Descending Vertically Vs Horizontally

[Dan Gibson]

Just to make the statement a little more correct for all circumstances, the starting bouyancy is also affected by the type of gas used. (i.e. Air or nitrox will generally cause the tank to be more negative than a helium based gas at the beginning.) The ending is the same.

The starting and ending bouyancy characteristics will be determined by tank composition, but the swing between the starting weight and ending weight is affected only by the amount of air used. Do you disagree with this statement?

 

[StSomewhere]

The starting and ending bouyancy characteristics will be determined by tank composition, but the swing between the starting weight and ending weight is affected only by the amount of air used.

Ding! Ding! Ding! Ding! We have a winner!

 

[TeddyDiver]

With 6lbs overweighted and low volume BC (20cbf) fully inflated you'll be negatively boyant at 80' with 7mm WS, with 7mm+5mm shorty at 60'!!
With 3mm there' no such risk.

Sorry! Should be 20 lbs of boyancy not 20cbf!!

 

[markfm]

That sounds like a gear mis-match.
My Reef Rider is a light travel BCD -- the S/M only has 24lb lift, the L/XL 32.
Something with only 20lb lift would have to be really small, a true tropical-water BCD.

 

[markfm]

On the original topic, I like horizontal -- feels neat. I also like H for ascent, basically breathing my way up, though have to be careful to dump properly.

 

[TeddyDiver]

That sounds like a gear mis-match.
My Reef Rider is a light travel BCD -- the S/M only has 24lb lift, the L/XL 32.
Something with only 20lb lift would have to be really small, a true tropical-water BCD.

Well, Thats about the size the most "club" type BCs are. Anyway I recalculated now with pen and paper and it's 26lbs BC allready having this issue in the same depths. Theres also a risk when having BC belts a bit too tight not allowing to use the total volume!
My biggest concern anyway is, that there's no common knowledge about the danger, not even among all instructors.

 

[markfm]

Edit: Erroneous data.

A 7mm compresses TO about 1.5 - 2 mm at 100', in other words losing about 75% of its volume due to compression.

(Not BY 1.5 - 2 mm)

A 7 mm suit (not including any additional core warmer) is on the order of 12l displacement (add up the volume of material), may weigh on the order of 1.5 - 2 kg, so it would be around +10kg buoyant in fresh water.

Take the suit, drop it to 4 atmospheres, 100'. It is mostly air, compresses down to around 3l displacement. It still actually weighs about 2 kg, so you are down to being about 3 - 2 = +1kg. You get a big swing, about 9kg in buoyancy.

(Teddy's post later is in reference to my earlier post. I've tried to correct the numbers).

 

[del_mo]

Newbie time. I thought one of the goals was to get your required weight down to the minimum. I've dropped enough weight now (185lbs, 3 mil, 8 lbs lead) that I can't descend by emptying my bcd and exhaling. I now have to fin down (assuming I don't have a line to descend with). At depth I give my bcd one quick burst and I'm good until I want to ascend. Then I dump the remaining air from my bcd and slowly fin up.

I've read on this thread about uncontrolled descents using the finning down technique. Would this be because of overweighting? I'm I looking for trouble descending in this manner? I do like the head first feeling.

 

[Wijbrandus]

I've read on this thread about uncontrolled descents using the finning down technique. Would this be because of overweighting? I'm I looking for trouble descending in this manner? I do like the head first feeling.

Primarily overweighting, sure. That's a problem for everyone who doesn't have their weight dialed in.

Another problem is if you dive in cold water, and in a thick wetsuit, you'll become increasingly negative as you go deeper due to compression of the neoprene. A cold water diver packs a lot of lead to compensate for the heavy wetsuit. When the suit no longer provides that bouyancy, then all that lead is quite happy to pull you down fast.

It's not a big deal. You just need to be aware of it and keep it all under control by adding a bit of air as you descend. I'd say that's a little complicated though for a lot of new divers, so a feet down descent allows you to kick to slow your descent while you deal with your bouyancy.

 

[ranger979]

You know back at about the second page of posts on this topic I was going to post a message asking about the whole steel vs aluminum bouyancy issue but I didn't what to hijack the thread. I guess I didn't need to worry about that.

I'm surprised what I thought would be a fairly straight forward question could turn into a 10 page debate.

I'm glad this thread is getting back on track --- sort of