Rule of Thirds & Shallow Rec diving

scubastingray · Nov 25, 2013

Hey guys,
I'd love to hear you opine about this rule in regards to "vacation divers." If you're under 50 feet do you really want to be getting back on the boat with 1000psi left in your tank?
I understand in tech diving it makes sense as it is an overhead environment, but this seems overly cautious when the surface is a cesa away. I'm not advocating running it empty, but when you dive tropical beginner level dives what psi do you want to return with?

MXGratefulDiver · Nov 25, 2013

It depends on the profile, the conditions and the circumstances. There are times when I don't mind ending the dive with 1,000 or more psi in my cylinder and there are times when I'll want to get the tank as close to empty as possible before I reach the surface.

Having a target psi to shoot for is a great idea for divers who haven't yet learned how to manage their gas supply. But let's not lose sight of the reality that (a) the objective is to not run out of air while below the surface, and (b) once you're on the surface your air supply is inexhaustible. Who cares how much air you have in your cylinder once you've ended the dive, as long as it exceeds your regulator's capabity to deliver it to your lungs.

... Bob (Grateful Diver)

Mofous · Nov 25, 2013

I have always finished my dives with ample air in my reserve while in OW dives. When on vacation I've experienced DM's turning the group and ending dives at 1000 psi, and I've also seen DMs let people breathe their tanks down under a couple hundred psi. People often just blindly put their lives/trust into others without much of a thought.

I think it is even more important as a beginner diver to ensure a solid reserve because if you have an emergency your air consumption will spike much more then someone who has been trained to manage these problems. Sure a CESA is available, but why not just ensure a healthy supply of gas on your back rather then risking injury from a CESA. Proper planning and gas/buddy management will ensure a safe return and does not require a CESA.

I would want to have 500 psi in my tank at the minimum when exiting the water.

tracydr · Nov 25, 2013

I figure mine like Bob. I take into account surface conditions and my exit when deciding gas. I want to make sure I have enough air on the surface if I need to exit through rough surf or swim to a boat with rough seas. Yes, I could get by without but I much prefer a functioning reg just in case I fall down in the surf zone or we have a long surface swim with 4-6 foot waves back to a boat.
If my exit will be simple or the seas are calm, I don't mind surfacing with 500psi, even with my AL63 tank,which I use in warm water. But, since I do dive a small tank at times, I have to be extra careful with gas planning.
I also want enough gas in reserve at all times to get me and my buddy back on the boat or shore in an emergency.
I don't remember ever having a DM tell me when I can or can't surface. I plan my gas, that's not the DM's job.

Teller · Nov 25, 2013

I do half plus 200 psi on rec dives.

scubastingray · Nov 25, 2013

Teller:
I do half plus 200 psi on rec dives.

so on an AL80 you get back on the boat with 1700psi?

I guess this isn't necessarily a beginner dive if gas planning is occurring, but in beginner dive conditions.
I don't want to rely on others for my planning.

rongoodman · Nov 25, 2013

For non-overhead dives, I think Minimum Gas, aka Rock Bottom, is a more useful way to think about gas reserves. There have been a number of discussions here about it.

Teller · Nov 25, 2013

I turn the dive at 1/2 plus 200. It also depends on the site and what I am doing. I dont think there is a rule that works great for every situation.

boulderjohn · Nov 25, 2013

The rule of thirds is really not appropriate to most recreational divers doing the kind of dives you are describing. Yes, you want to have a reserve, but 1,000 PSI is not necessary.

How you determine your needs for the reserve depends upon the kind of diving you will be doing. The rule of thirds is the least used in basic recreational dives.

iluvtheocean · Nov 25, 2013

also i would consider the conditions too... strong current, rough seas, -- i'd turn around with 1000 or so left -- otherwise i'd do 500 or whatever the min dictated by the boat since some have rules on that. but isn't psi a poor indicator of how much cu ft of gas you have left since it's dependent on tank size?

Rule of Thirds & Shallow Rec diving

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scubastingray

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