doubles, E7-100 or E8-130?

[lamont]

i'm starting to think about a future set of doubles (not in any rush at this point to start doing deco, but i'm starting to play with math). my interest in doubles and deco at this point would primary be in extending bottom time in the 120 fsw range. so, i'm trying to plan out what a dive would look like for me with double 100s and double 130s, based on max bottom time from backgas. my SAC rate kinda blows and is .6 on a good day, and 0.75 for planning ( even though i'm pretty comfortable in the water at this point):

2 x 100s @ 3500 psi =

rock bottom = 40 cu ft = 725 psi
thirds = ( 3500 - 725 ) / 3 = 925 psi
'turn pressure' = 3500 - 925 psi = 2575 psi
usable capacity = 2 * 925 psi = 1850 psi
usable cu ft = 1850 psi * 200 cu ft / 3500 psi = 105 cu ft
bottom time = 105 / ( .75 * (120 / 33 + 1) ) = 35 mins

2 x 130s @ 3500 psi =

rock bottom = 40 cu ft = 500 psi
thirds = ( 3500 - 500 ) / 3 = 1000 psi
'turn pressure' = 3500 - 1000 psi = 2500 psi
usable capacity = 2 * 1000 psi = 2000 psi
usable cu ft = 2000 psi * 260 cu ft / 3500 psi = 148 cu ft
bottom time = 148 / ( .75 * (120 / 33 + 1) ) = 50 mins

am i doing the air planning correctly here? if i am, there doesn't seem to be much point to double 100s for me, so should i start weightlifting to deal with the dual 130s?

and i understand that all my questions will be answered by a Tech I class, but i'd like to plan out a doubles config well in advance -- so I've got a little bit of a chicken-and-egg issue.

 

[novadiver]

i'm starting to think about a future set of doubles (not in any rush at this point to start doing deco, but i'm starting to play with math). my interest in doubles and deco at this point would primary be in extending bottom time in the 120 fsw range. so, i'm trying to plan out what a dive would look like for me with double 100s and double 130s, based on max bottom time from backgas. my SAC rate kinda blows and is .6 on a good day, and 0.75 for planning ( even though i'm pretty comfortable in the water at this point):

2 x 100s @ 3500 psi =

rock bottom = 40 cu ft = 725 psi
thirds = ( 3500 - 725 ) / 3 = 925 psi
'turn pressure' = 3500 - 925 psi = 2575 psi
usable capacity = 2 * 925 psi = 1850 psi
usable cu ft = 1850 psi * 200 cu ft / 3500 psi = 105 cu ft
bottom time = 105 / ( .75 * (120 / 33 + 1) ) = 35 mins

2 x 130s @ 3500 psi =

rock bottom = 40 cu ft = 500 psi
thirds = ( 3500 - 500 ) / 3 = 1000 psi
'turn pressure' = 3500 - 1000 psi = 2500 psi
usable capacity = 2 * 1000 psi = 2000 psi
usable cu ft = 2000 psi * 260 cu ft / 3500 psi = 148 cu ft
bottom time = 148 / ( .75 * (120 / 33 + 1) ) = 50 mins

am i doing the air planning correctly here? if i am, there doesn't seem to be much point to double 100s for me, so should i start weightlifting to deal with the dual 130s?

and i understand that all my questions will be answered by a Tech I class, but i'd like to plan out a doubles config well in advance -- so I've got a little bit of a chicken-and-egg issue.

there are other ways to plan this and it gets easier if you use a single deco mix. you really can't rock bottom unless you know your buddies sac anyway

 

[mweitz]

Lamont,

Are your dives really thirds dives or are they halves dives or even all usable? I would think in Seattle most of your dives would be half or all usable ....?? If you are doing (or are planning on doing) thirds dives then I would consider the bigger tanks. I think in the end you end up with more than one set of doubles. Double E7-100's (or PST HP 80's) are considered by many a good "all around" doubles set for OW / Tech 1 diving.

