Weighting and buoyancy questions

[Slonda828]

Hello everyone,

I've been reading this board for months and never had a question until now. I guess that speaks to the volumes of great information contained here. So for that, thanks.

I conned my local lifeguard into letting me use the YMCA's pool to figure out how buoyant all my gear is. I placed each piece of gear (or pair in the case of gloves and boots) in a mesh bag individually and weighted it until it became neutral, one object or pair at a time. I know many of us have done this from reading on the boards. Here's where I get slightly confused. I dove the springs all last weekend (fresh water, 5 dives, non-instructional) with only 4 pounds of lead and it felt great. I didn't have trouble hanging at my safety stop or anything, and my buoyancy felt rock solid. I had maybe two quick bursts of air in my BC at about 60 feet, but other than that I was aces. When I added up all my gear...I should be diving with about 11 or so pounds of lead.

Help me out and make sure I'm not retarded. Here's my gear load out, with how buoyant it is in pounds:

1st/2nd stage regulator with 7' primary hose, suunto cobra computer, and LP hose= -4
Zeagle Ranger LTD, size large= generally neutral
Luxford AL80 (500 PSI)= +2.4
ME (just me in shorts, 190 lbs, 72")= +1
Pinnacle Seal 3mm wetsuit= +9
6.5mm booties= +3
Mask, snorkel, safe second, wrist mounted compass= generally neutral
Atomic split fins= generally neutral
UK SL4 light= -.25

According to my math, provided I am not slow, that means I should weight myself with about 11 pounds of lead. If you go by the PADI guidelines in the AOW course then I should be using 9 pounds for my exposure protection minus 7 pounds for freshwater plus 5 pounds for the AL80 which is 7 pounds of lead. So how did I dive successfully with only 4? I understand that I'm a rookie diver (15 dives) but am I missing something? I used the buoyancy calculator excel file availible on the board here as well and it told me to use 11.5 pounds of lead.

I would understand if I was ballparking all of this, but I tried to be as empirical as possible. What do you guys think, aside from thinking about how I obviously have too much time on my hands between dives The only thing I can think of is that I was too light and didn't notice, but I wasn't having trouble staying down. The water was only 72 degrees, but as far as I gather water temperature does not have that much of an effect on the buoyancy of water. Oh, and I didn't forget to account for fresh water as all buoyancy tests and dives were conducted in fresh water.

Thanks in advance for feedback.

Slonda

 

[tstormdiver]

You forgot the weighting of 1 important thing,..... you! Most people are at a bit on the positively buoyant side, whilst a few are either neutrally or negatively buoyant. Depending on your body type, will determine whether you are positive, neutral or negative.

 

[vixtor]

The neoprene items compress with depth. This means that your wetsuit/boots/gloves will require less lead at 5 meters (15 feet) than they required at the surface.

If you weighted the equipment at a depth of at least 5 meters, you got the values you need. But if you weighted it at the surface (how much lead you need to sink them), it is not that relevant. You were probably positive in the first 5 meters, then the neoprene compressed and you went neutrally buoyant. At the begining of the dive, the air inside the tank also provided more weight to sink you.

 

[vixtor]

ME (just me in shorts, 190 lbs, 72")= +1

You forgot the weighting of 1 important thing,..... you!

he didn't

 

[digital_steve]

mate... just rig up your gear to a tank with 50bar in it (whatever that is in PSI, stupid imperial system) and jump in.
If you can hold your 6m stop then it's all gravy
Take someone with you so they can hand you weights so you can get it all sorted
***** weighing a whole heap of stuff in the pool!

 

[tstormdiver]

he didn't

Oops! You're right. Didn't catch that. Thanks!

 

[ishmaelcat]

Hi Slonda:

I actually did the same thing in a pool with my gear when it was new, with a similar discrepancy compared to the weight I actually need in the water. Several factors could be contributing to your results:

First and probably most importantly, Vixtor is definitely right about your neoprene compressing at depth. My 7 mm wetsuit took 20 lbs to sink at the surface, for example, but it accounts for maybe 13 lbs of the weight I actually wear (15 lbs in salt water with a steel 100 {about 7 lbs less bouyant than the typical AL80} and neutral BC).

Secondly, it's possible that there were some air pockets in your boots or suit when you sunk them in the pool. In particular, 3 lbs sounds a bit much for a pair of 6mm boots.

Thirdly,there may be a little extra weight in your tally because of culmulative rounding errors -- i.e. if it really took 2.5 lbs to make your boots neutral, and 8.5 to sink your suit, that would be another pound.

Finally, when you put your wetsuit on, if it fits well, it stretches, which I would think (having too much time on my hands to think about silly things like this) might actually compress the aircells somewhat even before you descend.

If I had to make a wild guess without any data, I would estmate you'd need about 5 lbs for a 3mm wetsuit and 1 lb for the booties, which would give you your measured 11lbs minus 6 lbs, leaving 5 lbs, which is pretty close to the 4 lbs that actually worked for you.

The one thing I'd make sure of is that you can easily hold your 15 ft. Safety Stop (or a stop at 10 ft or so if necessary -- you don't want to be forced to surface if you have to go up to 10 ft), with 500 PSI in the tank. Every 500 PSI weighs somewhere around a pound, so if you're holding the stop with 1200 PSI, you still could be a pound or two underweight.

Anyway, have fun; and I'm glad I'm not the only one to have done the dorky weighting my gear in the pool thing

Chris

 

[Slonda828]

See, I knew the dorky pool people were out there. It's also a cheap excuse to practice things like 25 meter underwater swims, ditch and recovery, 900 yard swims, etc. The reason I am trying to be so methodical is that I just assembled my BP/W and I'm curious how my weighting will have to change. Obvious my oxycheq BP/W is a lot less buoyant than my Zeagle ranger ltd was. My backplate is aluminum, and the dry weight of my BP/W is 7 lbs. I think this means I may not need any weight at all to sink in the springs, though of course I'm bringing plenty with me to troubleshoot just the same.

I think just for nerdiness' sake I may hop in the pool next time with the blasted wet suit and booties on and see how much it really takes to sink me. You're probably right though Ishmaelcat, I thought those numbers seemed a bit on the high side. I was using weights in increments of .5 lb in case you were wondering. I made my own .5 lb weights, it makes resolving trim issues incredibly easy and I definitely like having the ability to have exactly 5, 7, or 9 lbs of weight on my without having to affect my trim.

Thanks all. I'll let you know how the pool experiment turns out. Ideally, I'd like to know the specific buoyancy of my stuff that way it is easier to adjust my weighting when I change small things (like add my 6mm gloves for example).

 

[Slonda828]

mate... just rig up your gear to a tank with 50bar in it (whatever that is in PSI, stupid imperial system) and jump in.
If you can hold your 6m stop then it's all gravy
Take someone with you so they can hand you weights so you can get it all sorted
***** weighing a whole heap of stuff in the pool!

That's what I usually do man, I guess I just figured at some point I could begin to figure this out without having to re-weight everytime I changed something. It seems like that's the only way to get it done correctly though.

 

[don Francisco]

Hi- doing things your way is good for comparative differences if you change a piece of equipment but can yeild misleading results otherwise.

When you started your dive did you have a full tank? If so the added 6#s of air would have brought you to 10 total. At your safety stop your exposure protection would be compressed, adding a few extra pounds because of the reduced displacement.

Lastly, you have another variable 6#s or more to work with from your lung capacity. Most divers can easily manage a range of weighting, and automaticly compensate by adjusting their breathing slightly.