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

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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
You only do one transfill.
You have three 230bar tanks.
First dive breath two 230bar tanks down to 130bar.
Second dive breath the same two tanks down to 30bar.
Transfill one of those 30bar tanks from the third 230bar tank giving you two 130bar tanks.
Do the third dive with those.
 
Skip the transfil.
Sounds like your dive profile is like I was thinking. Recreational profiles in sidemount gear to get comfortable for eventual tech diving.
Back to the simple. Dive one, use a single tank with the other as a huge pony. If someone has an OOA, you have a huge pony to suck off of.
Two of you both have an OOA, WTF is going on? You would be completely screwed in normal recreational setup. You are not watching your gas usage. Take a step back and get gas management under control first. By the time you are trying sidemount you should be aware enough of your surroundings this won't happen. One of the two of you might screw up and do an OOA (or system failure) but not everyone. There is always the old school of sharing a reg, alternate breaths. OK, I have gone down this stupid tangent too far.
2nd/3rd dives start with 2 fresh tanks and practice real sidemount diving.

End of the first dive you have one empty and one full tank. End of second dive you have 2 half filled tanks. End of third dive you have all tanks empty.
Never going into the water with only a single tank of air, you always have two. The only bad is some unbalanced on the end of the first dive with one full and one empty tank.
 
Skip the transfil.
In your own words, sucking one tank down to nothing is not ideal, so what's wrong with doing three dives that are exactly the same, all with normal reg switching procedure keeping your trim right?
As I said, I'm new to this and just trying to learn.
 
@Mike1967 so if you transfill to a pair of 130bar, you are barely bringing any more gas than the full tank *260bar total*, so why bother with the extra bottle in the first place? Too complicated. Just drain one tank at a time and load the next one. Only downside is the slight imbalance which can be managed by offsetting the lead on your belt. I.e. if heavy tank is on the left and it's heavy by 5lbs, put 2lbs more on the right side and you won't notice a thing.
 
Bronco, tbone and Mike, thanks for your replies. Will go through them and digest further.

I do agree about a massive failing should two divers be OOA. Never happened before nor will I hope it does. Unlikely that my BM singles buddy has an equipment failure and then I do on one side too, you reckon?
 
why bother with the extra bottle in the first place?
You need the third tank to do three dives either way yeah?
if heavy tank is on the left and it's heavy by 5lbs, put 2lbs more on the right side and you won't notice a thing.
All my weight is in the centre of my back (XDeep) so that's not really an option.

There's also the OOA scenario. As has been said, it should never happen, but what about a failure resulting in losing the one you've left full?
I'm still not sure what's so complicated about doing the transfill but you're a million times more experienced than me lol :surrender:
 
@Mike1967 Transfilling isn't complicated, but it's about taking the largest quantity of gas on each dive when you start
Your way
Dive 1-2x full bottles
Dive 2-2x half bottles
Dive 3-2x half bottles
Less reserve if it goes sideways on dive 2.

My way
Dive 1-2x full bottles
Dive 2-2x full bottles
Dive 3-2x half bottles

You can always take a 2lb weight out and clip it to your drop d-ring, but on AL80's it's really irrelevant. You can feel it, but it's not bad, and you only have to deal with it on the first dive at the very end.

On the one you've left full, you are still checking it on each dive, but it's a risk you have on each dive regardless. In sidemount you should always start and finish the dive on the long hose because those are the two times you are at highest risk of having to share gas. There's nothing that says at the bottom of the line you shouldn't check your secondary, or at whatever point in the dive you want. It's on your neck, so it takes 3 seconds to grab it and take 3-4 breaths, then put the long hose back. We do that in doubles regularly for that reason.
 
@Mike1967 glad to help. Been at this for a hot minute. Scary to believe it's been 9 years since I've been sidemounting. I started at the very beginning of the commercialization of sidemount *three rigs were available at the time, Nomad, Razor-the OG using the MSR bag, and Armadillo*, so there's been a lot of trial and error and thought that has gone into it for me. Didn't have the ability to learn from the pros back then since they were still figuring it all out
 

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