New to BP/W. How much lead and lift do I need?

[halocline]

It seems all you guys are in agreement regarding the possibility of me being overweighted. You all seem to think that about 20 lbs of ballast should be enough. If that were the case, I think a 30 lb wing would be adequate.

I think you're getting a bit confused. You do not need a matching amount of lift to offset the weight of your ballast. They are related, but not directly. You need lift to offset the loss of buoyancy in your wetsuit at depth, and the weight of the gas you are using. That's it. If you are properly weighted, you need almost no lift while you're on the surface because the wetsuit is offsetting the ballast. As you descend and your wetsuit compresses, you need more lift. You should try tbone's suggestion of using a mesh bag to see how much lead it takes to keep your wetsuit submerged just below the surface; that's the total positive buoyancy of your wetsuit. Since it is impossible for a wetsuit to lose more positive buoyancy than it has, that amount of lead is a good conservative estimate of the maximum effect of suit compression on your buoyancy. Add to that 5 lbs for the weight of gas in a full AL80 (that's the tank you said you're using) and this is the maximum lift you should ever need. It's actually more, because your wetsuit will not lose 100% of it's positive buoyancy.

My favorite way to do a weight fine-tuning check is at the end of a dive. This way your suit is fully expunged of trapped air. After your safety stop, purge your tank down to 300 PSI or so (it's likely you will rarely, if ever, go below that on a dive unless you're not paying attention) and try to maintain a neutral position in the water at about 10 feet. Make sure all the air is out of your BC and you're not kicking. If you have a buddy you can work with, start handing him lead off your rig, one weight at a time, until you can no longer maintain depth. It's best to do this in a horizontal position. If you start to surface, swim back down to 10 ft and take back a weight, you can just hold it. (Make sure you don't drop any weights on an unsuspecting diver below!) Anyhow, after some experimenting you'll figure out the minimum amount of lead you need to hold a stop at 10 ft with a near empty tank. This puts you slightly positive on the surface with an empty tank, and a little negative or close to neutral with a full tank, depending on how spongy your wetsuit is.

If you can't get down with that much weight, it's probably more a technique issue. There's nothing wrong with swimming down to start a dive, especially when your gear is dry and likely trapping some air.

Anyhow, this gives you something to think about. It's tough for me to imagine a diver of your build (lean) needing 28 lbs of lead, even in a thick wetsuit. But I don't really know, nobody does, until you do a proper weight check.

 

[tbone1004]

one slight correction is that if all of your ballast is on your rig, you want your lift to exceed ballast because the rig has to be able to float itself. If you're wearing a weight belt then it's less, but the rig still has to hold itself up.

My personal preferred method of weight checking is to put the rig in the water and weight it with the luggage scale, gives you the ballast of the rig itself. Weigh a weight belt of an amount that is 5-10lbs more than you need in the water *note very few lead weights are actually the number they say they are, and then you get in the water, with your exposure protection, rig, and the excessively heavy weight belt, fully dump everything and sink until your head is about 3ft under. You'll be holding yourself up essentially on a piece of line connected to the luggage scale and your buddy can watch the weight on the luggage scale while you are breathing. Will have one number after you inhale, one after you exhale. Use the one after you inhale for now, and add in the amount of gas in the tank. I.e. if it's a full AL80, make sure you add 5lbs to the number on the scale to compensate for the air you can breathe. That number is the most accurate number you'll get. Just make sure your legs are crossed at the ankles, and if you're at a pool, bring your knees up a little like you're leaning back against a countertop to keep yourself vertical.

 

[cool_hardware52]

With thick suits, 7mm or a drysuit with a real under garment, and a normal sized single cylinder, 72, 80, 95, 100, 117, 120 , no 190's, I recommend a diver adjust their initial ballast so they are eye level at the surface with no gas in their wing and a full cylinder.

Suit compression from the surface (1ata) to their safety stop depth (~1.5 ata) will offset the weight of the gas they remove from their cylinder during the dive.

The nice thing about this approach is it will get you close, and you can do it at the start of the dive.

After adjusting your ballast as described go do an easy no stop dive. On the ascent with an near empty cylinder check to see if you can comfortably hold a shallow stop. If you feel "light" swim down to 20 ft and add a minute and do a slow ascent to the surface, wetsuits rebound slowly anyway. Add a pound or two for the next dive.

If you feel rock solid at 15 ft reach back and fell your wing. If there is gas in it you may be able to shed a pound or two. That may leave you very slightly positive for the start of the next dive, but swimming down a couple pounds to ~10 ft or so is no big deal.

Getting your ballast "spot on" pays huge benefits. It means less gas in your wing for all phases of the dive, and that makes buoyancy control easier. It's also safer.

Good luck,

Tobin