Negatively buoyant divers vs. ditchable weights

[boulderjohn]

With all this talk about the awesomeness of ditchable weights, I think it's worth pointing out that weight systems are also a point of failure, if the weight pockets that I occasionally find on the ocean floor at dive sites are any indication, which I am cleverly avoiding by not using.

I have also found weight pockets on the ocean floor, each of them using the velcro fastening system that appears to be obsolete. On the other hand, I have found more weight belts than pockets on the ocean floor. Anything ditchable is a potential failure point. The advantage of pockets as a failure point is that you are likely to lose only a portion of your weight rather than all of it.

 

[matt_w]

There's another scenario for ditchable weights that can occur with NO equipment failures, just a simple mistake. Imagine before the dive you turn on your air, then for some reason turn it off again. You gear up and check your pressure and it's good, and even a breath from a reg works. You jump in, deflate your BC and start descending. After a few breaths you discover:

- You are out of air
- Your BC is completely empty
- You are negatively buoyant and descending


If you aren't significantly overweighted you might be able to kick to the surface... If you are negatively buoyant with NO ditchable weight, what are your options?

- Dack

Why would this ever happen? Only if the BCD also failed. If the BCD/Wing failed and I was in a rig I could ot swim up, I would have some alternate form of buoyancy.

You are discussing a significant number of failures occurring at one time. I am sure my odds of being out of air with a wing failure are exactly 0. I am also sure I will never run out of air after inexplicably making my self negatively buoyant.

A piece of space junk could fall from the sky and maintain sufficient velocity to kill me after penetrating 80 feet of water too. (This is why I believe cave diving is safer...)

 

[Banyan]

There's another scenario for ditchable weights that can occur with NO equipment failures, just a simple mistake. Imagine before the dive you turn on your air, then for some reason turn it off again. You gear up and check your pressure and it's good, and even a breath from a reg works. You jump in, deflate your BC and start descending. After a few breaths you discover:

- You are out of air
- Your BC is completely empty
- You are negatively buoyant and descending


If you aren't significantly overweighted you might be able to kick to the surface... If you are negatively buoyant with NO ditchable weight, what are your options?

- Dack

Your hypothetical setup is actually what I do normally. I routinely put together my gear when I get on a boat and turn on the air for a few breaths to make sure it all works and i don't have any o-ring problems, before turning the air off again in case of leakage during a long boat ride. I've made it a habit to look at my SPG when taking a few test breaths just before entry. Even if the breaths seem normal, the needle will dive towards zero, indicating that you've forgotten to turn on the air.

The air remaining in a hose in this scenario is good for one normal breath and then maybe one very hard breath as the pressure plummets toward ambient. Assuming that you are doing a "normal" entry, you'll stick your reg in your mouth before jumping in and realize by the second breath that you have a problem. Certainly you would realize it by the time you finished your entry. The worst case of this scenario is if you were doing a hot entry with an empty BC with the intention of kicking straight down fast, like is done for dives directly onto Peleliu Corner, but even then you'd be at maybe 10 feet before realizing that you can't draw a third breath.

At the risk of putting numbers on things, let's think through my weighting in this worst case scenario. You get a 10 pound buoyancy spread using just your lungs. My AL80 loses 6 pounds over the course of my dive and I never touch my inflator during a normal dive. Since I can still descend at 500 psi by completely exhaling, let's say that the 6 pound range runs from the 0# to 6# range of my 10# lung range, so a 3# breath (or two-thirds capacity breath) is "neutral" for me at the start of dive. I take my first breath on entry and exhale completely to descend (at this point being -7 pounds) and then start my second breath, realizing that there is problem. (Half breath so I'm at -2.) If I'm very quick thinking, I continue inhaling my last breath as I change direction and start heading up, blowing bubbles from a full set of lungs. (+3 pounds of buoyancy!) If I'm stupid and pause to figure out what is going on and do a normal exhalation to one third capacity in what is likely my last full breath on the reg, I'm at -4 pounds, which is worrying, but certainly not going to prevent me from finning up and getting to the surface, probably in a big hurry. If a diver doesn't think that they could fin back to the surface from that depth, I would agree that they are overweighted and should rethink their buoyancy.

Of course, some divers on SB will tell you that you should have drilled OOG routines so many times that you shouldn't even have to think about the fact your breath is coming hard before your right arm automatically crooks over your shoulder to turn the tank knob as you continue your descent. I only dream of being that hardcore.

