Question about changing tank size to accomodate air consumption rate.

[Blue Sparkle]

I'd like to find out more about the idea of diving with a smaller or larger tank to accommodate lower or higher air-consumption rates.

For example, let's say I'm diving in Florida with a dive buddy who uses more air than I do, and we're renting tanks. The default tank is therefore an AL80.

I have had (qualified) people recommend that I use an AL 63, since I use less air and am fairly short waisted. I've also read on the forum where (qualified) people have recommended to someone who uses more air than their buddy to dive with a larger tank, so that they don't have to turn the dive early.

This sounds logical, but leaves me with a question:

I (thought I) understood that the buddy is supposed to provide backup air in case there is a problem with the other buddy's tank/regs/etc. So in that case the buddy pair would have to turn the dive when the "lowest" buddy was down to half the amount of air to provide emergency air for the "highest" buddy, right? So wouldn't we still end up having to return sooner because of air supply?

To illustrate what I'm trying to say (even I can see I might not be writing it clearly), let's say I'm using an AL 63, and my buddy an AL80, and that if all went well we would both have used half our air at the same time. Then (if I have this right and it's very possible I don't - go easy on me!) I would have about 31 cu. ft. of air left (1500 psi) and my buddy would have 40 cu. ft. left (1500 psi). Again, if all goes well we should plan to both be back at the surface at 500 psi.

But... what if in a worst-case-timing scenario, something happens to my buddy's air supply/regulators at exactly the furthest point into the dive? (I'm not saying we would have agreed to turn the dive at 1500 psi, but just giving example figures that are simple to calculate.) Then wouldn't "we" be short on air because my "half" is smaller than my buddy's "half" --- even though if both our dives went fine we would be surfacing both with about 500 psi left?

I suppose in an OW environment we would surface wherever we were, but...?

I'm guessing there is something wrong with my logic, so I thought I would ask here.

Sorry if I've got errors in my assumptions here; I'm pretty new to diving so I haven't done much more than very basic planning yet. In other words, on my last dive trip my buddy and I both dove with rental AL 80's, and we ascended so that neither of us went below 500 psi. However, that typically had me back on the boat with 1400 - 1500 psi. Hence my question.

B.

 

[fnfalman]

Let's keep it simple - you're doing fun recreational dive, NDL, no overhead. If something were to go wrong with your buddy's regulator or tank (O-ring blew out, 1st stage failing), unless one of you have screwed up your gas management planning and run the tank(s) too low, you should still be able to make an emergency ascension. May not have enough air for both of you to do a safety stop, but you should have enough air in both or either tank to safely make a slowly ascension to the surface. If you have already precalculated your return time and keeping to that schedule, there should be no problem.

If you're doing technical diving, then that's another story but then you wouldn't be asking this question because the tech people should and would plan their dives down to the very last PSI.

Plan your dive and dive your plan. Both buddies must buy into the plan and abide by the plan. When you start to deviate from the plan is usually when Mr. Murphy comes to visit.

 

[rstofer]

There are several strategies for turn pressure but one of the more well thought out is:
Rock Bottom and Gas Management for Recreational Divers

Although your SPG measures pressure, the better way to think about the problem is in terms of cubic feet. Naturally, the relationship between psi and cf is dependent on the size of the tank.

So, yes, you have to figure out how much air your smaller tank has to have to support 2 divers. Given the disparity in SAC rates, this gets more complicated when you use a smaller tank.

But, that's the way it goes. Work it all out in cubic feet and then back into psi.

Richard

 

[idocsteve]

Two ways to solve the imbalance between you and your buddy

One is for you to go to a smaller tank and the other is for your buddy to go to a larger tank. If you go to a smaller tank and you keep everything about the dive the same, there will be less available gas for your buddy in the event he needs it.

If your buddy goes for the larger tank then you'll have the same reserve for him that you would have before because there's no changes on your side of the equation, and you will enjoy a longer dive before your buddy's low gas supply forces you to turn the dive as is the typical scenario.

So of course the safest thing to do is the larger tank for him, not the smaller tank for you. Smaller tank gives you more comfort but less reserve buddy gas.

 

[TSandM]

Great question, Blue Sparkle!

