Your idea of ideal twins

[lamont]

I posted a thread a week or two back where I was trying to figure out roughly how much bottomtime doubled E7-100s would give me for Tech I dives. It came out to around 50 mins @ 120 fsw or 30 mins @ 180 fsw for 0.75 SAC. Also, since I usually come up with 1000 psi on my E8-130s, 200 cu ft of gas pretty much gets me two rec dives off the doubles as well. Double 80s would be a bit small for my purposes, and looking at doubling up my E8-130s I just wind up thinking how cold I'd get if I actually used all that gas (and how long I'd have to hang to deco...)

Last week I tried on a set of doubled E7-100s and the weight actually felt pretty similar to my single E8-130s. I think I'm sold on the double 100s...

 

[doole]

Last week I tried on a set of doubled E7-100s and the weight actually felt pretty similar to my single E8-130s. I think I'm sold on the double 100s...

Hmm. I kind of like the sound of that, too. Then I could keep the 120's for singles.

Who is the manufacturer?

 

[Munin]

I posted a thread a week or two back where I was trying to figure out roughly how much bottomtime doubled E7-100s would give me for Tech I dives. It came out to around 50 mins @ 120 fsw or 30 mins @ 180 fsw for 0.75 SAC. Also, since I usually come up with 1000 psi on my E8-130s, 200 cu ft of gas pretty much gets me two rec dives off the doubles as well. Double 80s would be a bit small for my purposes, and looking at doubling up my E8-130s I just wind up thinking how cold I'd get if I actually used all that gas (and how long I'd have to hang to deco...)

Dives within Tech 1 limits are limited by the volume of deco gas you are carrying, not your backgas. Both of the dives you mentioned exceed that limit, the latter by more than 50%. You will not need as much gas as you think, unless you are diving past the class limits. Generally, diving Al80s, the people I dive with turn all dives based on time rather than gas.

 

[nalfein]

I'm 6', 180 lbs and I dive twinned PST 130's from shore all the time. We're up and down over rocky coastline for entry and it's not so bad. Don't get me wrong.... they're heavy, but if you've ever done any time under a rucksack it's totally manageable. A guy your size should be able to handle those 120's with no problem, provided your knees and back are healthy.

 

[Green_Manelishi]

I am about to engage in a tech course and I am considering the purchase of a pair of new bottles. I am looking for recommendations in terms of capacity and bouyancy characteristics.
chris

Provided they don't create an unbalanced rig (i.e. way too negative at start) I suggest the highest capacity tanks you can carry. Backgas is like ammunition; as long as you can carry it, there is no such thing as too much.

 

[lamont]

Dives within Tech 1 limits are limited by the volume of deco gas you are carrying, not your backgas. Both of the dives you mentioned exceed that limit, the latter by more than 50%. You will not need as much gas as you think, unless you are diving past the class limits. Generally, diving Al80s, the people I dive with turn all dives based on time rather than gas.

Actually the limiting factor there pushing me towards the E7-100s instead of smaller doubles is that I want to be able to do two rec dives of a single fill on the doubles, equivalent to what I do now on my 130s. For that, the 100s might even be a little bit small (particularly if I get a short fill).

 

[Munin]

Actually the limiting factor there pushing me towards the E7-100s instead of smaller doubles is that I want to be able to do two rec dives of a single fill on the doubles, equivalent to what I do now on my 130s. For that, the 100s might even be a little bit small (particularly if I get a short fill).

I was just addressing the Tech 1-level dives you spoke of. I didn't mean to suggest that you should use 80s or shouldn't get the 100s. They look like a good choice, much better than 80s if you're using a drysuit.

 

[lamont]

I was just addressing the Tech 1-level dives you spoke of. I didn't mean to suggest that you should use 80s or shouldn't get the 100s. They look like a good choice, much better than 80s if you're using a drysuit.

Yeah, the Tech 1 level dives weren't really meant to be Tech 1 level dives, either. I was just trying to get a back-of-the-napkin idea of what to expect, so I could figure out what tanks to double before taking Tech 1. The conclusion I was going for there was just that the double E8-130s would turn me into a popsicle even without considering the deco obligation and that those profiles exceed my goals in what dives I'd like to do. If I can exceed Tech 1 limits on E7-100s, all the more reason that those tanks feel like enough, and the double 130s seem excessive for me...

 

[Daryl Morse]

Actually the limiting factor there pushing me towards the E7-100s instead of smaller doubles is that I want to be able to do two rec dives of a single fill on the doubles, equivalent to what I do now on my 130s. For that, the 100s might even be a little bit small (particularly if I get a short fill).

If by rec dives, you mean dives within NDL, I'd expect you would hit the limit easily with 100s.

My buddy dives with double Faber 95s and I'm still using a single AL80. He dives with a Suunto computer and I have an Apeks Quantum, both relatively conservative. Over the weekend, we did two dives to around 90-100 fsw and pushed pretty hard on NDL for both of them. (Okay, we crossed it. My bad.)

Both of my tanks were at 2900 psi once I got into the 46F water and at 500 psi after conservative stops each time.) My buddy uses more air than I do, but with the extra capacity, I was the limiting factor. If I was diving with 100s, I would have long since run out of NDL. (My SAC is around 0.6 CFM and improving.) Unless you've got a really liberal computer or you have a pretty high SAC, 100s should be ample for two "rec" dives.

 

[lamont]

well, on a 'good' multi-level shore NDL dive, I'll usually go from 3500 to 1000 psi on my 130s. that makes for: 2500 psi * ( 130 cu ft / 3500 psi ) = 92 cu ft -- which would only leave 16 cu ft * ( 3500 psi / 200 cu ft ) = 280 psi left in double 100s. cutting rock bottoms that close on doubles doesn't leave a lot of room left for SPG error...