 

[matt_w]

Your hypothetical setup is actually what I do normally. I routinely put together my gear when I get on a boat and turn on the air for a few breaths to make sure it all works and i don't have any o-ring problems, before turning the air off again in case of leakage during a long boat ride. I've made it a habit to look at my SPG when taking a few test breaths just before entry. Even if the breaths seem normal, the needle will dive towards zero, indicating that you've forgotten to turn on the air.

The air remaining in a hose in this scenario is good for one normal breath and then maybe one very hard breath as the pressure plummets toward ambient. Assuming that you are doing a "normal" entry, you'll stick your reg in your mouth before jumping in and realize by the second breath that you have a problem. Certainly you would realize it by the time you finished your entry. The worst case of this scenario is if you were doing a hot entry with an empty BC with the intention of kicking straight down fast, like is done for dives directly onto Peleliu Corner, but even then you'd be at maybe 10 feet before realizing that you can't draw a third breath.

At the risk of putting numbers on things, let's think through my weighting in this worst case scenario. You get a 10 pound buoyancy spread using just your lungs. My AL80 loses 6 pounds over the course of my dive and I never touch my inflator during a normal dive. Since I can still descend at 500 psi by completely exhaling, let's say that the 6 pound range runs from the 0# to 6# range of my 10# lung range, so a 3# breath (or two-thirds capacity breath) is "neutral" for me at the start of dive. I take my first breath on entry and exhale completely to descend (at this point being -7 pounds) and then start my second breath, realizing that there is problem. (Half breath so I'm at -2.) If I'm very quick thinking, I continue inhaling my last breath as I change direction and start heading up, blowing bubbles from a full set of lungs. (+3 pounds of buoyancy!) If I'm stupid and pause to figure out what is going on and do a normal exhalation to one third capacity in what is likely my last full breath on the reg, I'm at -4 pounds, which is worrying, but certainly not going to prevent me from finning up and getting to the surface, probably in a big hurry. If a diver doesn't think that they could fin back to the surface from that depth, I would agree that they are overweighted and should rethink their buoyancy.

Of course, some divers on SB will tell you that you should have drilled OOG routines so many times that you shouldn't even have to think about the fact your breath is coming hard before your right arm automatically crooks over your shoulder to turn the tank knob as you continue your descent. I only dream of being that hardcore.

Even in that case there is no reason for your BCD to be empty. You would want to be slightly negative or neutral. Someone who does not understand their buoyancy well enough to make that determination should not be making that entry.

All of the dire scenarios presented in this thread require either people being incredibly stupid or have unfathomably bad luck. It is ok to use procedures that presume people will dive within the limits of their training and experience.

Someone that would jump off a boat with their air off, extremely overweighted, and an empty BCD is going to find a way to kill themselves. In your unlikely scenario I would just turn the air on, it would only take half a turn beforemi could start breathing again and/or add enough air to my BCD to make myself neutral.

 

[aquaregia]

You are discussing a significant number of failures occurring at one time. I am sure my odds of being out of air with a wing failure are exactly 0. I am also sure I will never run out of air after inexplicably making my self negatively buoyant.

There have been fatalities due to this, and I've witnessed this exact situation.

When on the shore, you pressurized your rig to check the air, then turned it off to brief and prep.
You pop a bit of air in your BC and swim out
You get to your spot, empty your BC
You drop down a few feet, go to take your second breath, realize you're out of air
You are also negative

All the good arguments:
- I'm not overweighted so I'm not too negative, can swim up even with no air (hope you don't hold your breath)
- I can reach my valve (while plummetting faster and faster to an unknown depth and clearing your ears)
- I check my valve before I get in (I sure hope so)
- My buddy can turn it on (while you're plummetting away)
etc.

When I saw this, it was in conjunction with a buddy separation during the descent. They recovered from it just fine and had a bit of a laugh about it, but to me it was probably scariest thing I've ever seen on SCUBA, just for its potential. You might be a rockstar for whom the likelihood of this is "exactly 0", but I'm not willing to bet on that myself.

 

[matt_w]

There have been fatalities due to this, and I've witnessed this exact situation.

When on the shore, you pressurized your rig to check the air, then turned it off to brief and prep.
You pop a bit of air in your BC and swim out
You get to your spot, empty your BC
You drop down a few feet, go to take your second breath, realize you're out of air
You are also negative

All the good arguments:
- I'm not overweighted so I'm not too negative, can swim up even with no air (hope you don't hold your breath)
- I can reach my valve (while plummetting faster and faster to an unknown depth and clearing your ears)
- I check my valve before I get in (I sure hope so)
- My buddy can turn it on (while you're plummetting away)
etc.

