BP/W How much weight do I need

[WestCoastDivr]

I've been talking to Tobin, but I'm interested in getting the opinions of some more people. I'll post some stats first

Height: 6'4"
Weight: 175 lbs
Previously dived in: Cressi Aquaride Elite with aluminum tank and 30-35 pounds of weight
Exposure suit: 7mm wetsuit with 2-3mm shorty underneath, 5mm gloves and hood, 7mm boots
New dive bc: DSS SS long backplate with 35 lb wing and steel 100cf tank

I figure that by ditching the BCD I will lose around 5 pounds (Tobin suggested this) so I will be aiming for 25-30 pounds of weight. The BP is around -7 pounds according to Tobin and I purchased the -9 pound plates to attach, also my reg is around -2 and SS tanks if I am correct are around -10 lbs buoyant. This means that with no weight I am at -28 lbs. Does this seem like enough weight in a cold salt water dive site with 7mm exposure suit or will I need to add some weight in cam band pockets?

Thanks

 

[buddhasummer]

Tobin knows what he's talking about. What ever he says will be pretty close to the mark. Weighting is very individual. I dropped, IIFC, around 8kgs going from BC and AL tank to SS plate and steel tank. In a two piece 7mm, SS plate and Steel tank wear 2kgs on a belt. I'm lean, well skeletal actually.

 

[gcarter]

Start with current = 35
-5 for BCD
-7 for BP
-9 plates
reg = no change
so far we have 35-5-7-9=14 lbs of additional lead needed to stay the same.

Now the tanks: when I go from my AL80s to my HP100 steel, I take off an additional 6lbs. That is the difference between the buoyancy at 500psi between the tanks.

Example:

HP100 -10 full, -3.3 @500psi.
AL80 -1.6 full + 2.4 @ 500 psi

The difference at the end of the dive is what matters, in this case 5.7 lbs.

So if you are using the same tanks, 14-6 = 8 lbs still required in pockets or on a belt.

But different tanks have different characteristics, so your differences may differ.

If you were properly weighted before, take that as your starting point and only look at changes to get your estimated new starting point.

 

[Agility]

Just do a weight check with full tank and add 7lb (=the weight of 100cf air) for the dive.
At the end of the dive repeat the weight check and write your conclusion into your logbook for futurre reference.
This will have you slightly overweightet as it will be impossible to get rid of all air in your wetsuit, but a few pounds too many are better than ascending fast at the end of the dive.

 

[Colliam7]

I've been talking to Tobin, but I'm interested in getting the opinions of some more people.

Three thoughts:

1. Nothing wrong with getting second (and third) opinions. I also agree with buddhasummer, Tobin really does know what he is talking about and has probably worked with more divers on weighting - and specifically on the transition from a positively buoyant BCD to a BP/W rig - than many/most of us on SB.

I figure that by ditching the BCD I will lose around 5 pounds . . . The BP is around -7 pounds . . . I purchased the -9 pound plates to attach, also my reg is around -2 and SS tanks if I am correct are around -10 lbs buoyant.

2. I don't think the reg should figure into the comparison calculation, since that was part of your previous rig (although I may be misunderstanding your calculations). As for the cylinder, a similar comment. I would try to look at the buoyancy characteristics of the specific steel cylinder you will be using, in comparison to the buoyancy characteristics of the specific aluminum cylinder you were previously diving, to estimate the weight change. For example, using a publicly available table (Scuba Cylinder Specification Chart from Huron Scuba, Ann Arbor Michigan), if you are changing from a Luxfer AL80 to a Pressed Steel HP100, you are looking at an empty cylinder buoyancy change from +4.4 to -1 (so, a difference of 5.4 pounds) and a full cylinder buoyancy change from - 1.4 to -8.5 (so, a difference of 7.1 pounds). I normally weight for neutral buoyancy at 500 PSI, although some use 300 PSI, others may use another value, etc. But, using that cylinder comparison, the change in weight requirements associated with switching to the steel cylinder would be closer to 6 pounds, rather than 10. Just a thought.

