Side-mounting 2 different sizes (ex: 80cu & 40cu).

[MichaelMc]

This might come up is on travel as someone preferring sidemount. If the expected dive op only has 80s, you'd like redundancy, but do not want the weight of two 80s. Bringing a small pony could give you that redundancy. Not wanting to pay double for fills is another good reason.

The rental 80 on the left would let it feed the wing and the necklace. While the pony on the right was clipped off to the chest, just for emergencies assuming no dry suit. So less need for refills. Not an unusual breathing situation for sidemount, just normally it is not for the full dive. After the dive you could unhook the left tank and pass it up if you wanted to.

Or would people rig an 80 plus 19 differently, assuming they were side mounted.

 

[SlugLife]

The swinging cylinders and unstable mass shifting are actually harder on your back than a stable backmount cylinder aligned with your center of gravity. With <50 dives I would just concentrate on your core buoyancy and trim skills in a single tank and leave all your optimizing aside for another 50+ dives. You dont have the experience to evaluate what is or isnt really working or optimal

The ideas is I'd unclip both tanks before climbing the ladder, and then pull them up.

I think part of my problem was I was diving with a hand-me-down BCD that was somewhat floppy and probably not appropriately sized, likely contributing to the problem. I just (this week) got a backplate/harness/etc rig, and finally finished setting it up and sizing it today. I haven't dove with it yet, but already absolutely love it!

A place this might come up is on travel as someone preferring sidemount. If the expected dive op only has 80s, you'd like redundancy, but do not want the weight of two 80s. Bringing a small pony could give you that redundancy.

The rental 80 on the left would let it feed the wing and the necklace. While the pony on the right was clipped off to the chest, just for emergencies assuming no dry suit. Not an unusual breathing situation for sidemount, just normally it is not for the full dive. After the dive you could unhook the left tank and pass it up if you wanted to.

Or would people rig an 80 plus 19 differently, assuming they were side mounted.

If I brought my own backplate+bcd on vacation, I might roll 2x 80s, and blow through air like I don't care. That said, I am strongly considering getting both a 19 cu & a 40 cu, and the 19cu would be great for travel. If I'm renting equipment though, I'm pretty sure I'd just run it with a normal rear-configuration, with the 19 clipped to the front. I haven't thought a lot about travel-scuba yet.

 

[jale]

Hi
When I travel to some places to do single s80 tank dives, I take a minimalist SM system and dive single S80 in SM. Sometimes, if this is available, I take a s40 with oxy to "clean a bit" my body after a few day dives.
Is it balanced? yes. Cumbersome? Not at all.
Go for it

 

[halocline]

I recently started looking into a redundant air-source, and may suggested side-mounting. Then the idea of side-mounting both my main (80cu) and redundant (19cu or 40cu) came up. This seems like it would be great for the type of diving I do (boat with small ladder).

I think you would be much more comfortable with your main tank on your back and your bail out bottle slung. I don't think it would even be close. Sidemount comfort is all about balancing the tanks, both for weight distribution and redundancy in an overhead setting. Sidemount off a boat is also way less convenient for most people than backmount single tank.

I assume you are talking about recreational dives? Where you have access to the surface if you need it? No deco? If all that is true, then I would be asking yourself why it is you would want to use a technical diving set up. It won't be safer, (in fact there is a solid argument that it's less safe unless you train as a technical diver) it certainly won't be as physically comfortable in the setting you are describing.

I'm not trying to discourage you from trying sidemount as a dive technique sometime, but keep in mind it's used for specific reasons in technical diving, and there's quite a bit to learn if you want to be good at it. I dive sidemount exclusively in caves because it works for that environment, but I would never use it off a boat in OW for recreational diving.

One other thing, the idea of carrying a 40cft bail out bottle on a recreational dive where your primary tank is 80cft does not make much sense to me. If you are really doing dives where you would potentially need 40 cft of emergency gas, that does not sound anything like a recreational dive on a single AL80. In an overhead dive where each diver is carrying 160 cft, the standard rule of thumb would be to retain 53 cft (1/3 of your gas) for an emergency.

This doesn't mean its a bad idea to sling a AL40 instead of a more typical size pony (AL19) but it is definitely not necessary to do so. The only real advantage would be if you are interested in learning technical diving, you'll have a bottle than will work as a deco bottle later on.

Sorry if this isn't what you wanted to hear, but I think it's useful. Enjoy!

 

[Marie13]

As far as I’m concerned, the only reason to SM different sized AL tanks is for deco/stage bottles.

 

[napDiver]

Sounds like you never sidemounted before and a fairly new diver.

I recommend a course with a qualified SM instructor. At least a tutorial for a couple days in a pool with friend that knows sidemount.

Note you can make a bpw work with some mods work but a SM rig will be much better.

You should first get set up correctly with 1 al80. You'll have a tendency to rotate the tank side down. If your buoyancy is good, it'll take 30 min to a couple days to have it not effect you. Practice with tank at full and at empty. Easier to deal with when neutral tank is near middle full.

Once you get the hang of that do the same thing + AL40.

You'll have to figure out how to setup your regs correctly. Probably will have to change hose lengths. Make sure to you know how to turn on/off tanks underwater and can clearly ID reg to respective tank.

There's quite a bit more... that's why I recommend an instructor. If you want a really good online instruction, sidemounting.com is very good. Not as good as real life instruction though. GL

 

[napDiver]

In your case, it'll be a lot easier to backmount the al80. You can sling the 40.

Once your comfortable with that you can sidemount the 40 instead. It'll be more streamlined. Just a simple mod to your bpw.

In the future if you want to learn real SM with a SM bcd. You can do al80+40 SM if you like. To be honest, you'll probably just SM 2x80 at that point. Very few cases people wouldn't

 

[Marie13]

Single tank backmount with an 80 or similarly sized tank (HP80 in my case) and AL40 is very easy. Slinging the 40 was fine. It disappeared under water. You do have to have a BC with D rings in the proper places to sling a 40.

 

[rick00001967]

inmho if you are looking for redundancy, and are not needing any special deco gas, you could dive two of the same tanks in double sm, or keep the main on your back and sm the spare gas. i would not bother using two different tanks for double sm. but thats me. there is no law that says you could not use two different tanks.

if you are only looking to add a smaller tank with a deco mix, i would stick with backmounting your main and sm the deco tank.

 

[lostsheep]

If the general idea is ridiculous, let me know. That's part of why I'm asking.

2x LP50s? That would mean buying 4x tanks, for 2 dives. That would also be 4 fills for 2 dives.

I do think asymmetric side Mount tanks would look odd and I might laugh at you. That being said I do all kinds of weird stuff myself and I’m sure I get laughed at too. Do whatever makes you happy.

It’s rare these days I dive with a single primary tank but when I do, it goes on my back and I sling / SM an aluminum 40 for emergencies.