When I saw this, it was in conjunction with a buddy separation during the descent. They recovered from it just fine and had a bit of a laugh about it, but to me it was probably scariest thing I've ever seen on SCUBA, just for its potential. You might be a rockstar for whom the likelihood of this is "exactly 0", but I'm not willing to bet on that myself.

As I added to my previous post, it only takes half a turn of the valve before the problem is over.

Your post seems to suggest there is no hope and this will happen. It is an accident that is easily prevented. It is also not too hard to solve for an experienced diver, but I can see where a new diver might be in trouble.

There are very few situations where a diver would want a very negative descent. All of those situations are for very advanced divers. If you are carrying enough weight that a fast descent is even possible, you should understand buoyancy enough that you will not just dump all of your air...

 

[MisterPG]

Another reason that non swimmers should not be diving. If you can't tread water or swim to stay afloat you need to be in a life jacket and maybe some of those water wings for kids. Not on scuba.

...Which is why I signed up for swimming lessons... My first lesson is tonight

~m

 

[windapp]

The way I do a buddy check reduces my chances of entering the water with the valve off to almost zero. The last step of my buddy check before the final over-all check is to check my SPG while breathing off both the oct, and primary second stage regulators. If I take my equipment off after that, a new buddy check is performed. Unless someone else turned off my tank valve without me knowing it, I couldn't enter the water without it being on.

 

[Bubbletrubble]

The way I do a buddy check reduces my chances of entering the water with the valve off to almost zero. The last step of my buddy check before the final over-all check is to check my SPG while breathing off both the oct, and primary second stage regulators. If I take my equipment off after that, a new buddy check is performed. Unless someone else turned off my tank valve without me knowing it, I couldn't enter the water without it being on.

@windapp: Your system of checks sounds like a good one. Take care not to be too over-confident, though. I've seen divers committed to very diligent buddy checks make some very silly equipment-related mistakes while diving (prior to entry). All it takes is a momentary loss of focus. The probability of this happening increases when you operate outside of your routine...for whatever reason. It's pretty common for an experienced diver to help out a newbie with gear and then forget to do something critical on his own gear. Ask any DM or instructor. Another scenario which causes mistakes is when the boat crew begins to rush a buddy team to get in the water. Divers are particularly vulnerable to making mistakes when the dive isn't the first one of the day. It's so easy to get complacent with the checks because earlier dives during the day were completed successfully.

I've also seen boat crew inadvertently turn off the tank valve of a diver just prior to entry. Divers aren't always aware that the boat crew are manipulating their tank valves. It happens.

 

[jimthediver]

I have a real life example of why taking the time to get you and your rig balanced for neutral bouancy is important and the value of having ditchable weights. Last week my wife, daughter and one of my daughter's diving friends were down in Grand Cayman for sunning, snorkling and scuba. My wife had hurt her ear snorkling the previous couple of days so she was not diving the day they went out to scuba. My wife is well connected in Cayman so they were on a small boat with just the three of them, a dive master and the boat handler. The girls hadn't been diving for quite awhile so the dive master took the time to have them do a proper bouyancy check. They jumped in with empty BCs and no weights. He got them neutral on the surface then added just enough weight to counter balance their AL 80s when empty. They then proceeded to do their dive.

They surfaced right behind the boat after their safety stop. At that point my daughter grabbed her inflater to pump some air into her vest before taking her fins off. She couldn't get the vest to inflate! The inflator was working but no air was going into the vest. There was no problem, she just grabbed the ladder, doffed her fins and climbed up on the boat. Her buddy and the DM were right there the whole time so she was never in any real peril.

However, under different circumstances the situation could have been very hazardous. If she had been overweighted instead of neutral at the surface. Less confident in the water and farther away from the boat and or her buddy.

When they got home she showed me the inflator and hose (now broken completely off the vest). Boy, that made my heart skip a beat! I asked her "didn't they notice the hose leaking during their dive?" She said no, she only put a tiny puff of air in when they first got to the bottom (she's like her dad, only dives in a short sleeve rash guard) and then never touched it again until they were behind the boat. I asked her if she thought about ditching her weights when she discovered her vest wasn't working. She said yes, but because she wasn't having any problem staying on the surface and they were right by the boat she decided it wasn't necessary. She added if they had been a long way from the boat she would have dropped them for sure. She only has 50-60 dives but she is a pretty squared away recreational diver!

By the way the vest was an old ScubaPro with the plastic (fake corrogated) hose that used to belong to her mother. Its in the trash now. We also had some discussion about doing a more thorough equipment check before diving, especially when the gear hasn't been used for awhile (goes for Dad too, seeing as how he was the one who packed the gear for their trip!).

Alls well that ends well....and some lessons learned to boot!

jimthediver