I would also not look at them at 2 x 100, I would do the calculations as 1 x 200 @ 3500 PSI. Less math and less chance of error.

Your math looks correct though. EDITED <SNIP>

For rock bottom I was taught to use 1 CF per minute. If you calculate on that and do the math on your average depth on a 100' ascent you *should* be OK. Of course, with a thirds dive it could be different.

Mark

 

[teknitroxdiver]

It looks like the weight difference would be about 20 lbs, depending on your manifold and bands. They would be about 70lbs and 90lbs, respectively, in doubles setup. I'd go with the 100s for now, they're lighter and easier to move around but will still give you good enough bottom time for now. Once you get used to the 70#ers and move up to Tech 2/3 then get some 130s or whatever is the best when that time comes.

 

[lamont]

Lamont,

Are your dives really thirds dives or are they halves dives or even all usable? I would think in Seattle most of your dives would be half or all usable ....??

I think this gets to the issue of where my knowledge is really lacking. What I'm thinking of is just getting more bottom time on non-penetration wreck dives around 120-130 fsw using 30/30 as a backgas and a deco bottle of EAN50. Its nothing more than I do now with a single E8-130 of EANx, only with a longer runtime and some light deco. If I don't have to plan on thirds for that, it makes the double 100s look a lot better.

Yeah, okay, I think I see where I'm confused. I'm making a hard analogy between cave diving and the "soft overhead" of decompression. There's a fundamental difference though between having a "deco" overhead and a rock overhead which is that with the rock you can't arbitrarily decide to head up and start doing deco, you must exit the cave first...

Aha, *lightbulb*...

Cool, thanks for the help all...

 

[teknitroxdiver]

If you've already got a 130, you might consider getting another 130 and go ahead and learn to deal with the extra 20 pounds. But it would be nice to have a 'light' set of doubles for when you do a dive that doesn't require a lot of gas.

 

[mweitz]

Just as an FYI, full E7-100's doubled with manifold and bands are right around 90#. Add in your BP, wing, regs, and any weight you need and you are looking at 100# or so.

I would really consult with whomever you are going to do your Tech 1 with. Since they are familiar with your environment and your diving style (presumably) they would be the most qualified to answer.

Best of luck,

Mark

 

[Warren_L]

You can also consider the E7-120s. I recently doubled mine up, and they're about 12lbs lighter than the E8-130s doubled. I think my rig weighs about 120lbs total the last time I checked, and I've managed to trim them out properly too. And with the same bands you can always switch back to the E7-100s as well.

 

[lamont]

i've already got four e8-130s, so when i start heading down this road i'll probably double them up at some point anyway. but looking at the specs it looks like the e7-100's are 24# lighter offset by 4# more buoyancy (for the set), and i'm thinking that 20# less might help getting used to doubles. it'd only be about 1.5 times as much tank as i already carry now instead of 2 x the weight. and i'm not sure right now what i'd be doing with almost 90 mins of bottom time at 120 fsw other than turning into a popsicle...

also, for recreational shore diving, i usually wind up with around 800-900 psi or more left in my e8-130s and if you work the numbers i should be able to get two of those dives off double 100's...

thanks for the help...

 

[TwoTanks]

Just to throw some more numbers at you...

35 min at 120 feet on EAN30 means:

30 minutes of deco time without a deco gas

or

15 minutes of deco time with EAN80 (strokemix) deco gas.

So unless you want long deco stops, you should take a Deco / adv Nitrox class, and in addition to getting certified for greater than 40% O2 you will get trained in how to deal with all the tech issues.

I wouldn't consider 30 min of deco a "soft overhead". If you just "arbitrarily" head up and do a drifting deco you could end up miles from the boat.

By the way 35min at 120 with 30 min deco with a sac of .6 = 134 cf, add 1/3 = 200 cf.
So you can do it with 2 E7-100s.

All of the above calculations were done using Deco Planner.

TT