Any reason to have both double 72's and double al80's?

[Jimmer]

Ok, here is the situation, I have a set of double 72's that are my favourite, and I have 4 al 80's. I've been thinking of setting up a pair of the 80's as doubles as well, but when I really think about the situation, I dont see a time when the al80's would really be any better suited than the 72's to dive with. Anyone have any good reason for having them both?

Jim

 

[Gombessa]

Since the gas difference is only about 10cf in doubles, it sounds like the main benefit to having two sets would be for different mixes. If you use a lot of exposure protection, won't the dub-80s add quite a bit of extra lead?

 

[mark99]

I found a whole thread about 72's vs. 80's here:
http://www.scubaboard.com/forums/tanks-valves-bands/196601-steel-72s-v-aluminum-80s.html

I started with a set of AL80's and now also have a set of LP85's. I can use the 85's with minimal weight on the belt, whereas the 80's require a substantial amount. I actually carry more out of water weight with the 80's, even though the cylinders themselves are lighter. In the water I counterbalance the belt weight on the 80's with 1# ankle weights around the neck of each cylinder (to trim to horizontal).

So I'd say you'd have no particular advantage/disadvantage with either, other than having to weight and trim differently. I suppose the question should be whether you'd have any advantage having two sets of doubles, say for two different dives on the same day with different mixes, for example.

 

[jamiep3]

Unless your 72's are the newer "Mid Pressure" models, or "Plus" Hydrod, the actual difference is about 24 Cu/ft. Most AL80's are actually 77 Cu/Ft and most older 72's are 65 Cu/Ft.

Unless you often get into a situation where you really need the extra gas, the smaller size and smaller profile of the 72's would be my preference.

Of course your mileage may vary.

 

[TSandM]

The Al80's are nice tanks for warm, fresh water, where you're wearing minimal exposure protection and don't want to be too negative.

 

[Jimmer]

Unless your 72's are the newer "Mid Pressure" models, or "Plus" Hydrod, the actual difference is about 24 Cu/ft. Most AL80's are actually 77 Cu/Ft and most older 72's are 65 Cu/Ft.

Unless you often get into a situation where you really need the extra gas, the smaller size and smaller profile of the 72's would be my preference.

Of course your mileage may vary.

I do have mine plus hydro'd and they are usually filled to 2500 to 2600, giving me no real gas advantage over 80's. Really the only reason I was thinking of putting the 80's together is that I have 4 of them, and 2 of them just sit there and are rarely ever used. I'm starting to think I might just sell off the matching set and invest in a set of larger doubles, so I'll have my 72's and something bigger.

 

[Blackwood]

160CF is considered by many parties to be the minimum for stage deco dives. Unless you're considerably overfilling your 72s, they won't cut it. So, if you have any aspirations towards deeper diving, doubling up your 80s is a viable option (though selling some off and getting bigger tanks is even better).

 

[Jimmer]

160CF is considered by many parties to be the minimum for stage deco dives. Unless you're considerably overfilling your 72s, they won't cut it. So, if you have any aspirations towards deeper diving, doubling up your 80s is a viable option (though selling some off and getting bigger tanks is even better).

Well I used to have a set of LP108's, but unloaded them with all the intentions of purchasing new HP doubles to replace them. Then I lost a job, was out of work for a couple weeks, lucked out and got a new job pretty quick, but had to put that idea on the back burner and was thinking of using the 80's since I already own them. But yeah I think I'll sell two of them and put that towards a set of bigger HP tanks.

 

[DA Aquamaster]

Gas planning wise, the difference between double 72's and double 80's, for me, is 20 minutes at 150 ft versus 23 minutes with a 1/3rd reserve in both cases. So the difference is really pretty minor.

In my opinion saying a set of 72's won't cut it for staged deco as they hold less than the arbitrary 160 cu ft is dogmatic. What is required really depnds on the diver and the situation. The irony of course is that same dogma would also exclude AL 80's as they only hold 154 cu ft. Of course dogma probably does not prevent dogmatic divers from being 6 cu ft off on their gas calculations from the get go just because they read or heard from someone important that an AL 80 holds 80 cu ft.

Realistically, if filled to 2600 psi as mentioned above, a steel 72 holds 75 cu ft compared to the 77 held by an AL 80 at 3000 psi and 71.8 psi held by an AL 80 at 2800 psi. Or in other words, a properly fillled plus rated steel 72 holds exactly the same amount of gas the average hot filled AL 80. Chew on that for awhile before you decide the steel 72 will not cut it while an AL 80 will.

All that said, double steel 72's make great tanks for offshore dives along the mid atlantic coast in the 100-150 ft range where any more than mild deco may not fit well with the overall plans for the rest of the divers on a more or less recreationally oriented boat. AL80's on the other hand make great stage bottles and are worth keeping in that regard. If additional gas is needed an AL 80 stage is not a bad way to carry it along with a 40 with your deco gas.

Weight wise you need an 8lb V weight with the double 80's to maintain the same buoyancy and inmy opinion the steel 72's trim better. Also steel 72's have the same tank factor as my X7-100's and have close to the same bouyancy traits.

There is also nothing wrong with having several sets of doubles. At the moment I have two sets of of steel 72's, and 3 sets of X7-100's. One set of 72's has air, the other has 32%, one set of 100's has trimix and the the other two have 30%. Its nice to not know where you will go next but to have enough sets of doubles to know you have the right gas to go there.

 

[Hoosier]

I have one set of AL80, but if your local diving is pretty much limited in a cold and dry suit diving, there might be no chance to use it.