Most common steel tank size for doubles

[landlockeddivingdoc]

As the weather is getting colder in the midwest, my goal over the next few months is to become proficient in drysuit diving, in colder (fresh) water.. Part of that, of course, is modifying weight and trim, and I'd like to minimize the amount of extra lead I carry on my BPW...so I'm going to take the plunge and purchase a steel tank.

Eventually (ie, over the next several years), I would also like to transition to backmount doubles, and then tech diving. (I've looked into sidemount, but around here most introductory tech courses seem to use backmounts, so I'd like to get as used to the configuration as possible). My basic question is: what is the most popular tank for backmount doubles?

I've seen several sizes and pressures referenced, and so if I'm going to make a purchases with future goals in mind, I would again like to try to acquire and learn inside and out the equipment that I'd be using. The plan would be to acquire at least one tank and use it singly for now, and then get a manifold and mate for it when I need to pursue doubles.

An additional practical consideration would just be what is too big (dimensions, not capacity) for doubles, and/or too big for me (i'm only 5'4", 145lbs)? I usually dive with an AL80, and carry it with ease.

Any advice offered would be very much appreciated.
(additionally yes, all my other gear is cold water rated, regs are DIN, and LDS can fill HP tanks)

 

[inquis]

HP100

 

[Tracy]

HP100 is the most popular size of doubles in the US.
LP85s are also common, as are LP95/HP117, and LP104/HP130s.

As open circuit trimix diving has been phasing out in the last few years, the big doubles are also losing popularity outside of cave diving.

 

[Whitrzac]

With your size, 85s/100s would be plenty.
Some lp72s might be a good too

 

[landlockeddivingdoc]

Appreciate the help from everyone. HP 100 it is!

 

[USdiver1]

Check out some vintage steel, Voit 50s make great doubles, or go the standard 72 doubles like the US Navy uses [although at 5'4" they may be a tad long and heavy].

 

[TravisD]

One thing to keep in mind though - Apparently the finished dimensions of cylinders can vary from one to the next. If you buy them separately, it's possible that you might end up with two cylinders that are slightly different heights. They'll still band together, but might lean to the side some when stood up. When you order new cylinders you can say that they're for doubles and they might get you a more closely matched pair.

 

[Johnoly]

... what is too big (dimensions, not capacity) for doubles,

Since you asked the question..........

I've dove with my jacked up LP121 in cave country and boardwalk spectators rightfully called me nuts. But then again the guy behind me had a snorkel and no one laughed at him.

So I'd say LP121's are the 'un-popular' tanks.

 

[Rilelen]

I'm your height and weight. I can dive HP100s, but vastly prefer double LP85s (I'm also in Florida though, where I can get cave fills). I won't go larger than that - if I need more air, I'd rather carry a stage than struggle with LP104s.

 

[NothingClever]

@landlockeddivingdoc

I'm making an assumption you’re female. If so, keep reading. If not, disregard.

Make sure to check in with other female twinset divers (who are diving the conditions and profiles you envision doing) for what size twinsets they're diving to ensure you don’t unnecessarily purchase more tank than necessary. Most women are much more efficient than men with their gas consumption and you don't want to get inadvertently convinced to buy "up" because the salesman's point of reference is the average male's RMV. IOW, you could buy a set of HP100s, go diving with others in HP100s and find at the end of the dive you have 50-70 bar more than the others. You may be able to accomplish the same exact dive as the others but only need the volume an AL80 twinset provides because you process oxygen much more efficiently.

I'd just use the steel tank you're purchasing now, rack up a bunch of dives, rent a few twinsets when you take your tech course and THEN purchase a purpose-built twinset that's matched like @TravisD advised.