Odd Numbered Dives - Pay For A 4th Tank For The 3rd Dive?

[MARK1992]

Hi guys,

Making 3 dives a day isn't uncommon. The first two can be made with the first pair of tanks no problem. What about the third and last one? Swap out both for a third and fourth tank? Move a 2nd stage over and dive with one?

 

[Mike1967]

I'm new at this SM game and haven't done three SM dives in a day yet, but what about bleeding half of the third tank into one of the empty tanks from the first two dives, then doing the last dive with two half tanks?

 

[broncobowsher]

What kind of dives are you doing? Need some idea of what kind of profile you will be running on these 3 dives.

I'm going to be doing some guessing here, probably out diving with others on singles doing standard recreational diving. Close?
I would probably treat the first dive as a single with a very large pony and the 2nd and 3rd dives as sidemount. Not ideal, but safe and workable.
I am sure others will have other opinions.

 

[tbone1004]

if you need the redundancy, you need the 4th tank.
Most people don't, and those that want to dive sidemount, but don't need to dive sidemount, should be diving single tank sidemount.

If you're going to dive 2-tank sidemount anyway. Looks like this
Dive 1-don't bother with the whole alternating every x-psi nonsense. Breathe the right tank down to 700psi or whatever it ends up at. Highest risk of sharing gas is usually on the first dive so you get primary donate out of that guy.

Dive 2-new tank on the right, alternate as you see fit

Dive 3-same tanks, same procedure as dive 2

@Mike1967 transfill whip isn't a good idea in this case. Since the tanks won't be breathed quite that far, the transfill is going to leave a lot of gas behind. Best to breathe one tank to "dead" and load a new tank in there.

 

[MARK1992]

Sorry, should have mentioned, yes recreational SM. I'm looking to build experience diving SM before considering tec diving. As a result I'd probably like to try to dive with a setup and conditions that would inculcate proper SM & tec diving procedures and habits.

Mike, I haven't seen a tank to tank transfer before. Might check with my friend who runs an operation to see if that's possible.

broncobowsher & tbone, seems like the idea I had is almost identical to the suggestions from you both, but wondered whether breathing one tank down, and have my buddy in a OOA situation breathing off my other tank would leave me without much gas for myself. I've got a friend who suggested single tank SM on the 3rd dive (without moving 2nd stages over i.e. only one 2nd stage) but that sounds even worse tbh.

Thanks for the input guys!

 

[tbone1004]

@MARK1992 do the math on the OOA scenario for one diver, and set that as your rock bottom pressure.
100ft-4ata, 1cfm, 3 minute ascent, 5 minute safety stop, 5 minute situation resolution/get back to the anchor. 2*1*3=6cf for ascent, 5*1*1.5~7cf for safety stop. 5*1*4=20cf for the bottom resolution. 6+7+20=33cf=1300psi in an AL80. Obviously substitute whatever variables you are diving in for depth/required distance back to the anchor line or whatever, but that's rough math.

What that says is you need to keep the left tank above 1300psi at all times to be able to get you up to the surface. The goal for "safety" is to maximize the amount of gas you have on you at all times. Left bottle is always "my" bottle, so I prioritize that bottle above all else.

So let's say the boat has some arbitrary be back on the boat with 500psi rule.
The problem with the way I proposed above is on one of the dives, you'll have a 5lb gas differential on the left side. It's totally manageable, we put all stages on the left in backmount, but it's noticeable and some people don't like it. It's the only way to maximize the gas you are going to consume though. Depending on your buddy, you can do your ascent/safety stop on the left bottle to make that delta a bit smaller, but in either instance, one of your bottles is going to have more than the other 2, and you want that bottle to be the left bottle on the last dive.

 

[Mike1967]

Mike, I haven't seen a tank to tank transfer before. Might check with my friend who runs an operation to see if that's possible.

I made a whip up with a SPG hose and a nipple. Then connect that into a HP port on each reg.

@Mike1967 transfill whip isn't a good idea in this case. Since the tanks won't be breathed quite that far, the transfill is going to leave a lot of gas behind. Best to breathe one tank to "dead" and load a new tank in there.

Not sure what you mean. After two dives both tanks will be as good as empty, say 30bar. If you transfer from a 230bar tank, won't they both end up being 130bar?

 

[tbone1004]

@Mike1967 no. They will equalize.
Assuming tanks are identical in size
Tank 1=30bar
Tank 2=30bar
Tank 3=230bar

After first transfill
Tank 1+3=130bar
Tank 2=30
After second transfill
Tank 1+2=80bar
Tank 3=130 bar

 

[Mike1967]

@Mike1967 no. They will equalize.
Assuming tanks are identical in size
Tank 1=30bar
Tank 2=30bar
Tank 3=230bar

After first transfill
Tank 1+3=130bar
Tank 2=30
After second transfill
Tank 1+2=80bar
Tank 3=130 bar

Yeah only do one transfill then dive with the two 130bar tanks.

 

[Landau]

Yeah only do one transfill then dive with the two 130bar tanks.

After one more transfill you won't have two 13 bar tanks. You will have:

Tank 1 = 80 Bar,
Tank 2 = 105 Bar,
Tank 3 = 105 Bar