Single vs. double tanks

[Dryglove]

I am currently using a OMS 55lb wing and will be going to a 45lb in the next few weeks as the 55lb wing seems a bit overkill. I dive cold water with a drysuit. I think a 45lb wing will suit my needs pretty good.

 

[DA Aquamaster]

roakey once bubbled...

Oh my gawd, I thought we taught Archemedes principle in OW, I guess not.

What a rude remark, especially from a regulator. I would have thought you would have had both the tact and the brains to express yourself in a more civil manner.

If you are using a SS backplate and integrated weights and you want your stuff to actually float on the surface without you in it, you need the extra lift.

Safely diving doubles is one thing, doing it practically and comfortably is another. It's nice to be able to float yourself a little higher out of the water in rough seas, especially if it just ain't your day and you have a flooded dry suit. Also, having equipment that will float on it's own greatly expands your dive boat and exit possibilities.

 

[Karl_in_Calif]

lal7176 once bubbled...
I have two sets of steel dbl 72's. ...The down side is that i have to pay twice as much for fills.

Don't you also get twice as much time in the water for your twice as much for the fills???

 

[Karl_in_Calif]

ColdH20diving once bubbled...
I met a diver recently who was diving with double 72's. He told me he used to dive with a larger single tank, but found that the shorter 72's offered better balance and he was able to maintain an almost level position as opposed to using a larger single tank. I like diving my single 112 but have considered going to a smaller set of doubles especially if it helped maintain a level position underwater. I've always heard that double's tend to increase your drag and take some getting used to.

Double 72s are quite popular. I have a number of friends who dive with them.

By the way, the new Faber 72s are service rated to 3000 psi, and you can fill them to 3300 with the "+" rating, which loads them with 79 cu ft of air, similar to an HP 80. The Faber 72s are almost the same dimensions as a PST HP 80. The buoyancy characteristics are totally different, however.

Twin 72s give you about 140 cu ft of air or nitrox.

That equals about 70 mins dive time at 33 fsw,

35 mins at 99 fsw, etc.

Your time will be slightly more or less, depending on your RMV.

You definitely need to watch your dive computer or dive slate to make sure you do not overstay your NDL with this much dive time available to you at the deeper depths.

The Navy and NOAA dive tables use a 120 sum factor for NDL limits at the deeper depths for air. If you dive conservatively, then you could use this 120 sum factor for EAN32.

So at 100 fsw with EAN32, your NDL limit would be 20 mins. (100 + 20 = 120). But your twin 72s hold way more gas than this. In fact, you would have a 15 minute reserve after 20 mins at 100 fsw.

Twin 72s are definitely a nitrox rig, if you are diving conservatively. Nice, because they give you complete redundancy.

The only drawback is that two tanks that weigh 35 lbs each are going to weigh a lot together out of the water. You need a good strong back if you are going to dive like this.

 

[ColdH20diving]

Karl_in_Calif once bubbled...


The only drawback is that two tanks that weigh 35 lbs each are going to weigh a lot together out of the water. You need a good strong back if you are going to dive like this.

My Faber 112 empty comes in at 41 pounds and is -1 pound buoyancy when empty. I could handle the additional weight but it most likely would leave me with no ditchable weight, which I assume is common when diving doubles. My 112 offers plenty of bottom time, I usually reach my NDL before getting low on air on deep dives. I have been carrying a 19cuft pony bottle on deep dives, I could eliminate that with a double tanks. My LDS has a surplus of 72's maybe I can talk him into letting me try before I buy.

 

[pescador775]

I agree with Roakey on this one. 'Aquamaster' should have listed his menu of stuff before stating that a 45 pound lift is required for double 72's. Diving in Cozumel and other tropical places I always used double 72's. No weight or extra buoyancy is required but a vest should be worn for emergency flotation. A word of caution, compared to a single 72 the doubles have significantly more drag. There are annoying currents in the Yucatan strait and swimming with doubles will require more effort and 10% more air consumption compared to a single 72. Be aware that single tanks larger in dia than a 6.9" 72 also carry a penalty, albeit less.

 

[roakey]

DA Aquamaster once bubbled...
What a rude remark, especially from a regulator. I would have thought you would have had both the tact and the brains to express yourself in a more civil manner.

Wow. We're human after all I guess. We can contribute the the board just like anyone else can, my moderator hat's off when I'm simply contributing.

This massive amount of lift misinformation crops up from time to time an I'm real tired of seeing it again and again and wish dive shops that teach this stuff would finally figure out that they're doing every one a disservice by pushing it. Of course the reason they're pushing it is because they get to sell more gear...

DA Aquamaster once bubbled...
If you are using a SS backplate and integrated weights and you want your stuff to actually float on the surface without you in it, you need the extra lift.

Let's see, how to I phrase this... You're right about what you want to do, but wrong about how much lift you need to do it...

Let's do the math with my 104 setup. I'm diving dry, in fresh water, and all my weight is in the backplate/cylinder combination. Backplate: -6 pounds. Full 104s: about -15 each in fresh water or -30 total. Manifold, call it -2. 6+30+2 = 38 pounds.

Though not a huge margin, the 45 is just fine...

DA Aquamaster once bubbled...
Safely diving doubles is one thing, doing it practically and comfortably is another. It's nice to be able to float yourself a little higher out of the water in rough seas, especially if it just ain't your day and you have a flooded dry suit. Also, having equipment that will float on it's own greatly expands your dive boat and exit possibilities.

Ok, already coverd the equipment floating on it's own. Higher out of the water? You're calling for almost DOUBLE the lift required? Are we talking about floating ourselves higher out of the water, or if diving trimix are we talking becoming a dirigible and flying back to shore?

Note the requirement for lift is DECREASED with 72s versus 104s, by, taking a SWAG, about 5 pounds, even if all your weight is on the BP. This should make the 45 pound lift wing have plenty of extra for floating you higher in the water without going overboard.

So, if a 45 pound lift wing can float a dual 72 kit with room to spare, it can just as easily float you in a flooded drysuit as well, don't you agree? (and if not, why not?)

Roak

 

[Karl_in_Calif]

Based on the NAUI dive tables for reference (no particular reference is sacred), here is what I get for ideal tank sizes (based on total air consumption) given the following planned depths and assuming a 0.75 surface RMV:


100 ft

Air21 = 76 cu ft
EAN32 = 100 cu ft
EAN36 = 130 cu ft (note your ppO2 at 100 ft will be in the high range)

pony = 13 cu ft (or add this to the above amounts for your buddy if no pony is being brought)


50 ft

Air21 = 160 cu ft
EAN32 & EAN36 = off the scale / the NDL is over 3 hours

pony = 7 cu ft (or add this to the above for your buddy if no pony is being brought)


The point being, twin 72s are way too much gas for anything but really shallow diving. You would exceed your NDLs with twin 72s at any depth deeper than about 50 ft, OR ELSE be hauling around a lot of spare gas that you do not need.

Therefore, depending on your planned depth, for NDL diving, twin 40s would make a lot more sense than twin 72s, if you really want to wear twin tanks, and gain the advantage of the redundancy and trim.

Twin 40s are going to weigh a lot less on the shore or boat than the twin 72s as well.

 

[Karl_in_Calif]

roakey once bubbled...
Though not a huge margin, the 45 is just fine...

I wish the equipment makers would figure this out for themselves, and then stop making B/Cs with lift LESS than 45 lbs.

 

[Dryglove]

Karl_in_Calif once bubbled...


Don't you also get twice as much time in the water for your twice as much for the fills???

I sure do