Number one cause of diving fatalities?

[boulderjohn]

That may be, but the converse isn't true: if I am interestred in holding a safety a safety stop with small-but-sufficient-num psi in my tank and no air in my bc, I do not want to be weighed so that my head bobs above water unless I exhale completely and cease breathing.

You are very right that an optimally weighted diver will have trouble holding a safety stop when the tank is almost completely empty. With the tank completely empty, you will have trouble submerging. That was my initial point, though, wasn't it? If divers with some air in the BCD (they must have had SOME during the vie before they went OOA, and that must have expanded during the ascent) cannot keep themselves on the surface and will subsequently sink out of control, then they are not only not optimally weighted, they are very much overweighted.

Allow me to describe a demonstration I do in the pool for my students to illustrate this. The purpose of the demonstration is to teach the impact of breath control (chest volume) on buoyancy. I start by telling them that, as an instructor, I always dive overweighted with students so that I can descend rapidly or hold on to a struggling student. That is important because what I do would be MUCH easier if I were not overweighted. I go to the bottom of the 12-foot pool and put a random shot of air in the BCD. I then adjust my breathing until I am neutrally buoyant. I then inhale sharply and begin to ascend. I inhale and exhale so that I ascend slowly, without using hands or feet, all the way to the surface. I then float on the surface briefly, the top of my head out of the water, with the air I had in my BCD at the bottom still there (and having expanded as I ascended). At our altitude, that is a 40% increase in BCD volume. I then exhale sharply and begin to sink. I inhale and exhale as I descend slowly. I stop for a while half way down and hover. Then I make a sharp exhalation and go to the bottom.

Summary: Even overweighted by several pounds and having my BCD inflated enough to keep me afloat on the surface, I can descend simply by exhaling. Once I am at the bottom, I can achieve neutral buoyancy with that same amount of air (now compressed) in the BCD. Even though the air inmy BCD had a 40% increase in volume from the bottom of the pool to the top, my chest has enough total volume that I can overcome this change through my breathing.

BTW, every pound of excess lead requires a volume of 15 fluid ounces of air in the BCD (or lung volume) to compensate.

As for me, while diving I prefer a few extra pounds beyond optimal weighting because I like being able to dump a tiny bit of air at the end of the dive without having to go through a gymnastics routine to get the air bubble at the exit point.

 

[boulderjohn]

Yes, but I mention it because (I'd guess that) 99.9% of people who'd go to DAN's site are divers/in a field that relates to diving. We know a bit about "what's what;" perhaps more than many involved in the investigation into the death.

I still don't get your point. Are you saying that divers are better able to recognize signs of a heart attack during an autopsy than coroners?

 

[boulderjohn]

with tank being -1

I am not sure of the point here. The weight and /buoyancy of a tank only matters to the degree that it affects the total weight and buoyancy of the diver and gear. I a diver dives with a steel LP 85, which weighs a heck of a lot more than an AL 80, and if the buddy starts with the AL 80. if they both use the same amount of gas during the dive, they will both be the same amount more buoyant at the end of the dive than they were at the beginning.

 

[wgmontgomery]

No, I'm saying that a large number of coroners don't know the first thing about diving. That can lead to a COD (in the eyes of the uninformed anyway) that really doesn't address WHY the diver died.

 

[dmaziuk]

You are very right that an optimally weighted diver will have trouble holding a safety stop when the tank is almost completely empty. With the tank completely empty, you will have trouble submerging. That was my initial point, though, wasn't it?

Yes, mine was more about "float with head above water" and "if I'm OOA I don't care to be weighted properly for SS" comments around it. The former's just silly while the appropriate response to the latter is "do you plan your dive so as to be OOA and weight yourself accordingly?"

I am not sure of the point here. The weight and /buoyancy of a tank only matters to the degree that it affects the total weight and buoyancy of the diver and gear.

I may have misread @2airishuman, he came up with "a pound of air" and my mental arithmetics doesn't work well in cubic feet per square hands over millifirkins.

 

[boulderjohn]

No, I'm saying that a large number of coroners don't know the first thing about diving. That can lead to a COD (in the eyes of the uninformed anyway) that really doesn't address WHY the diver died.

If you go to the DAN reports, they address this in every issue. The cause of death is almost always listed as drowning, and that really is usually the cause of death. DAN has to reply on other information to find out--when they can--what led to the drowning. If they can't tell, they say so. If there was an autopsy that showed a heart attack, then the fatality is included in the list of fatalities associated with cardiac problems. A coroner can certainly identify a heart attack, right?

 

[boulderjohn]

Yes, mine was more about "float with head above water" and "if I'm OOA I don't care to be weighted properly for SS" comments around it. The former's just silly while the appropriate response to the latter is "do you plan your dive so as to be OOA and weight yourself accordingly?"

As far as I know, pretty much all agencies have taught the weight check the same way for decades--possibly since the beginning of scuba instruction. I have been a member of ScubaBoard for 12 years now, I can't begin to estimate how many threads have discussed how to do a weight check. I am pretty sure that this is the first thread ever in which someone has said it is wrong, let alone said it was silly. You must have a tremendous amount of self-confidence to make such a proclamation.

As for your "do you plan your dive so as to be OOA and weight yourself accordingly?", well, that is something I find silly. We teach people to dive properly weighted so that they can have the best possible experience during the dive. Diving and especially buoyancy control are easier the more optimally you are weighted. As a happy coincidence, if divers do end up OOA at the end of the dive, they will find it easy to float on the surface. That is not the goal, though; that's just how it works out.

 

[wgmontgomery]

bj-I would hope so. Yes, DAN investigates the WHY, but most people do not read DAN's site.
There was a time when an "Andrea Doria diving ban" was discussed (it couldn't really be done as the wreck is in int'l waters) due to the number of people "drowning" or dying of "air embolisms" on the wreck. The USCG looked only at the HOW & not the WHY people were dying, hence the "ban" was discussed.
That kind of sums-up my point.

 

[wgmontgomery]

"Good talk!" Ttyl

 

[boulderjohn]

I may have misread @2airishuman, he came up with "a pound of air" and my mental arithmetics doesn't work well in cubic feet per square hands over millifirkins.

He is right in that a AL 80 with 500 PSI has one pound more air than an empty AL 80. Once again, though, the key factor is the total weight. A diver with an empty AL 80 will have the same total volume as the diver with 500 PSI, so the same weight of water will be displaced. The diver with 500 PSI will weight one pound more and will thus be one pound more negatively buoyant. I could not figure out what that had to do with your calculations.