Buoyancy Question

[Archer_Pensacola]

We did our first open water checkout dive yesterday. Before we suited up (only mask and wetsuit) we did our buoyancy check, and I determined I needed 12 lbs on the belt. I suited up (2300psi AL80 tank, BC, mask and fins) and we went down about 15-20'. I had to release all of the air from my BC to stay down and if I put more than a puff or two in the BC I began to float up. At the bottom I felt like I was able to swim up or down as desired, and hover in place if I positioned myself with my arms and fins.

My questions are:

- Does this sound like I was at proper buoyancy? The lack of some air in my BC was a little unsettling, but since we were only 15' +/- down and I had other things (checkout tests) to worry about, I didn't bother with it. How will this change when we start going down 40-50' and need to make a safety stop at the end of the dive?

 

[The Kraken]

Just as a guesstimate . . . you may need to add 4 pounds or so due to the buoyancy shift of the Al 80 at the end of your dive.

At that same 15' at the end of your dive with 500 or so psi your tank is going to be 4 pounds more positively buoyant - which, of course, is going to send you popping up like a cork.

the K

 

[BrianS]

I find it odd you did your initial test with just that on. I would think they would have you wear your Fins, BC and a tank with about 500 psi of air in it as well.

We did our first open water checkout dive yesterday. Before we suited up (only mask and wetsuit) we did our buoyancy check, and I determined I needed 12 lbs on the belt.

 

[spectrum]

It's an OK first estimate. If you could still stay down with a puff or two in the BC that loosely represents the loss of mass you will experience as you breathe the tank down. As you go deeer it will becme less relavent since the suit will loose bouyancy and you will be adding to the BC volume. If you were very over weighted then you'd have excessive air volume and it fwuld fluctuate wildly with only moderate depth change making control difficult.

Frankly for your first dives if it works just go with it until yor respiration settles down.

You don't say where the 12 pound number came from. If it was from the bobbing at eye level with an average breath method then you're close. The two disadvantages to doing this pre-dive are that you must do the math to add weight for anticipated air weight loss and your gear will still have entrapped air (bouyancy) that will demand needless weight.

 

[Archer_Pensacola]

You don't say where the 12 pound number came from. If it was from the bobbing at eye level with an average breath method then you're close. The two disadvantages to doing this pre-dive are that you must do the math to add weight for anticipated air weight loss and your gear will still have entrapped air (bouyancy) that will demand needless weight.

Yes we determined weight by settling in about 10' of water and weighted ourselves until we were eye level. I started off with 24 lbs and sunk like a rock. I'm going to keep a closer eye on it this weekend on our next checkout dive.

 

[spectrum]

Yes we determined weight by settling in about 10' of water and weighted ourselves until we were eye level. I started off with 24 lbs and sunk like a rock. I'm going to keep a closer eye on it this weekend on our next checkout dive.

Assuming that you were in the same water (salt/fresh) and someone accounted for the loss of air mass (just over 5 pounds for an AL-80) at the end of the dive you're as close as can be for the moment. Be sure to repeat every 5-10 dives or when ever you get to run your cylinder down to 500 PSI or so.

Pete

 

[cummings66]

- Does this sound like I was at proper buoyancy? The lack of some air in my BC was a little unsettling, but since we were only 15' +/- down and I

Well, at the start your BC will probably be empty of air to go down and that shouldn't cause you grief, in fact how do you go down? You let air out.

Now, as you get deeper you'll start adding air to adjust buoyancy and that too is normal. If you have a wetsuit you'll find it's a different amount of air than without.

As to how it will change at 50 feet, well you'll have more air in it there and as you come up you'll release that air, or you'll pop up like a cork.

Personally I think you were told how to check for weighting wrongly, your check didn't tell you much about the rest of the gear such as tanks, knives, lights, etc. You should be fully suited up and ready to go, if you have a full tank of air then add around 4 lbs to what you have found to be neutral assuming you have an AL80 tank.

What did you find it like as you breathed the tank down, could you stay at 15 feet with the weight you had or could you only stay down by swimming around?

If you could hover at 15 feet and you had around 500psi or so left your weighting for now is probably fine. For me I have to exhale as well as empty the air in the BC to go down. Once I go down and play around when I come up with a near empty tank (500 psi) I find that it's easy to maintain any depth I want all the way up to the surface. Part of that is when you descend those little pockets of air in the wetsuit get pushed out and so you're a bit less bouyant due to less trapped air in things.

 

[spectrum]

Before we suited up (only mask and wetsuit) we did our buoyancy check

Holly cow, how did I miss that, everything I said but with all of your gear on!

 

[mark99]

You are underweighted, if you were wearing the 12 lbs during the buoyancy check. When your tank is empty it will be 4 pounds less (as noted above), and you could have trouble holding a safety stop. I would suggest adding 4 lbs next time (16# total), and do a "buoyancy check" at the safety stop (i.e let all air bc out and see if you float or sink). If you are still sinking even on inspiration , then try taking off 2# (14# total) on the next time and so on.

It is more dangerous to be underweighted, as you could get into an uncontrolled ascent, and that is not good...

 

[TorontoDive]

I suited up (2300psi AL80 tank, BC, mask and fins) and we went down about 15-20'.

You started your dive with 2300 psi? Also if, you achieved perfect bouyancy without any gear then you would probably have to add a little to compensate for the 5-ish pounds of +ve bouyancy from a 500pci AL80.