Doubles Cylinder Selection

[FireInMyBones]

I am interested in purchasing a set of doubles. As I progress in my diving, I find redundancy imperative.
My question is if there is a preferred cylinder selection (i.e. HP vs LP). I currently dive a single HP120 and desire to advance into Advanced Nitrox/ Decompression training via TDI.
My primary goal in diving doubles will be bottom time and technical training.
I have been looking at double steel HP100s.

 

[Blackwood]

Many people call ~160CF the minimum for dives in the "technical 1" (i.e. 150-160) range, but beyond actually holding enough gas it's largely personal. Depending on your body type, you may trim out better in short and fat tanks (often HP) or long and skinny tanks (often LP).

Best thing would be to get together with some divers and try a variety just to see how much work you'll have to do in order to fly them with ease.

 

[Atomic_Diver]

I dive with Double LP 108's for caves but i would reccomend it for OW too

 

[RoyN]

The HP100 or LP85 seems to be the average tank size for most people. Shouldn't go wrong with those tanks.

 

[ivobj]

Well I use HP120 (actually it is the Worthington X8-119) and am very satisfyed, I use it mostly for wreck diving and caves.

 

[StreetDoctor]

I dive double 130's and have no problem. Heavy on land but I manage at the quarry. Boat diving is easy. If you're calculating rock bottoms you'll be suprised and at how fast double 100s go even at 100'.

 

[TSandM]

1) Look at the dives you want to do, and figure out how much gas you need to do them. That tells you the volume you are looking at.

2) Look at what your buddies are using. Figuring gas supplies is easier if you're all on the same size tanks.

3) Dive a couple of candidate sets. See what you are comfortable in and what you can trim out. Believe me, some tanks will work, and others just won't.

 

[FireInMyBones]

My buddies dive mostly LP85s and one that dives LP95s. I like the potential to get more gas out of double LP 95s if cave filled than unsafely overfilling HP100s. I have a great SAC, so I will mostly be deterred from bottom time by who I dive with. Looking at V planner last night with the guy that dives the LP95s, I saw that just to have contingency to dive 150ft for 30min saying something goes wrong with my stage bottle, I would need to have LP95s or bigger so I can have 1/3rd left for my buddy.
Thoughts?

 

[Blackwood]

My buddies dive mostly LP85s and one that dives LP95s. I like the potential to get more gas out of double LP 95s if cave filled than unsafely overfilling HP100s.

Why are you okay overfilling one set and against overfilling another?

I saw that just to have contingency to dive 150ft for 30min saying something goes wrong with my stage bottle, I would need to have LP95s or bigger so I can have 1/3rd left for my buddy.
Thoughts?

Diving thirds for a deep square profile doesn't necessarily make sense to me, nor would I likely assume 2 failures (your buddy's backgas and your stagebottle). Why does your buddy need 1/3 of your gas? Do you need to make it back somewhere? If you do, sure, reserve 1/3 for him AND whatever you need for you to both ascend after returning to that location.

For a non-penetration dive to 150 feet for 30 minutes, I reserve about 1000PSI in my HP100 doubles, which is somewhat close to 1/3 (out of mere coincidence), but only half of it is for my buddy.

 

[FireInMyBones]

I do want to thank everyone for the comments. They are quite enlightening.

As far as over filling, this is my understanding: HPs are rated to an already high pressure and the gas rating is designated for the higher pressure. LPs have their gas rating at the lower pressure and have the ability to be overfilled.
Now understand, this is one of the reasons I asked the question in the first place: I have limited knowledge of cylinder workings and what I do know is information given me from other divers and here on the board.

I am just trying to get in the habit of planning dives vs just computer diving like I used to. The profiles we ran were to show me that if I were to make dives that were penetration that I would need to keep other things in mind. In all likelihood, neither regs nor cylinders should fail, though I am not opposed to plan contingency options in case of those things going wrong. I know it is conservative, but it is not unheard of.