Near disaster; dropped weights - what to do?

[Lost Yooper]

I think the "neutral at 15' with empty tanks" is simply a rule of thumb. The key here, really, is the "empty tanks" part, I think. You're never going to have empty tanks unless you've done something foolish, right?

That's how I've always thought of it, anyways.

Mike

 

[Rick Murchison]

Originally posted by Lost Yooper
I think the "neutral at 15' with empty tanks" is simply a rule of thumb. The key here, really, is the "empty tanks" part, I think. You're never going to have empty tanks unless you've done something foolish, right?

That's how I've always thought of it, anyways.

Mike

We train our students "neutral with no air in the BC, 15 FSW, 500 psi (for an AL 80)." (That's 13 CF, right at a pound heavier than "empty")
Rick

 

[GP]

I remember in open water class they taught us to be neutral at the surface with a full breathe of air and to sink when you exhale. This was with a full AL80. I don't understand why they taught us that way with AL tanks. When we were on our fun dive on the last day of class once we got down to under 1000psi left and got up to around 15 ft or a little less we'd start to uncontrollably ascend. Since being certified I've done all of my diving with Steel tanks and haven't had that problem.

 

[Bob3]

2 pockets (3lbx1 and 6lbx1) plus 3lb

Guess we'd better ask Louie about the lead, I thought that meant 9 lbs in each pocket plus a 3 pounder in the back for a total of 21lbs.

I used the "empty" (with the quotation marks) to denote your minimum air 300-500 psi. I don't know too many folks that suck vacuums on their tanks in order to get every penny's worth. I haven't done the vacuum cleaner trick since I started using a SPG 'cuz there are no surprises from whacking the J-valve into the down position.

The actual buoyancy change between your 10'-15' safety stop and the surface is only going to vary a couple pounds, more than what the tidal capacity of your lungs is. If you're weighted "perfectly" you'll float at the surface with an extreme full set of lungs, or sink with an extreme empty set. Averages out to floating with "normal" tidal volume. The next time you're at the DR for a physical, have him let you blow into one of those gizmos that measures your tidal volume (lung capacity) Every litre translates into 2.2 lbs. You'll probably be surprised at the total buoyancy swing just from your lungs.

The lack of stress on proper weighting is one of my pet peeves. If you read through the fatality reports, you will see that a very significant number of deaths are directly attributed to overweighting.

Don't EVEN get me started on "pushbutton diving", something that goes hand in hand with said overweighting. :nono:

There are times you'll need to be grossly overweight, like diving in a ripping current or training students in a pool. The weight needs to be adapted to the type of dive.

If integrated weight manufacturers want to get a look at how to make a DEPENDABLE set of quick releases, they should check out the modern parachute containers. An accidental opening of the container inside the plane has been known to pull the jumper out through the side of the plane, not a good thing. A combination of velcro and a pin/loop arrangement would be the best to use.
I had my parachute rigger make my weight harness.

 

[Se7en]

Rick
You are correct, I should have said the greatest relative pressure change vs distance. I admit that I was thinking about the result and assuming you knew what I meant... lazy typing.

You then go on to point out that the final few feet of ascent are the most critical for bubble growth - which is why we want to take it slow.
Great.

However, you then say;
"We train our students "neutral with no air in the BC, 15 FSW, 500 psi (for an AL 80)."

Why do you train your students to do something which you have just explained the dangers of?

As far as the idea that you won't get positive enough to matter goes, two things to consider;
1. With a 7mm wetsuit, after doing a 3minute safetry stop, if I don't control my ascent (which you cannot do if you were neutral with no air in BC at 5m), the ascent rate is enough to set my Alladin beeping away unhappily.
2. There are documented cases of people dying from lung expansion injuries in a 3ft deep pool.

Can anyone in favour of the 'neutral with no air in the BC at 5m' concept provide a reason WHY this is safer than being neutral at the surface?

(And I don't count 'all students are overweighted anyway' as a reason to teach them to do something else that is incorrect)

Mike
(Who spent the last 10 minutes of his last dive hovering in control at 10fsw over a reef...)

