Weighting for doubles

[WD8CDH]

Your weight swing from the beginning of the first short dive to the end of the second short dive will be the same as the swing from the beginning to end of a long single dive so weigh your self the same for the first short dive as you would for a long dive. That will familiarize you to the rig beginning of dive characteristics.

For the second dive, If you are primarily trying to familiarize your self to the beginning of dive characteristics, add weight equal to the air used for the first dive (about 6 pounds for 80 CF) preferably as a V weight to the tanks.

If you are primarily trying to familiarize your self to the end of dive characteristics on the second dive, don't add the weight.

 

[Rick Inman]

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.

This is the answer, IMHO, whether you are doing "deco" stops or not. I believe rjack is referring to even recreational stops/ascent profile.

... usually I have other bottles and reels and lights and... that I don't require any extra lead however I like to clip a couple 2lb weights on the upline in case someone should need them including me.

I never count anything I am clipping on, or in my pockets as weight. I need to be able to lose all my stages, light, reel, etc. and still stay down. And anything on the upline I consider never having at all, because I might never get back to it.

I have buddies/instructors I dive with who have a ton more experience who trust the upline for O2, etc., but I always think that if it isn't on my person, I can't plan for it - even as a contingency.

 

[jamiep3]

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.

I like your approach, easy to follow.

Switched to BP/W last year, have been doing redundant regs on my single tanks for a couple of years. Trying to build up to this in baby steps. Tried the tanks out in the pool, 3mm suit, and my regular mask and fins, and was I'm guessing about 4-5lb heavy with the tank full. Hoping to get to the quarry next week.

 

[cool_hardware52]

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.

The easy way to calculate weighting is to FIRST determine the buoyancy of your exposure suit.

The goal of course is to have your total ballast (all the things that don't float like back plates and regs and can lights and tank bands and manifolds etc.) equal to the buoyancy of your suit at your shallow stop with empty tanks.

Once you know the buoyancy of your suit the rest is simple addition or subtraction.

Tobin

 

[DA Aquamaster]

That's basically the same approach - if you know what you need in a single tank in a given exposure suit, it is just addition and substraction to figure out the change in buoyancy and how much weight you need to add or subtract with a given set of doubles.

 

[WD8CDH]

Once you have weighted your self with your single, weigh the single with a fish scale while the rig is submerged. Then weigh the new rig the same way and simple addition or subtraction will give you a starting point.

 

[b1gcountry]

I won't echo the advice you were already given, but will add that as you are getting used to doubles, it may be easier to start with a 1500 psi fill. It's less gas weight to be carrying around, and you'll have a smaller bubble to deal with as you're getting used to everything else.