Tanks for transition from Mexican Cave SM to cold ocean SM?

[Florinda]

I learned to dive SM in Mexican caves, wearing a drysuit with thin undergarments and two AL 80s. I use 14 lbs of lead in that configuration. I like that AL 80s are light underwater and easy to unclip, unbungee, and manipulate in other ways.

I am now looking to switch from BM doubles to SM doubles for cold water ocean diving at home in California. I wear the same drysuit as in Mexico but with much thicker undergarments. I'm planning to do deco/tech dives with this setup (I have LP 50s for rec diving at home). I'm trying to decide what kind of tanks to use, taking into consideration issues around weighting, gas volume, ability to manipulate underwater... what else? What tanks do other cold water ocean divers use? Should I just stick with the AL 80s? LP 85s? I have the XDeep Stealth Tec, if that matters.

I have tried side-mounting a single LP 85, and though it was ok when diving, it was too heavy for me to unclip or do much with underwater. They also don't work with sliding D-rings, obviously. I have also tried side-mounting a single AL 80 in my cold water gear. Oddly, I needed 30 lbs of lead in both cases. Does weighting for AL vs. steel work differently in SM vs. BM?

 

[Brett Hatch]

I don't dive sidemount or in caves, so take what I say with a grain of salt. I can't speak to the overall best tanks for sidemount in cold water, but I think I can answer the weighting part of your question.

If you have a config with a known weighting for your exposure gear, you can figure out the weighting for the new config by comparing the buoyancy of the tanks when empty. LP85's are about 2.5 lbs buoyant when empty, and AL80s are about 4 lbs buoyant when empty. So, the weight would be about the same. Maybe 3 lbs difference for double AL80's vs double LP85's. Both types of tanks will be a little bit negative when full, and a little bit positive when empty.

But, different steel tanks have very different buoyancy characteristics. Contrast this with a typical Faber HP100 tank for example, those are about 1.5 lbs negative when empty. So double HP100's needs about 11 lbs less lead than double AL80s. And HP100s are much more negative when full, which I guess SM divers will care about more than BM divers would, since you manipulate the tanks more, and will feel the weight when clipping/unclipping/handing off

Since you have a known working BM doubles config for local water, I think that will be your starting place for your SM weighting. Once you select your SM tanks, find the difference in empty buoyancy vs your BM doubles tanks, and add or subtract that much lead. If the backplate is very different between them, you'll need to account for that too. And the BM rig will have a little bit more metal on it for the bands and manifolds, so maybe add another 2 lbs to account for those missing on your SM rig. If I were you, that would be my starting place for weighting, because it's much easier to look up the difference in tank buoyancy, than the difference in exposure gear buoyancy.

Here's a list of tank buoyancy characteristics collected by a member here on ScubaBoard: Updated SCUBA tank specifications list -- in PDF and spreadsheet
And a similar list from Huron Scuba, a dive shop in Michigan: SCUBA Cylinder Specifications – Huron Scuba, Snorkel & Adventure Travel Inc. PADI 5 star IDC in Ann Arbor, MI
Hopefully between the two, the tanks you are looking at for SM will be on there

 

[Marie13]

LP85s hit the sweet spot for trim and buoyancy for SM. They are lovely tanks and I really recommend them. A pair of 85s with drysuit and heavy undies requires 16lbs of lead for me. A new 5mm wetsuit and AL80s is about 6lbs of lead (that’s from full cave in MX last year). For FL caves with the drysuit and thin undies and the 85s is about 4lbs of lead.

 

[Bald Wookie]

I dive a little north from you, in BC, and most here are either diving LP85s or AL80s (I am a rare exception diving LP72s, but I got them for super cheap soooooooo...). If you already have experience with AL80s it may be simpler to continue along this route and add 2lbs for your additional undergarments.

 

[lostsheep]

FWIW, my preferred cylinders for NorCal SM ocean diving are PST HP100s, I need the weight.

The trade off is that they are heavy in the water, that said you are not going to be taking tanks on and off to squeeze through anything so unless you just have to have that poser shot "gunning" a tank or whatever they call it, the weight is largely irrelevant. The exceptions to this would be if you planned to pass a cylinder to someone which I do not or donning cylinders off of a boat (but why are you sidemounting off of a boat anyways?)

Best course of action would be to find multiple cylinders and see what you prefer. I had SM LP85s and I still have LP50s...I prefer my HP100s hands down .

As always YMMV

ETA: Someone will be along shortly to go OT and tell you not to dive SM in the ocean...I look at it differently and say don't SM off of boats. Ocean diving SM is not always BS, if the entry is crap, the approach is tough, and you gotta haul so much stuff that setting a float is needed anyways, SM is just fine.

 

[VikingDives]

I have tried side-mounting a single LP 85, and though it was ok when diving, it was too heavy for me to unclip or do much with underwater. They also don't work with sliding D-rings, obviously.

Why would you need to unclip underwater? Restrictions? Otherwise, the tanks should just be on your drop attachment points, right?

 

[Florinda]

The exceptions to this would be if you planned to pass a cylinder to someone which I do not or donning cylinders off of a boat (but why are you sidemounting off of a boat anyways?)

I don't know if I might need to pass a cylinder to someone, or if there are other use cases here. That's part of why I'm asking. No restrictions. However, I do plan to dive SM off of boats. I assumed someone would come along and say, "Just dive backmount," or whatever. But probably that someone is a large man, able to walk around with 4 tanks on him on a wobbly boat. And maybe my buddy is also a large man, so he needs all this extra gas I'm carrying.

 

[Bald Wookie]

Maybe that person has very flexible shoulders that allows them to shut down tanks on their backs quite easily

 

[Florinda]

Why would you need to unclip underwater? Restrictions? Otherwise, the tanks should just be on your drop attachment points, right?

Yes, when I dived with the single 85 it was just on my drop attachment points. But since it was a single tank, I was able to giant stride off the boat with it. Also, that boat had an elevator, so I could keep the tank on the whole time. I guess I'm thinking that with two tanks, I'll need them handed to me. And most boats in CA don't have elevators, so I'll need to hand them up, too. I also needed to unclip the double ender on my chest D-ring to bungee the tank in, which meant some manipulation in the water.

 

[lostsheep]

I don't know if I might need to pass a cylinder to someone, or if there are other use cases here. That's part of why I'm asking. No restrictions. However, I do plan to dive SM off of boats. I assumed someone would come along and say, "Just dive backmount," or whatever. But probably that someone is a large man, able to walk around with 4 tanks on him on a wobbly boat. And maybe my buddy is also a large man, so he needs all this extra gas I'm carrying.

Apologies if I came across as crass.

For what it's worth, I continue to take a lot of **** for diving SM in open water and I will dive SM off of boats when needed; I just have to concede it is less than ideal. Most SM divers are not efficient at dealing with their rig and they take up too much space.

Don't listen to the naysayers, myself included, do what you want.

I suspect you are in SoCal but if not I am happy to loan you 85s, 50s, or 100s to play with.