Fin pivot and weighting question...

[GregA2k6]

Hello all,

We did a weight check in my 3rd pool dive the other day. (salt-water) I had a six pound weight belt and when I deflated my BCD and held a normal breath, I was actually around a foot below the surface. (I know it is supposed to be mask level) My instructor said that would be okay since I had a full tank at the time and that he was more concerned with my rate of descent when I exhaled.

When we did the fin-pivot however, I didn't need to put ANY air in my BCD to get myself to rise and fall. (most other people did) I asked the instructor after the class if I should be carrying more weight and he said I should try some extra weight next time.

My instincts say that I should need to put at least SOME air in my BCD while doing the fin pivot but am not sure. Any opinons? From what I understand, it is better to have a bit of extra weight so you can control your ascent and I'm pretty sure I was underweighted.

Thanks for any input!

GregA

 

[Guba]

I have a tendency to be a bit conservative, but I think I'd add a little weight on your next dive, then fine tune your weighting again at the safety stop when your tank is nearly empty. You can always make the change before your next dive after that.

 

[Ron Brandt]

Perfect weighting......for a real dive. Add two more pounds so you can play with the power inflator.

Ron

 

[pir8]

Sounds like you are a little light, you will need to compensate for an empty tank of course. Try not to put too much more on though.

 

[loosebits]

Yep, if you had a full tank, you were probably a little light (as the tank empties, you would have become positive). If you were below 1000 psi, you were weighted just perfectly. Either way, you were pretty close; congrats, it takes most divers a long time to be anywhere close to being correctly weighted (why do single-tank divers need BC's with 60# of lift? So they can float their 40# weight belts).

 

[WJL]

Greg,

You don't need to have a certain minimum amount of air in your BCD. If you can get under water just by exhaling you are not "underweighted". You don't need "extra" weight. It doesn't make you safer, it doesn't make it easier to control your bouyancy; to the contrary, it makes it harder to control your bouyancy. Carrying extra weight makes diving harder.

The reason why is that your lead weight is incompressable, and the main way you keep the weight from sinking is to use air, a gas, to bouy it up. The gas is compressible. As you go up and down in the water column the density of the gas is changing. Close to the surface the density changes relatively more rapidly than at depth. The more gas you have in your BCD the harder it is to control. So, the goal is to minimize the gas and thereby minimize the bouyancy issues it creates.

Don't add weight unless you can't get under water with an empty cylinder and an empty BC just by exhaling. If you can get under, shed weight until you can just barely do it. Then you will be correctly weighted.

 

[TeddyDiver]

Hello all,

My instincts say that I should need to put at least SOME air in my BCD while doing the fin pivot but am not sure. Any opinons? From what I understand, it is better to have a bit of extra weight so you can control your ascent and I'm pretty sure I was underweighted.

Thanks for any input!

GregA

You had perfect weighting . Only reason to take couple of pounds more when having exercises on bottom to stay in one place, but after that go back to same weights you had.

 

[Marek K]

I'm not sure Greg had perfect weight... though I agree he was pretty close; much, much closer than the average OW course student.

Sounds like the weight check he did was at the beginning of the dive. If so, then he should add (have added) a couple extra pounds to compensate for the air he was going to use up during the dive. Otherwise, of course, he'd be too light at the end of his dive, and wouldn't be able to maintain buoyancy.

He also didn't say when during the dive he did the fin pivot. If it was toward the beginning of the dive, then that would confirm that he was a bit light for the whole dive.

--Marek

 

[boulderjohn]

Perfect weighting......for a real dive. Add two more pounds so you can play with the power inflator.

Ron

I agree.

 

[GregA2k6]

Thanks so much for all of the GREAT information! As usual, the members here have come through again!

I neglected to mention that the fin pivot was very close to the end of the dive and (if I recall) I had around 900 psi. So, from what I've read above, my weight is pretty darn close.

I guess the smart thing would have been to do the weight check at the end of my dive when I had 500 psi and see if I am at mask level and able to drop by exhaling. I will definitely remember to do this for next time! I guess I am just a little paranoid about runaway ascents!

Thanks again everyone!

Greg A