Reku
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Glad to see people love lp85's as much as I do! - heck even in the north I can get a semi cave fill to at least 3200-3400.
What Size Doubles?
- Thread starter Bigeclipse
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Michael Guerrero
Contributor
Lots of suggestions here.
My recommendation would be based in part on your size. Taller people tend to prefer longer tanks and feel that they trim out better. Shorter people seem to prefer more compact tanks. There are plenty of links to tank charts on the internet to compare. Of course, you should also consider the kinds of diving you're going to do and whether you're going to try and match with any buddies for planning purposes.
I'm 6'3" and have a set of LP85s and a set of HP130s. Both are longer tanks, which seem to suit me well. And getting a cave fill on a set of LP85s can let you do some decent dives.
People often end up with several sets of doubles because different dives call for different tools.
My recommendation would be based in part on your size. Taller people tend to prefer longer tanks and feel that they trim out better. Shorter people seem to prefer more compact tanks. There are plenty of links to tank charts on the internet to compare. Of course, you should also consider the kinds of diving you're going to do and whether you're going to try and match with any buddies for planning purposes.
I'm 6'3" and have a set of LP85s and a set of HP130s. Both are longer tanks, which seem to suit me well. And getting a cave fill on a set of LP85s can let you do some decent dives.
People often end up with several sets of doubles because different dives call for different tools.
Reku
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Yeeessss double water heaters!!!!!!
Michael Guerrero
Contributor
I really like my 130s...once I'm in the water 
I've been doing the dive planning for my AN/DP checkout dives. My instructor told me to plan 2 dives, on successive days. Both to 120' for 35 minutes. I'll be using double HP120s and an AL40 for deco gas.
After going through this exercise, I am very glad I did not get doubles that are any smaller.
I ended up going back to my instructor and proposing we shorten the dives to 30 minutes because, essentially, I was gas limited by the 120s. The dives are long/deep enough that my planning showed I would need well more than half the gas in my deco bottle. That, in turn, means I have to accommodate a lost deco gas scenario in my planning - i.e. have enough back gas to do all my deco on that, if necessary.
If I were diving, for example, double 100s, I would have needed to shorten the dive even more. I think 30 minutes at 120' isn't really that long, so I am definitely glad I don't need to shorten the dive any further.
Of course, if everyone took an AL80 for deco gas, instead of a 40, it wouldn't be an issue. Then I could probably do the 35 minute dive on double 100s. But I don't really want to step up to carrying a deco bottle that big just yet.
And, yes, my planning is using very conservative numbers for consumption. These will be my first real deco dives, so I don't mind being extra conservative in my planning and having 5 minutes less bottom time as a result.
Just another thought on the size of doubles...
After going through this exercise, I am very glad I did not get doubles that are any smaller.
I ended up going back to my instructor and proposing we shorten the dives to 30 minutes because, essentially, I was gas limited by the 120s. The dives are long/deep enough that my planning showed I would need well more than half the gas in my deco bottle. That, in turn, means I have to accommodate a lost deco gas scenario in my planning - i.e. have enough back gas to do all my deco on that, if necessary.
If I were diving, for example, double 100s, I would have needed to shorten the dive even more. I think 30 minutes at 120' isn't really that long, so I am definitely glad I don't need to shorten the dive any further.
Of course, if everyone took an AL80 for deco gas, instead of a 40, it wouldn't be an issue. Then I could probably do the 35 minute dive on double 100s. But I don't really want to step up to carrying a deco bottle that big just yet.
And, yes, my planning is using very conservative numbers for consumption. These will be my first real deco dives, so I don't mind being extra conservative in my planning and having 5 minutes less bottom time as a result.
Just another thought on the size of doubles...
I haven't had any tech training (yet), but from what I understand, stage bottles can be employed in some scenarios--perhaps more readily in cave diving than ocean diving--to alleviate the need for larger tanks. So, we divers of modest stature may be able to get away with smaller doubles for cave diving. I have a set of backmount LP 85s that I am trying to learn to manage. I'm 5'-8" in height, and I can't imagine humping longer and/or heavier tanks than that on my back. (Yeah yeah sidemount, whatever.)
What was said earlier about "bums (English meaning) and opinions". My personal favs are LP steel 72s held together with a solid manifold, two steel bands and a cotton harness. When combined with a double hose US Diver Mistral or Aquamaster, they just about trim out perfectly, and depending on how I am suited up can allow for no weights / weight belt or BC. Diving at its purest. It's such a good combination that the US Navy went diving this way for decades, and still trains its divers in how to use this rig. Before you knock it, give it a try.
Yeah, ocean and cave seem very different in this regard. I have no cave training at all. My current level of training is such that I would only do a dive where I planned to carry everything I could need, in order to get back to the surface safely, with me at all times. At this point, if I wanted to do a longer dive (as in my above scenario), I would either carry bigger backmount tanks or carry a bigger deco bottle. I don't feel qualified at this point to make a plan that involved carrying additional cylinders (beyond 2 on my back and 1 slung deco), much less stashing one anywhere. But, obviously that would be an option for extending bottom time - when I get more trained up.
We have very commonly use stages of bottom gas on trips. If you want to get in 4 deco dives in the 120-160ft range that are hours from the nearest compressor, you can do it with 2 sets of doubles and 4 ali80 stages plus some ali40s of deco gas. Basically we use the bottom stage completely plus a small amount of gas from the doubles, repeat for dive 2. At the end of day 1 you have a set of doubles that are 1/2 to 2/3rds full and 2 empty stages. Switch to the second set of doubles and repeat for dives 3 and 4.
portable compressors don't work for a group of 6 using 200+cf of gas each per day. You'll need a huge truck to haul the O2 and helium supply bottles and all night to refill them
If I have 2 back to back 30+m dives with deco planned, I take a bottom stage for the first dive (ali 80). I generally get about 30 minutes on the stage, and switch to my backgas for the final 10-15 minutes and deco. I still have plenty of gas for a 40 minute bottom time+ deco for the second dive. My twinset is a set of twin 12s (roughly equivalent to a set of HP100s). They are, by far, the most common size twinset in the UK.
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