What size is good for doubles?

[grazie42]

Hi all!
I´m looking to buy a doubles set up...
My question is about tank sizes...

During my training I dived 9 liter doubles @200Bar/64cft...(on dives to 160 feet/ 50+ meters)
I thought I´d get 12 liters/80cft but talked to an instructor/tech diver this weekend who suggested I get at least 15 liters/106 cft...

I´m also thinking about getting tanks that are @232BAR which would make it:
98cft/12 liters
or
123 cft/15 liters...

I have a RMV or is it SAC? of 14 liters/min @1 atm
or 0.5 cft/min @ 1atm

Sorry about all the numbers but as I´m imperially impaired I´d like to enable responses from "my" people as well...

What would you suggest I get?
Obviously I´d like to get as small tanks as I can (why pay/carry more then you need to?) but at the same time I dont want to save money now only to have to buy bigger tanks later...

What do you use?
Do you find that sac/rmv changes depending on mix? (my figures are based on trimix) The reason for my question is that He is expensive and hard to "score" here...

Looking forward to answers...

 

[rblackburn]

I personally dive PST LP 104s... (2640psi rated preasure).

I like them... But do have LP81s for side mounting.

 

[jagfish]

Would the intended use have any bearning on choosing a tank size?

JAG

 

[grazie42]

jagfish: Would the intended use have any bearning on choosing a tank size?

You´re right of course...

I´ll be using the doubles for coldwater, wreck-diving (dry).
As I´m diving dry I don´t think the cold water will impact much so any advice from warmwater-divers should still be applicable...
Depth around 50m/160 feet to start out but as there are other wrecks, locally, @ 70m/210 feet thats propably something I´ll end up diving as well.

Deco-diving with 50%, maybe 100% deco and at least rule of thirds...

 

[Don Burke]

Here is what Decoplanner gave me for bottom gas consumption using .7 rmv on the bottom and .6 rmv in deco:

160 feet using 18/45 and going to deco gas at 70 feet
15 minutes 74cuft
20 minutes 96cuft
25 minutes 115cuft
30 minutes 137cuft

160 feet using 18/45 and deco on bottom gas
15 minutes 116cuft
20 minutes 162cuft
25 minutes 212cuft
30 minutes 270cuft

210 feet using 15/55 and going to deco gas at 70 feet
10 minutes 78cuft
15 minutes 104cuft
20 minutes 138cuft

210 feet using 15/55 and deco on bottom gas
10 minutes 141cuft
15 minutes 230cuft
20 minutes 340cuft

 

[grazie42]

Thanks Don Burke...
I did my 160 ft dive on 25/19 (50% deco), is there a good reason for lowering the 02?
(I know all about ox tox but doesent seem to apply here...)

I´m thinking of getting 15 liters/123cft doubles, any comments?

 

[Don Burke]

Thanks Don Burke...
I did my 160 ft dive on 25/19 (50% deco), is there a good reason for lowering the 02?
(I know all about ox tox but doesent seem to apply here...)

I´m thinking of getting 15 liters/123cft doubles, any comments?

I come up with a PPO2 of 1.47 for that mix at 160. That is a little hot for my taste.

160 feet using 25/19 and going to deco gas at 70 feet
15 minutes 74cuft
20 minutes 94cuft
25 minutes 115cuft
30 minutes 137cuft

160 feet using 25/19 and deco on bottom gas
15 minutes 92cuft
20 minutes 126cuft
25 minutes 161cuft
30 minutes 195cuft


You need enough gas to stay with your buddy. If those 15 liter tanks do that and your back can handle them, they might be right for you.

I would not put my back through it. Your mileage may vary.

 

[grazie42]

Thank you Don Burke...
The guy who´s selling them said it was because of backproblems so you make a good point.
Still...getting the doubles for 375$ (incl bands and whatnot) is a convincing argument all of its own...

 

[kuster500]

The big thing is deciding what kind of diving you are doing, that will decide the volume you need...as everyone has been pointing out.

Personally I just divie air at this point, so I limit myself to 180 foot range (thats were I feel comfortable). I will take double faber 95's (which normally get overfilled) utilizing 1/3's and an AL80 with 36% ( which I go to at 110), and then 80-100% O2 in an AL40.

My RMV is smaller too at .4...

 

[db8us]

I dove with double 15´s a few times and for me they are bulky, heavy and a PITA.
Today i either use Double 12L or D18 for the cave and deeper-stuff.
It also depends on your heigth. I am 1,91m and so long tanks like D20 are also no problem.
Also for boat-diving i think the D12 are great. If you go below a certain depth, D12 is not a lot of gas and i would never penetrate a wreck deeply on a set of D12 if it is let´s say in 75m...

Sorry for all the metric stuff