Weight drop from Rig2 to BP/W

Sloeber · Apr 8, 2011

I'm trying to run some numbers through my head, but I'm not the best at working with numbers :idk:

Sometimes it works best for me if I spell it out in front of me.... you guys can then correct my errors hahaha.

I'm trying to figure out how much lead I may need on my upcoming BP/W. It'll be a bit before I can test it out, but I'm anxious to have a "good guess" of where I'll fall in line.

Currently I dive a Dacor Rig 2. I added lot's of personal weight this year after an accident, so my lead requirements on my most recent trip were mostly guesses. I used 16lbs, felt overweight, but didn't concern myself with dialing it in. The Rig2 is a beast, but I do believe it is positively buoyant. I guess I need to head down to my local pool to find out just how much lead it takes to sink it. For the sake of arguement lets say it is 5lbs positive. If by switching to a 5lbs SS BP and 2lbs harness kit, does that effectively change my lead requirements to 4lbs? Or am I missing something?

Bubbletrubble · Apr 8, 2011

If those are the right numbers, then, yeah, your lead requirement would be 16-5-5-2=4 lbs. This assumes that the only variable you are changing is the BCD and that the 16 lbs. with the Rig2 was determined properly.

I've never used the Rig2, so I have no idea whether it's really +5 lbs. buoyant. Most conventional jacket BCDs are +2 to +4 lbs. buoyant. +5 sounds reasonable, I suppose.

As you already know, all of this is really just an academic exercise. The true test is to do a proper weight check with all of your gear once you get your BP/W setup.

Have fun with the new gear...

DevonDiver · Apr 8, 2011

Yeah, if the numbers are correct, then you should be able to drop that weight.

BP&W allows far more flexibility in weight placement, so you will really have the opportunity to fine-tune your requirement and get your trim perfected.

When you get your BP&W, plan to spend some time in shallow water getting everything perfected; including your weighting, trim and the adjustment of the harness. Once you've got it sorted and locked-down, it will be an absolute pleasure to dive.

If possible, seek out an experienced BP&W diver in your local area... as mentoring will help you short-cut a lot of the experimentation to get it right.

Sloeber · Jun 10, 2011

OK, was able to get to the local pool tonight with my old gear and new gear. I only had weights in 2lbs increments, but my Rig2 floated with 4lbs lead and sank with 6lbs lead. I'd say 5lbs is a solid workable number.

Naturally, my new SS BP/W sank like a rock. Is there anyway to make a fair estimate at how negatively bouyant this device is?

It was very, very nice to not jump into the water tongith with 14lbs lead around my waist.

TSandM · Jun 10, 2011

Well, most manufacturers list the weight of their backplates. They don't displace enough water to adjust that -- so a 5 lb backplate can be treated as though it is 5 lbs negative. The hardware on the harness may add a pound or two, which makes up for any displacement of the backplate. In my experience, considering the BP/W rig as negative by the weight of the plate plus the empty buoyancy of the tank works fine.

CPH · Jun 10, 2011

I went over to a wing recently and was advised to start with about 5kilos. I have always been a bit bouyant so I added 7 kilos.
What a crap dive that was. I dropped my air down to 20bar and tested my weight. I managed to drop ALL my weight.
I now dive with no lead at all.
As said, there is no real accurate way to guesstimante what you need. You are already diving overweighted (in your words) so why not chuck a load on, do a dive and at the end do a proper weight check.
Best way.

Bubbletrubble · Jun 10, 2011

CPH:
As said, there is no real accurate way to guesstimante what you need. You are already diving overweighted (in your words) so why not chuck a load on, do a dive and at the end do a proper weight check.
Best way.

Or, even better, do a proper weight check before your next dive and compensate for the amount of gas in your full tank (for a full AL80, that's about 5-6 lbs.). At least then you would get the benefit of proper weighting for the dive in question. I can't tell you how many times I've had to explain this to divers I've encountered. (Typically they'll tell me: "I can't do a weight check now because I won't have an empty tank until I finish the dive.") If you know the tank buoyancy specs (buoyancy when full vs. buoyancy when empty), you have enough info to do a fairly decent weight check with a full tank. (Yeah, I'm aware that once wetsuit neoprene is compressed at depth during a dive, it doesn't re-expand entirely by the end of the dive. If one is concerned about the magnitude of this effect, it wouldn't be difficult to do another weight check at the end of the dive to determine how much more weight could be shed.)

CPH · Jun 10, 2011

Wot e said

alundy · Jun 22, 2011

I agree that a proper weight check before your next dive is in order (the math doesn't always seem to work out perfectly it seems like!). Personally, I used 20 lbs of lead with a jacket style BCD. I just got a DSS bp/w with the SS backplate, and when I did my weight check and a few dives last weekend I was down to 12 lbs of lead and that was perfect. All other variables were pretty much the same (steel tank, same neoprene thickness but newer suit, etc).

AbyssalPlains · Jun 22, 2011

I went through the same thing to figure out what wing size I needed before I switched. I even went to the trouble of taking all my gear into the pool, attach a liftbag to every single item and titrate its buoyancy by adding weights until it was neutral. Took me two hours and turned out a complete waste of time. My theoretical weight calculations were completely off once I got my BP/W. I strongly recommend you save yourself the pain and spend the time on having a beer or two with your best diver buddies while you wait for the BP/W arrives. Then take it to the pool and try it out. Everything else just doesn't work. It's all theory, and it'll be wrong.

Just to give you a taste: My Jacket-style BC was neutral or MAYBE one pound positive in the water. So with everything else being equal, theory would have predicted that by switching to a BW/W, I should need pretty much the same kind of weight. But I didn't. I needed much less. Why? I have no idea.

Weight drop from Rig2 to BP/W

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