In cave diving, we have to do what's called tank matching. If I'm diving a smaller tank, I can't use more than a third of what's in my tanks. My buddy, diving larger tanks, can't use more than a third of what's in my tanks, because that's all I've got extra to get him out of the cave. So he won't get to use a third of what he's carrying, no matter what my SAC rate is.

In open water diving, each of you needs to have enough reserve to get you and a buddy up to the surface from wherever you are. Those calculations are done using very conservative estimates of gas consumption (assuming it will be very high throughout the entire ascent). After you've put that amount of gas aside, you can either use all of the remaining gas (drift dive), half of the remaining gas (shore dive where you CAN swim back on the surface if you have to) or one third of the remaining gas (dive where the surface is not an option, which is verging upon not being a recreational dive). In the first two cases, you are not carrying gas for an underwater swim, but just for an ascent, and you put that aside before you figured your turn pressure, so there is no need for tank matching. Whichever of you hits turn first, turns the dive.

I've never seen the point in asking for a SMALLER tank. If they've got 80's, I dive them. If they've got 100's, I dive them. Nobody ever had a problem from having too much gas!

 

[Bubbletrubble]

I think you've identified at least one of the salient aspects of gas management. At any given point during a dive, a two-person buddy team has to have enough gas to deal with catastrophic loss of one diver's gas supply. This means that the team needs to have enough remaining gas to get both divers to the surface, allowing for safe ascent rates, mandatory deco stops, and safety stops (ideally).

A separate but related issue is selecting the appropriate turn pressure for a dive. I'll let others address this.

[Edited later: OK, TSandM. I give up. Is your avatar a Borg cupcake? ]

 

[Blue Sparkle]

Thanks for the replies so far. This makes sense, and I appreciate the input. I see that things are more flexible on an OW dive when you can usually surface whenever you need to.

Still, I like to understand the concepts, and this has been rattling around in my head ever since I first mentioned that I had used an AL63 (on a dive in 2008), and the person I was talking to explained why that was technically a no-no (meaning, it could be a no-no depending on the situation). Before that I had just thought "Oh, right, I use less gas so I can carry less."

I'm the sort of person who likes to think over details and concepts, and so, rather than making things less pleasant for me, it's actually something that "gets me into" it more not that I think about things to the exclusion of actually doing them though

The original reason for the AL63 was because my BC was fitting so horribly that the tank was wobbling all over the place, bonking me, and etc. I have now taken care of that problem, thanks to SB

I, too, want to know what's on top of the cupcake, TSandM. Shoot, now you'll be inundated for diving requests from those with a sweet tooth

B.

 

[Bubbletrubble]

[Edited later: OK, TSandM. I give up. Is your avatar a Borg cupcake? ]

I, too, want to know what's on top of the cupcake, TSandM. Shoot, now you'll be inundated for diving requests from those with a sweet tooth

Man, am I disappointed! A quick Google search for "borg cupcake" reveals the following picture:

Very low probability that Lynne made the cupcake herself.

 

[kanonfodr]

[Edited later: OK, TSandM. I give up. Is your avatar a Borg cupcake? ]

Lol, that's awesome. TSandM, showing SB the sweeter, tastier side of diving universe assimilation??

Btw, cupcakes are cool but the dark side has cookies!!

Peace,
Greg

 

[scubafanatic]

Ideally, the buddy with the higher gas consumption should use a larger tank....WHEN AVAILABLE, which isn't always the case, sometimes the only thing available is an AL80, for everyone. I've known cases where buddies donate the octo to even up gas consumption, it's not uncommon for 'couples' to do this, or for DM's in Cozumel to do this...just sayin' Also, have the diver with higher gas consumption dive shallower, say 10' shallower, I've done this myself, to help even the odds. Finally, as you're not a techie diver/overhead diver/deco diver, you need to be working your way shallower as the dive progresses anyway, remember, 500 psi lasts a lot longer at 20 or 30 ft than it does at 100', so you don't have to go overboard and leave an oversized reserve in your tank as long as you're working you way up to shallower depths as the dive progresses. There is nothing wrong with you also using an AL80 just as your buddy is, as long as you're comfortable with an AL80, there are smaller/shorter women for whom an AL80 is oversized (too long/too heavy) and if that describes 'you' then I wouldn't dive an AL80 just for the extra gas, UNLESS it makes no difference to your dive comfort, then why not, as mentioned, no such this as too much gas !