Pool-to-Open Water Conversion

[MSilvia]

What ever weight you end up with remember to add a few for the empty tank which will pull you up towards the end of the dive.

If you figure it out at the end of a dive (ie. using a tank with 500psi) you won't need to add anything. The extra weight of the air you're carrying at the beginning of the dive will only make it easier to submerge in the first place. As your suit compresses at depth, you'll have more than enough to stay down, and when you ascend, you'll know you have enough weight to stay under, because you figured your weighting out with end-of-dive pressure at the surface.

Figuring out your weighting with a full tank requires a little guesswork with regard to how much you'll need to offset the bouyancy of an "empty" one. Why not figure it out empty and be sure?

Mark's suggestions about breathing are excellent. His comment about struggling is good to keep in mind to... struggling often means kicking without realizing it, and kicking up when you want to go down is obviously not helpful. When I've had trouble descending on occasion, such as in choppy conditions, I have found that crossing my legs, exhaling, and putting my hands over my head helps quite a bit.

 

[Scuba_John]

Sorry I did not mean not to figure it out at the end but as new divers it may be difficult to find an 500psi tank unless they have purchased equipment. There is also the dive op to consider when they come in from the dives most will head for the dock to start the clean up and refilling and other duties that need to be done for the next dives, that is not to say they will nnot take the time but it may not be logisticaly posiable. It is true that a lenght of time will be need for checking this at the end and also I keep in mind that what is good in one ocean area is not good in another. Partly due to salt content is not the same over the world What is good on the GBR is not in the galapagos. Other factors also affect ones person from day to day, what one eats can have a big affect. Where rice and beans are common in Central America it is not in the UK or Africa. With that said it is a good idea to use and tank with 500psi if you can get you hands on one but keep in mind it will change depending an where you dive

 

[GrierHPharmD]

That is a good point. Again, I suggest diving with your local Conchs and doing a bouyancy check just off the beach.

 

[jbd]

I've sifted through a TON of posts this morning but I haven't been able to find the answer to my question (so I'm sorry if it's a repeat -- I really didn't find it!).

I have heard that there's an equation to use to figure out how much weight you'll need out in the ocean if you know how much weight it takes to get underwater in a pool. But I haven't been able to find it yet.

Also, does anyone have any tips on how to reduce the weight you'll need to get down? I know I'm carrying a LOT of weight (20+ lb) and that was the one critique my instructor had for me was to try to shed some of it.

Appreciate any suggestions, but definitely want to try to get ahold of that formula! Thanks heaps!

-Stacy

Stacy,
Any time you change gear you can use this method. String several weights on a belt and lay it on the bottom in about 5 to 6 feet of water. Your BC must be completely empty and if it is possible use a tank that has about 500psi in it. You will not have the regulator in your mouth. Take a normal to slightly normal breath and hold it. Dive down to the belt and pick up one end of it and hold close to your belly. Totally relax and become absolutely motionless. What will happen is that you will ascend until you have raised the the amount of weight off the bottom that makes you neutrally bouyant with that specific gear on. I use 4 lb and 2 lb weights for this and pick up the end with the 4 lbs on it. Set up the belt with the correct amount of weight and then using the same breath hold techique above see if you float with the water level just above your eyes(maybe an inch). If so place the regulator in your mouth, clear it and then when you are breathing normally exhale completely and you should start to descend.

With this done, if its at all possible to weigh yourself and all the gear used at this time, you can figure out how much water you displace and then make the appropriate calculations to switch between salt water and fresh water and vice versa.

I have seen somewhere that when going from freshwater to saltwater you should add 2.5% more weight.

Hope this helps

 

[Pandie]

Wow, you guys are so great! This is all good advice!

I'll definitely have to take you guys up on your offer to help me out on the next Conch dive. Depends entirely on when I can get out of work today if I can make the dive tonight.

I just felt really bad about holding everyone up during our beach dive that I probably over-weighted, cause I was trying to just get down to the bottom to do my skills.

Grier, I just hate making people wait for me, ya know? I'd much rather be in the water than checking to see if I float or not

 

[yc92602]

If you can weigh yourself with all the gears (including the weights for pool or fresh water), let's say the result is Wf pounds. Then your actual volume (or displacement) in fresh water is Vf = Wf / Df, where Df is the fresh water density.

We can get the similar thing for salt water: Vs = Ws / Ds. Since the volume difference is very little, we can say that Vs = Vf, and therefore we have Ws / Ds = Wf / Df, or Ws = Wf * (Ds / Df). As we all know Ds / Df is about 1.03, thus Ws = 1.03 Wf, which means the weight you need to add for salt water is about 3% of Wf (total weight for fresh water).

 

[O2BBubbleFree]

I posted a chart on this thread:

http://www.scubaboard.com/t61010.html

It gives a rough starting point based on suit thickness and other stuff. It has worked for several people.

It's only a starting point, though, as a buoyance test must still be done.

 

[Walter]

If you are weighted correctly in fresh water and if you are using the same equipment in salt water, you can calculate how much weight you'll need to add. Weigh you and your equipment (including the lead you used in fresh water). Take that total and multiply by .025. The product is the amount of lead you'll need to add in salt water.

For example, let's assume you are petite and you weigh (with your gear - tank, BC, reg, weights, everything) 150 lbs. 150 x .025 = 3.75. You'll need to add 3 lbs, 12 oz or round up to 4 lbs.

If you change any gear (different wet suit, different tank) at all, you'll need to start over and weight yourself in the water.

 

[scubatoys]

4 to 5 lbs is normally about right. We can assume the BC and Tank are about neutral... Negative when full, positive when empty (assuming aluminum) so we basically ignore them. So then you add your weight and the weight you're wearing, divide by 62.4 and multiple by 1.6.

Huh??

Ok, a cubic foot of water weighs 62.4 lbs. So let's take someone that weighs 180 lbs, and needs 6 pounds in fresh water - total 186. Divide by the weight of a cubic foot, 62.4 and you get 2.98. that means this individual is taking up 2.98 cubic feet of space. Since salt water weighs 64 lbs per cubic foot, or 1.6 more per cubic foot, we multiple the 2.98 by 1.6 for 4.768 lbs needed to maintain the same buoyancy in salt water. Phun with Physics!

So yea... like everyone else said... experiment - but probably about 4 to 5 lbs... Just wanted to show I could do the math!

 

[teknitroxdiver]

It's always better to be under weighted than over weighted.
db

Obviousy you've had a lot more experience than me, but I disagree with that. I would much rather carry a little too much than I needed so I would be sure that I would be able to be neutral at my safety or deco stops. One dive after my divemaster suggested I take off some weight, I ended up hanging from the anchor line feet-up for the safety stop--not fun.