 

[The Chairman]

wetsuit compression begins as SOON as pressure is applied, I have to concur with se7en. I would rather have students neutral on the SURFACE with 300 psi than neutral at 15 ft. WHile I could probably control my ascent at that point, newer divers are having troubles with EVERYTHING. Why give them another problem to resolve if they don't need it? Teach them control ALL the way up, including 30 f/min MAX ascent rate and a 3 to 5 minute safety stop!!!

 

[Rick Murchison]

Originally posted by Se7en
However, you then say;
"We train our students "neutral with no air in the BC, 15 FSW, 500 psi (for an AL 80)."

Why do you train your students to do something which you have just explained the dangers of?

Now *that* is an excellent question, and begs a very fine definition of "neutrally buoyant." We mean "with a full breath" - and what we're striving for is the least weight necessary for a safe dive, but enough to be able to finely control the last few feet of the ascent too. We still want to be able to descend from the surface with a full exhalation. I also train to take a full minute to get from the safety stop to the surface. I feel strongly that many "undeserved" DCS hits probably occur after a pop-up from the safety stop - and I believe that there is a general feeling among new divers (and lots of not-so-new divers too) that the dive is basically over at the end of the safety stop. I've been running a casual survey around the world for the past five years, timing how long it takes the average diver to surface after leaving the safety stop... care to hazard a guess? Would you believe 7 seconds? Over 120 fpm? Scary!
An interesting set of numbers to run is to calculate a constant delta P over P - a 30 FPM rate of ascent at the surface, for example, is equivalent to a 60 FPM rate at 33 feet, and 120 FPM at 100 FSW. Looking at that another way, if we need to be down to 30 FPM ascent rate by, say 33 feet, then we need to be down to 15 FPM for the last few feet just below the surface. Something to think about.
Again, an excellent question.
Rick

 

[Lost Yooper]

That follows my my observations too, Rick. You can help make yourself feel a lot better after a dive by simply making that last 20' take a few minutes after a deco or safety stop. So many people don't know about this "trick".

Mike

 

[Se7en]

Rick - Cool.

It seems that you are getting the important message across - ascend slowly to the surface.

I also come from learning to dive back when the rule was "as fast as your smallest bubbles, and get the hell out of deep water" Which I was taught, and I accepted as recieved wisdom from an instructor repeating current agency dogma.

It's only recently, when I actually started to think about what I was doing (ie once I got over the belief that I was immortal, and could not be narced or bent) that I realised the importance of slowing the ascent near the surface. (I was also playing with calculating the integral of pressure over depth giving the same ideas as you expressed)
Strangely enough, I began feeling a lot better after dives with the slow ascents... even managing to stay awake. Even though I was doing deeper and longer dives.

However - I still think the idea of neutral at 15 fsw is plain dumb, even given your convoluted definition of neutral.

IMH(I try to be)O
Neutral implies that as you breath in, you ascend, as you breath out, you descend. That goes as far back as PADI 101 and fin pivots.
If you are stationary in the water column with a full breath, you are not neutral, you are negative, and compensating for it using your lungs as boyancy control.

So to teach this 'neutral at 15 fsw' nonsense, you are also trying to teach your students that neutral means with 1/2 full lungs at depth, but at 15 fsw it means lungs full?

How about taking the simple, straightforward approach, and teaching to be neutral at the surface? (which is essentially what you are doing anyway, you are just confusing the issue with this whole neutral with full lungs at 15 fsw malarky)

Mike

 

[Bob3]

This is sounding like we're quibbling over 1½-2 lbs. of lead here; I'll change that much just from belching & farting.:devious:

The being "not overweighted" thing is pre-PADI; it comes from before there were BCs (and PADI), when divers HAD to pay attention to their weighting because pushbutton diving hadn't been invented yet. The production line training/gear marketing consortium had a lot to do with several of the bad habits floating (or sinking) around.

Now, lets take a look at the UK's fatality reports and count the number of people that might be alive yet today if it weren't for being overweighted:
http://216.150.74.119/techserv/increp01/intro.htm
:reaper:

Slow ascents are nice, and a pound or 2 isn't going to change things all that much.
Express elevators to the bottom where a person can't swim their way up without the BC crutch are NOT nice.