double 104's

[Don Burke]

FIXXERVI6 once bubbled... After reading these posts, thank you all, its been helpfull, I won't be buying 104's, or probably even 95's. I talked to a local that will let me jump in the pool with 98's but suggested a double bladder wing.

so now I've changed my mind down to double standard 80's and a 45# wing.

I think you'll do fine. The standard 80s will leave you with enough ditchable weight.

There are many divers for whom the no-stop limits aren't much of an issue as long as they dive a single 80. With double 80s and EAN32 below about 60 feet, the tables are your limit, so profiles start becoming very important. That "safety stop" that the agencies recommend becomes very important and should be considered a decompression stop when you get near the edges. Spending a few minutes at 20 feet is a pretty good idea too.

If you are keeping yourself above about 90 feet, EAN36 is an option. EAN36 and double 80s go together very nicely.

Something I found is that reducing the concern for running out of gas reduced my gas consumption, so get used to the idea of coming off the bottom with a lot of gas on many profiles.

 

[Spectre]

lal7176 once bubbled...
It doesn't take a rocket scientist to see that a person wearing double 95's, 104's, etc using 32% gas could easily exceed the NDL of 25min at 109 ft. It does ease things a bit compared to 21% at 110ft for 15min NDL.

See, that's because you're using the wrong mix. If you would stop being married to 32%, you'd see that you can't exceed NDL limits if you dive the optimal mix for that depth.

I feel so enlightened!

 

[Dryglove]

I feel so enlightened!

Ok, me too

 

[Spectre]

Karl_in_Calif once bubbled...
When you learn how to do math for EAD NDL calculations, come back.

MOD 127.

160/33 = 4.85. 1.4 / 4.85 = ~ 28%

4.85 * .911 = 4.4 = 113.

Ok.. I'm back.

So a dive to 127 on the optimal mix has an EAD of 113. Now what was your little trick? 120 - 113 = 7 minutes at depth.

Hmm. Lets say we go with 15 minutes.

0.7 * 15 * 4.85 = 51 cu ft

51 * 1.5 = 76.5.

So you're saying I can do a 20 minute dive to 127, with an aluminum 80 safely?

Ok... cool.

So we've got 25.5 cuft for our ascent.

127 would be about a 4.33 ascent @ average 63.5 feet, or 8.86 cu ft.

So if my buddy has a problem, We'd be using 17.73 cu ft to get back to the surface. We had 25.5 cu ft.

So we've got 7.77 cu ft to spare! That's over 300 psi on my aluminum 80!

Wow! Wonderful Method! I never would have dreamed that an aluminum 80 would be safe for a dive to 127 feet! Cool!

 

[Karl_in_Calif]

Spectre once bubbled...


MOD 127.

160/33 = 4.85. 1.4 / 4.85 = ~ 28%

4.85 * .911 = 4.4 = 113.

Ok.. I'm back.

So a dive to 127 on the optimal mix has an EAD of 113. Now what was your little trick? 120 - 113 = 7 minutes at depth.

Hmm. Lets say we go with 15 minutes.

I do not recommend to air or nitrox divers to dive deeper than 100 ft. Forget 127 then. Burke was talking about 100 ft dives. Thats where I drew the line as well.

Try my math, and see what you get. If 40 mins on EAN35 at an EAD of 80 ft makes you nervous, try NAUI's 35 mins instead. You will then have a slightly larger reserve, with the same sized tanks.

P(total) at 100 fsw is still 4.0 whether you are breathing nitrox, air, or anything else. I cannot say that I see where all of your confusion is coming from. Rule of thirds is plenty of gas to get back to the surface on, from 100 fsw, diving NDL.

Your nitrox dive computer dialed into 35% is going to tell you to surface a lot sooner than 35 minutes, as well.

 

[Karl_in_Calif]

lal7176 once bubbled...


Also be careful as it is very easy to go beyond NDL limits and not even realize it when you are diving doubles. Be very aware and constantly monitor your gauges/computer to make sure that doesn't happen. I also advise against pushing the limits of NDL as you may find yourself in a deco dive or bent.

OK I will do the math for you.

EAN 32 @ 100 ft = NDL 30 mins per NAUI (off the EAN32 table)

72 + 72 = 144 cu ft gas

100/33 + 1 = 4.0 atas at 100 fsw depth

RMV ~ 0.75 swimming/working assumed

30 x 0.75 x 4 = 90 cu ft consumed during full NDL period

144 - 90 = 54 cu ft reserve

54/144 x 2400 psig = 900 psi remaining.

I suspect he would turn around when either of the following first occurs:

1) his nitrox computer started beeping at him within 30 mins

2) his spg reads 1200 psi (one-half of 2400)

Sure, if he totally fails to pay attention to his dive computer or his SPG, then yes he could venture into decompression. But it is not likely with nitrox, since both instruments are telling him it is time to turn around and go home, before he reaches the NDL limit.

The NDL limit would roughly occur when he has 900 psi left, by which time he should have already turned around and headed up.

With air, however, you are absolutely right. The NDL comes up fast. And then your SPG can be deceiving.

The moral of the story is: If you do the math, you find out where your disagreement or misconception is. NAUI's NDLs are a lot more generous than the ones you are using, LAL.

 

[Karl_in_Calif]

Don Burke once bubbled...
If you are keeping yourself above about 90 feet, EAN36 is an option. EAN36 and double 80s go together very nicely.

Exactly right!


Because:

P(total) = PO2/FO2 = 1.4/0.36 = 3.9 ATAs MOD for EAN36

(3.9 - 1 ) x 33 = 96 ft MOD

EAN36 NDL per NAUI at 90 ft is 50 mins

80 + 80 = 160 cu ft

RMV assumed ~ 0.75 cu ft / min

160 / 3.9 / 0.75 = 55 mins (computed for 96 ft)


However, you cannot be married to EAN32 if you want to compute and utilize the "best mix."

Best mix = FO2 = PO2/P(total) for any given depth.

 

[Soggy]

Karl, what is the point you are trying to make? You've proven that you can do math that most of us learned in our OW or AOW classes.

As for the original question. Double LP104s are way too heavy in the water for a wetsuit....you'll be a big anchor. LP72s aren't much better, however, as their in-water buoyancy characteristics aren't much different. Double steel tanks and a wetsuit are a bad idea, basically.

 

[Karl_in_Calif]

Soggy once bubbled...
As for the original question. Double LP104s are way too heavy in the water for a wetsuit....you'll be a big anchor. LP72s aren't much better, however, as their in-water buoyancy characteristics aren't much different. Double steel tanks and a wetsuit are a bad idea, basically.

Agreed.

It looks like we are all telling him to go with twin alum 80s for his wetsuit diving, keep his depth to 100 or shallower, and breathe nitrox.


P.S. If SSI taught you that in BOW or AOW, then you had one awesome instructor! I would venture you learned it in your nitrox class. But even that is pretty impressive for a basic nitrox course, since basic nitrox is usually table oriented, and people forget the formulae pretty fast, if indeed they ever learned formulae.

 

[chickdiver]

Karl_in_Calif once bubbled...


Agreed.

It looks like we are all telling him to go with twin alum 80s for his wetsuit diving, keep his depth to 100 or shallower, and breathe nitrox.


P.S. If SSI taught you that in BOW or AOW, then you had one awesome instructor! I would venture you learned it in your nitrox class. But even that is pretty impressive for a basic nitrox course.

Uhhhh nooo.. Its the fact that he had a COMPETENT instructor- the agency is irrelevant. BTW_ I teach this in Basic courses- where it SHOULD be taught.