Does this seem like enough weight in a cold salt water dive site with 7mm exposure suit or will I need to add some weight in cam band pockets?

3. Ultimately, you are really looking for a starting point for the new rig, from which you can fine tune your weighting in-water - that is the best approach to determine what you need. Also, to say you are currently using '30-35 pounds' seems to be a bit general. Is it 30? Is it 35? Is it 32? What kind of gear changes do you normally make between dives that would cause your weight requirement to vary by ~14% (35 pounds less 5 pounds) or ~16% (5 pounds in addition to 30), depending on how you look at it? That is actually quite a bit.

 

[fnfalman]

The only way to know the buoyancy characteristics of the tank is to find out exactly which tank it is (manufacturer and model) then go on line and look at the manufacturer's data. Also, pay attention to the negative buoyancy of the tank when it's empty and not when it's full. I could be wrong but I don't know of any current manufacture tank from Worthington and Faber that would give you -10lbs when empty. The heaviest ones would be the Faber M-series and those HP100 & HP120 go for around -6.5lbs empty.

I personally think that with a 7mm wetsuit, you're well overweighted at 35-lbs right now. I can't imagine that the Cressi BCD is that floaty. 6'4" and 175lbs don't make you a fatty.

My guess is that you shouldn't need more than 20lbs of lead to be properly weighed for your 7mm wetsuit (1-piece or 2-piece?). Can you find a swimming pool and do a weight check just with yourself, your wetsuit, fins/mask/snorkel, no BC, no tank, no nothing? Do that and add 6lbs for extra buoyancy in saltwater.

Then we can look at your gear configuration more intelligently.

Many times people swore to me that they're properly weighted and within one dive I can tell that they're easily 4lbs overweighted or more.

 

[halocline]

I agree that someone 6'4" and 175 lbs is likely to be pretty lean, and 35 lbs sounds like a LOT of weight even for a 7mm wetsuit.

Ultimately, you need to do a weight check to figure out what you'll need. If you're moving to a steel plate with a weight plate on your back and a steel tank, that's all your weight moving from your hips to your back. You're going to feel a huge difference in trim. I'd start in a pool.

 

[fnfalman]

Just for comparison's sake, I'm a chubby dude weighing at 5ft6 and between 205-210lbs depending on the time of the year. In my brand new 1-piece 7mm wetsuit, I need 16lbs of lead (or the equivalence of negative buoyancy from my gears) to be properly weighted.

The last time I dived with a poodle jacket & aluminum 80 tank, I needed 20-lbs of lead on my weight belt. Which calculated out just about right for 16lbs of lead plus 4 extra lbs to compensate for the floatiness of the AL80 when it's empty.

 

[Mike126]

Trust Tobin. I went through this last fall. He suggested checking the bouancy of my wetsuit in water. I ended up rolling my 7 mm and hood and gloves and weighted it down with dive weights until in began to sink. It was at 21 pounds. Using his calcs on the plate, reg etc I came up with my weight requirements of around 15 pounds. I dive with the torus 26 and ss long plate. I'm 6'4" and 205.


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[eelnoraa]

I agree that someone 6'4" and 175 lbs is likely to be pretty lean, and 35 lbs sounds like a LOT of weight even for a 7mm wetsuit.

Ultimately, you need to do a weight check to figure out what you'll need. If you're moving to a steel plate with a weight plate on your back and a steel tank, that's all your weight moving from your hips to your back. You're going to feel a huge difference in trim. I'd start in a pool.

I think it really depends on the wetsuit itself. I am 5'6", 135lb. In relatively new 7+7 farmer john/jacket, my total balast is 28lb. This seems ridculouisly high considered I only need 22lb TLS350+DUI400g thinsulate. Yes, I did proper weight check. In fact, I was in so disbelieve, I did wieght check after each dives (2 total), yeilded same result. The comfort level between these two set up is also dramatically different.

But yes, the only way to narrow down the weight is a weight check. Over the internetl, if we can guestimated within +/- 5, I think we did good.