Rock Bottom- Rethinking the 500psi Gas Rule
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.
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. 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
James Mott
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Nice blogg sound perfectly reasonable to me
Funnily enough the above has always been my approach to air consumption. – Maybe I just like spending too much time chasing the fishes.
So what do you think of the idea of completing your planned profile and then wandering around in the shallows (as a photographer that can be quite productive) and then surfacing on the last gasp?
As a DIR Instructor I expect you to shake your head in disgust at this formula. Where is the gas reserved for my buddy – usually (again being a shooter) I don’t have one because they get bored out of their minds following me around. I see the head shakes getting more animated
The thing is, some of my best friends dive by the DIR numbers. That is definitely their choice but after thousands of not particularly aggressive dives I’m happy to break conventional rules because I don’t think they’re always important.
Hi Andy,
Planning for Rock Bottom will always give you a compelling advantage over not planning. It will give you the understanding and awareness as to what level of risk you are choosing to take. In reality that is what all diving really is. Risk Vs. Benefit. What are you willing to risk vs. what pictures can you add to your slideshow.
Breaking conventional rules that don’t apply to your unconventional situation isn’t really breaking any rules. Planning Rock Bottom for 100’ and then spending the dive in 20’ of water is not breaking any rule and I’d agree… have fun, be safe and do your dive.
The advantage of knowing what you need to get two divers up to the surface from max depth, doing a proper ascent and accounting for all minimum deco/safety stops, will only improve your awareness underwater. If you choose to dive solo, divide that number in half and that’s the minimum amount you need to surface from that depth so that you can wander around on your own, taking pictures and surface on your last gasp.
A great article. I’m interested to know how you work out the consumption rate for the entry level students?
Hi Alex,
To work out the consumption rate, you can simply take your students to 33ft/10m and have them swim a course for 10 minutes and log the amount of gas used. This figure then needs to be associated with a tank factor that corresponds to the size tank used, because 800 psi from an AL80 is not the same amount of gas as 800 psi in twin 104’s. In the end, they will have a number that they can transpose to any tank they will use.
Most divers have a Surface Consumption Rate of about .6 and .8 cubic feet per minute, less on deco and more during emergencies. None the less, these are simple lessons that any new diver can easily understand as a part of proper dive planning.
Feel free to contact me at jm@unifiedteamdiving.com if you still have questions, our you can visit our website at http://www.unifiedteamdiving.com