a safe reserve of 500 psi when reaching the surface. Also check out the video interview I did with James at the festival.

By James Mott

Throughout the world, divers are told again and again to return to the boat with 300-500 psi in their tanks.  Understandably, most competent divers stretch their bottom time out as long as they can.  They smile as they show their pressure gauges to the dive master upon surfacing and then compare gauges with other divers in order to see who the closest one to 301psi is.  Getting the most bottom time underwater is a fun game to play with buddies and I’m not saying that divers shouldn’t use as much of their tanks as possible.  However the question becomes, “Is this the smartest way to plan gas?” What exactly is the goal of leaving some air in our tanks?  To help a buddy in need, to keep water out of our tanks, to inflate our BCD’s at the surface?  Many divers use the 1/3’s rule, but even this plan has numerous flaws.  So where do we start?  Is there a plan that works for deep-diving technical divers and shallow-water recreational divers alike?  What is wrong with the idea of surfacing with a safe amount of gas, like 500psi?
The answer to the 500psi problem is that being on the surface at the end of the dive with 500psi does not answer the more important question for scuba divers, which is, “When do I have to leave the bottom?  If we have an emergency and we need to share air, “How much air will I need to bring me and my buddy to the surface safely?  This is the question that should start all gas planning.
Doing It Right (DIR) education teaches the unified team to plan for the worst possible emergency before the dive starts.  We always ask the question, “What happens if at the worst possible moment, the deepest part of the dive, the furthest distance from home… my buddy runs out of air… How much gas do I need to bring both of us to the surface without any incident?”
Calculating rock bottom is easy enough to do in your head before the dive and it is taught in all entry level DIR courses.  How long will the ascent take, multiplied by two divers, then by the average depth and then by a consumption rate, equals rock bottom.

Rock bottom is calculated by adding up the time it would take to ascend from a given depth.  For example, from 60 feet, we would normally ascend at 30 feet/minute and have a three-minute safety stop.  This normal ascent would take us four minutes.  We then add one minute for the air-sharing emergency to take place and have a five-minute ascent, requiring 10 minutes of total air with two divers breathing during that 5 minute time frame.  We assume we have an elevated breathing rate of about 1 cubic foot/minute.  Our average depth during the ascent is about 30 feet or 2 ATA (atmospheric pressure one ATA is atmospheric pressure at sea level) where we consume gas at twice the surface rate.

SCUBA Math

5 minutes X 2 divers X 1 cfm X 2 ATA = 20 cubic feet of gas

20 cf of gas on an AL 80 is about 750 psi.

Rock Bottom for 60 FSW on an AL80 is 750 or for most gauges, 800 psi.

Rock Bottom for 100FSW on an AL 80 is 1600psi.  (Math is not provided but can be, just ask.)

Once rock bottom is determined, the remainder of the useable gas is then divided into a logical plan.  Maybe it is a drift dive on a Caribbean reef where we can use everything. Maybe we are diving on a shipwreck in the Great Lakes and need to get back to the mooring line, or we might be doing a penetration on this shipwreck where we will need enough gas to get out of the wreck plus enough to get to the surface.  Different dives will require different gas plans, but rock bottom must always be accounted for, before the gas plan is made.  DIR education teaches the unified team to plan their gas in accordance for the specific dive, so that each diver can get the most fun possible out of their diving and still be safe.
Being at depth below rock bottom is irresponsible and will not give us enough air to safely ascend.  The only emergency underwater is running out of gas, everything else is just an inconvenience.  Once the out of gas diver is breathing again, we move from emergency to management.  Just because someone ran out of air, does not mean that we rush to the surface, exceed safe ascent rates, skip safety/deco stops, or anything else we know about safe diving protocols.  The only option is to remain calm, think, communicate and finish the dive.
For more information about rock bottom or other gas management options, contact me at jm@unifiedteamdiving.com or www.unifiedteamdiving.com or Sea The World Scuba Center in Farmington Hills at 248-478-6400.