Weighting for doubles

[jamiep3]

Just put together a set of AL80s, really want HP100s, but I had all the stuff to assemble the 80s and figured I could start with them and then move up.

My question is, if you are planning multiple dives on the same set of doubles, do you weight yourself for both dives at the start, or weight yourself based on expected gas consumption for the first dive and then adjust before the second?

Really hope this is not a dumb question, just seems to me if you weighted for both dives at the start, you would have a lot of excess lead on.

 

[rjack321]

You need to weight yourself to be able to hold your deco stops after a complete gas loss. So yes esp. with double al80s you need more lead. Not as much as you'd think since you have more valve brass and some SS bands compensating a little.

 

[Kevrumbo]

Just as a starting reference Rule-of-Thumb, take half of the weight you would normally use for a single tank AL80 and your exposure suit, and then add 4 to 6 lbs more.

For instance, I use 26 lbs for a single AL80 tank in a drysuit: take half (13lbs) and add from 4 to 6lbs (which becomes a total 17 to 19lbs weightbelt range for my double AL80's).

 

[jamiep3]

You need to weight yourself to be able to hold your deco stops after a complete gas loss. So yes esp. with double al80s you need more lead. Not as much as you'd think since you have more valve brass and some SS bands compensating a little.

Hadn't considered a complete Gas loss. Won't be getting into deco stops for a while, but would not want to blow a safety stop.

Planning on shallow dives in the quarry and a lake to get comfortable first, then trying them off shore in some wreck diving, then taking intro to tech and advanced Nitrox. There where some guys on a boat with me last year doing advanced classes that had not gotten used to the doubles before the class, and I noticed they where having a lot of problems.

 

[Cave Diver]

You need to weight yourself to be able to hold your deco stops after a complete gas loss. So yes esp. with double al80s you need more lead. Not as much as you'd think since you have more valve brass and some SS bands compensating a little.

I agree. You need enough weight to be able to hold a safety stop with near empty tanks even if you dont plan to have near empty tanks.

 

[DA Aquamaster]

I take a different approach to figuring the change in weight needed from a soingle AL 80 to double AL 80's.

The manifold, bolts and bands will add 7 lbs of weight and nearly that much negative bouyancy over the single a single valve on a single AL 80. The second AL 80 adds 4.4 pounds of positive bouyancy if if is near empty.

-7 + 4.4 = -2.6 pounds net loss in bouyancy, so you can reduce your normal AL 80 (neutral at the end of the dive with almost no gas in the tank, none in the BC) lead weight requirement by about 2 to 3 lbs. First stage regs differ, but on average are probably 2 pounds negative, so take 2 more pounds off for the extra first stage and you should be very close to perfectly weighted with double AL 80's - 4 to 5 pounds total weight reduction compared to a single AL 80.

If switching from a recreational BC (often 2-3 pounds positive) to an aluminum back plate (-2 pounds buoyancy on average) or stainless steel back plate (-6 pounds buoyancy on average) can mean taking off another 2 to 4 pounds of lead, bringing you to 6 to 9 pounds total reduction in lead weighting.

+ 2 or 3 with the need to weight for the worst case almost empty tank condition as you need to be able to hold a 10 ft deco stop even with near empty tanks. They may in fact be near empty and many deco bottles are close to neutral when empty and a near empty AL80 will be slightly positive even with the reg and stage strap attached as it approaches empty.

That means you will have 10 pounds more negative buoyancy at the start of the dive as the full double AL 80's will be 10 pounds more negative than their empty condition. That, the additional mass of doubles and changes in trim related to doubles and often a newly purchased BP/wing can combine to make transitioning to doubles difficult, especially with the task loading learning new things in a tech class. Consequently, I agree with the idea of divers getting comfortable with doubles in recreational settings before attempting a tech class.

 

[NorthWoodsDiver]

on my aluminum 80's I use a 7lb V-weight in my thin undergarments and a 11lb in my thick undergarments. usually I have other bottles and reels and lights and... that I dont require any extra lead however I like to clip a couple 2lb weights on the upline in case someone should need them including me. My backplate is also a 9lb SS version.

 

[halocline]

My question is, if you are planning multiple dives on the same set of doubles, do you weight yourself for both dives at the start, or weight yourself based on expected gas consumption for the first dive and then adjust before the second?

Really hope this is not a dumb question, just seems to me if you weighted for both dives at the start, you would have a lot of excess lead on.

It's an interesting idea in specific situations, like a shallow rec dive where you wouldn't be in trouble if you accidently lost all the gas and ended up a few pounds positive. Worst case you might spend a few minutes upside down at your stop swimming in place, but I've done that on a single tank after handing off too much weight while DMing.

In general, though, as the others have said, you have to weight yourself for empty tanks, and the more gas you carry, the more negative you'll be at the beginning. For me it's part of the learning curve to getting comfortable with doubles before taking them into caves.

 

[rjack321]

Hadn't considered a complete Gas loss. Won't be getting into deco stops for a while, but would not want to blow a safety stop.

Planning on shallow dives in the quarry and a lake to get comfortable first, then trying them off shore in some wreck diving, then taking intro to tech and advanced Nitrox. There where some guys on a boat with me last year doing advanced classes that had not gotten used to the doubles before the class, and I noticed they where having a lot of problems.

You also consider just taking the intro to tech type course earlier, after maybe 2 or 3 lake/quarry dives. The class is there to help you get the weighting/trim and all this kinda stuff worked out. Its not a class where you are expected to have worked these personal issues out (offshore) beforehand (like how much lead do I really need).

 

[Doc Harry]

V weights are they way to go.

At the end of the dive I dump down to 300 PSI and ensure that I can stay down at 10 feet. Add or subtract weight accordingly.