double 104's

[roakey]

Don Burke once bubbled...
That assumes you are going to breathe them flat. I normally leave 400-500 pounds in the tanks.

2640/500 means you'll have about 20% of the capacity left when you get to your minimum pressure. That leaves about 12 pounds of swing when you dive the tanks.

You're right, you should never breathe them flat, but you should ALWAYS set your weighting so you can.

If, for example, you blow your depth and/or time, you may be forced to breathe them flat, and the LAST thing you want to be doing on top of an emergency situation is to be struggling to stay down.

Always adjust your weighting for as empty a cylinder as possible.

Roak

 

[Don Burke]

Karl_in_Calif once bubbled... You stay shallow, right?

Not unless you consider 100 feet to be shallow.

 

[Karl_in_Calif]

roakey once bubbled...
...Case in point, AL80s that swing -3 to +3. Double them, add two pounds for the manifold/bands and you have a kit that swings from -8 to +4, only 5 pounds more - at the start than a single cylinder, and (just barely) more positive at the end.

Roak

So for warm water, thin wetsuit diving, twin aluminum 80s with a manifold and some kind of backplate, would end up being neutral near the end of the dive. Thanks for the calculations, Roak.

If FIXXERVI6-in-Dallas is diving lakes in Texas, he still needs a drysuit. Lake water gets fairly cold.

If he is going to dive warm water in Mexico, he can rent twin aluminums there. And he does not need to buy steel twin tanks.

Either way, trying to dive with twin steel tanks any size, and a wetsuit, is probably not a good idea. If he has a good reason for buying tanks, and if he really wants to buy twin steel, then he should probably buy a drysuit first. N'est pas?

That then is probably the answer to FIXXERVI6-in-Dallas' original question.

 

[Karl_in_Calif]

Don Burke once bubbled...
Not unless you consider 100 feet to be shallow.

Not exactly "shallow" no.

100 ft is 4 ATAs, so the fine bubbles in your wetsuit are going to be crushed even finer to 1/4th their usual fineness. The suit is going to have zero buoyancy. You are going to be wearing a lot of lead to neutralize the suit at the surface (unless you weight yourself positive and then kick down or something old fashioned like that). The suit is going to have zero insulation. You are going to be cold and maybe start to shiver. I hope you are diving in warm water, and its a thin suit.

The NDL at 100 ft is 20 mins max. 160 cu ft (twin 80s) would last 40 mins or more at 100 ft. That is way too much gas to take down to 100 ft for an NDL dive.

You would be better off with a single tank and a stage tank as a back up if you were diving at 100 ft NDL.

FIXXERVI6-in-Dallas would have no reason to buy doubles under this scenario. He/she would be better sticking to a single tank.

 

[Don Burke]

roakey once bubbled... You're right, you should never breathe them flat, but you should ALWAYS set your weighting so you can.

If, for example, you blow your depth and/or time, you may be forced to breathe them flat, and the LAST thing you want to be doing on top of an emergency situation is to be struggling to stay down.

Always adjust your weighting for as empty a cylinder as possible.

Roak

I disagree with that. The last thing I want to do is be underwater with flat backgas tanks.

There are chambers for bent divers. There are no chambers for drowned divers.

 

[Boogie711]

There are chambers for bent divers. There are no chambers for drowned divers.

I think Roakey's just talking about weighting here, Don...

Say you overstay your dive... are in a Deco obligation... you're at 15 feet... you want to breath your tanks as long as you can at 15 feet. You don't want to come to the surface.

If you're underweighted, you're coming up. If you're weighted for empty tanks, you can easily maintain neutral buoyancy until you need to come up. Worse comes to worse, you can CESA from 15 feet.

It would be a real shame to come up under those circumstances with 500 psi in your tanks ( close to 40 cubic feet of breathing gas!!!) because you weren't weighted for it.

 

[Karl_in_Calif]

Don Burke once bubbled...
There are chambers for bent divers. There are no chambers for drowned divers.

This is a phrase we use to describe the trade off between omitted deco versus going back down after omitted deco.

I am not sure it has any other valid applications.

 

[Don Burke]

Karl_in_Calif once bubbled... Not exactly "shallow" no.

100 ft is 4 ATAs, so the fine bubbles in your wetsuit are going to be crushed even finer to 1/4th their usual fineness. The suit is going to have zero buoyancy. You are going to be wearing a lot of lead to neutralize the suit at the surface (unless you weight yourself positive and then kick down or something old fashioned like that). The suit is going to have zero insulation. You are going to be cold and maybe start to shiver. I hope you are diving in warm water, and its a thin suit.

The bubbles in foamed neoprene don't compress quite as fast as a free bubble and the gas is only a part of the suit, so the suit is quite a bit thicker than 1/4 of the surface thickness. The buoyancy and the R-value don't go to zero at only 4 ATM in any case.

At the surface I have ten extra pounds of gas to help me get under. It hasn't been an issue.

I wouldn't call the water all that cold. Not all of us live in California.

Karl_in_Calif once bubbled... The NDL at 100 ft is 20 mins max. 160 cu ft (twin 80s) would last 40 mins or more at 100 ft. That is way too much gas to take down to 100 ft for an NDL dive.

I guess you have never heard of EAN.

Try these numbers: tanks that are really about 77 cubic feet, the no-stop time for EAN32 at 100 feet, and planning to leave the bottom with 1000psig to deal with unforseen problems.

Karl_in_Calif once bubbled... You would be better off with a single tank and a stage tank as a back up if you were diving at 100 ft NDL.

I'm not much for independent doubles, which is what a single with a stage really is.

I'm interested is seeing what your figures are using _my_ inputs.

 

[Karl_in_Calif]

Don Burke once bubbled...
I'm not much for independent doubles, which is what a single with a stage really is.

A single with a stage backup works great. Your back tank can be steel. Your stage is aluminum. You avoid the non-ditchable weight issue of twin steel tanks.

If you like EAN32 or EAN36, a 100 cu ft tank works fine. Even a 120 cu ft single will have less negative buoyancy than twin steel would.

The stage gives you redundancy during your ascent. You plan your whole dive including the egression and safety stops with your back gas, then you carry a stage with enough gas to substitute during the egression and safety stops. You sling this tanks for easy access, and keep it turned off until you need it.

If you load EAN32 into your back tank, you can load EAN36 into your stage tank, for more efficient off-gassing of N2.

A single tank with a stage backup gives you lots of flexibility, which you do not have with twin steel tanks alone.

 

[Don Burke]

Boogie711 once bubbled... I think Roakey's just talking about weighting here, Don...

Say you overstay your dive... are in a Deco obligation... you're at 15 feet... you want to breath your tanks as long as you can at 15 feet. You don't want to come to the surface.

If you're underweighted, you're coming up. If you're weighted for empty tanks, you can easily maintain neutral buoyancy until you need to come up. Worse comes to worse, you can CESA from 15 feet.

It would be a real shame to come up under those circumstances with 500 psi in your tanks ( close to 40 cubic feet of breathing gas!!!) because you weren't weighted for it.

I see what Roakey is saying. I just don't buy into the logic. I can easily fin the couple of pounds that the 500 psig will require if the need arises. I'm not the least bit interested in dealing with the extra volume in my wing on all of the dives that I don't have to breathe my backgas tanks flat on.