Doubles...

[PfcAJ]

Back in open water days, things were simple. Al80s were it. Now, its time to look into a first set of doubles.

Background: 5'7" male, 145lbs, pretty good consumption rate. Most of my diving with these tanks will be in Fl. caverns/caves. Salt water tastes funny..I don't like it. I plan on taking fundies within the next 2 months (in a single). I know that learning the way of the double cylinders will take a while.

I'm mostly looking into the PST E series, E7-100's. What is the norm for DIR divers? I think that it'll take a while before I outgrow them, but maybe I'm missing something?
How are they in the water? Do you like them? Are they good for a cave 1 class?

Thanks for your replies!

 

[Reinoud]

Back in open water days, things were simple. Al80s were it. Now, its time to look into a first set of doubles.

Background: 5'7" male, 145lbs, pretty good consumption rate. Most of my diving with these tanks will be in Fl. caverns/caves. Salt water tastes funny..I don't like it. I plan on taking fundies within the next 2 months (in a single). I know that learning the way of the double cylinders will take a while.

I'm mostly looking into the PST E series, E7-100's. What is the norm for DIR divers? I think that it'll take a while before I outgrow them, but maybe I'm missing something?
How are they in the water? Do you like them? Are they good for a cave 1 class?

It sounds a bit like you are rushing things. You have not even done DIR-F yet, but already you know that you will be using your doubles mainly in caves and caverns?
I would advise you to take your time. Practice with a single tank first, and ask any questions during your DIR-F. I think you will get much better advice there than on any Internet forum. (even better advice than this one! ";-)

 

[Dive_Girl PNW]

My suggestion - try out as much equipment as you can and confir with experienced divers or your instructor before opening your wallet. My first spin in doubes was instantly more valuable then every piece of info I ever read on the subject.

 

[JimC]

Double up your 80's for now. Its cheap, and you can re-use the manifold and they will last you untill Cave2/Apprentice.

Don't waste money on E7-100's to double up, because you'll need LP104's/E8-130's to match the team and Cave2 level. I know your a small guy, but I know women smaller than you who use 104's. Even if you breath less than everyone, its the guy with the smallest tanks that restricts the dive.

 

[PerroneFord]

JImC, where are you finding LP104s?

 

[MXGratefulDiver]

Even if you breath less than everyone, its the guy with the smallest tanks that restricts the dive.

Hmmm ... are you saying that DIR doesn't factor individual consumption rate into the equation when determining turn pressure?

... Bob (Grateful Diver)

 

[Meng_Tze]

I would also say to take it easy into doubles / caves. Caves may not be what you expected.

I started with double AL 80's (heck I still dive my AL's into caves often). These you can break up later if you wish to be come stages, or singles again for those 'leisure' dives. More gas is better obviously for longer dives. But you dont HAVE to go with 104's. The e7-100 would do fine, but as mentioned, its the guy with the least gast that calls it first usually. Carry a stage and guess what....... you are not the guy with the least Gas anymore. But thats not what cave diving is for (or any kind of diving). Its not who stays down longest, but it does play an important part in planning. The decision of tanks you should make, but be aware of what the decision means to you and your team.

Get fundies first, it will help you in the caves (or any kind of diving) and will set you on the right path for trim, bouyancy, team awareness, gas management, propulsion etc.... and take it slow on the cave path. I have seen many cave aspiring divers who have some serious issues in overhead environments. It's annoying to come out of a cave and having to go through another team's dust clouds where they kneeled down to check spg's........or whatever they did...

Start at the basics (fundamentals) and work from there. Dont rush, there is enough time to speed up.

 

[vlada]

Hmmm ... are you saying that DIR doesn't factor individual consumption rate into the equation when determining turn pressure?

... Bob (Grateful Diver)

Nah, he's saying exact opposite - while diving with dissimilar tanks, you've gotta take everyone's individual consumption into account, so that at the turn point, the guy with the smallest tanks would have enough gas for himself as well as his buddy who may have a much higher consumption rate. Sure, he may not be the one calling the dive, but the turn point would be driven by the size of his tanks.

I went from 95s to 130s for the same reason - buddy had a much higher consumption and much bigger tanks and gas planning was a nightmare as we were bumping against limits all the time.

 

[JimC]

To answer that I'll have to get into a bit more of a discution about thirds. So here goes:

Basic idea of gas management. Allways have enough gas to get you and your buddy out. In caves this means 1/3rds for an outflowing cave. 1/3rd in, 1/3rd out, 1/3rd for buddy.

Senario #1, no tank size matching:
Lets call the cave a squair profile to 102ffw (4ATA) and ignore deco.
Diver 1: double 100's filled to 3400psi. SAC rate on this dive is 0.5cf/min.
Diver 2: double 130's filled to 3400psi. SAC rate on this dive is 0.75cf/min.

Doing 3rds without regard to size matching:
Diver 1: 1100psi (64cf) third, 2300 turn
Diver 2: 1100psi (83cf) third, 2300 turn

They dive into the cave and exactly at 28 minutes, diver 2 hits his turn (2300psi) and diver 1 has used 56cf and has 2440psi (141cf) left. Diver 2's back gas is instantly vaporizes. Diver 1 and diver 2 need 147cf to get out, they have 141cf left. You will note the rounding up here, if they dove to true thirds instead of rounding up to the next 100psi they would be worse off.

So, you have to limit the penetration to the smallest 1/3rd on the team. In this case, diver2 must use 64cf for his third. Diver 1 does nothing, Diver2 must recalculate his third.

Diver 2: 800psi (64cf), turn 2600psi.

Same dive, except dive2 turns the dive at 21 minutes and both divers survive.

 

[Dan Gibson]

I'm sure you actually meant to say it's based on the team member with the smallest volume of gas at the start of the dive, not size of tank.

Sure, he may not be the one calling the dive, but the turn point would be driven by the size of his tanks.