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Not sure if this is suitable for this forum, but here goes anyways. On my dive last night of the Conestoga (St. Lawrence river, near Cardinal, Ontario) in bit of a current and some choppy waters, I was going along fine at about 20' of depth with about 1000psi in my tank, nearing the end of the dive. Being a newb with about 5 dives under my belt, I have issues with bouyancy control and trim, but try to stay on top of it as best I can. For me, the most difficult part of staying neutral comes at less than 15', it can be a real struggle, especially on a near empty tank. Anyways, I started to ascend, but when I hit my BCD deflate button, it didn't help, nothing came out. I tried raising it, turning my body to get the hose more elevated, but nothing. Soon I was at the surface much faster than I had wanted.
My buddy said the current was somewhat strong, but not as strong as it could be, so the positioning of my body may have contributed to this. Having my angled body with my head above my feet into the current may have caused my body to act like a wing. Duly noted for next time.
I'm pretty concerned about this, because even though the dive I did last night was fairly shallow (max depth of 24'), at deeper depths this could have been a more serious issue. The worst part was I went from comfortably being at about 20' to at or near the surface in maybe 10 seconds, it happened very quickly.
I also have fairly new equipment, I've only done 3 dives with it (new BCD, regs). Perhaps being more familiar with it, I could have used one of the BCD dumps...?
Anyone have any advice, apart from keep practicing? Was there anything else I could have done differently?
Perhaps you need more weight. If you are in a similar freshwater environment without the current diving the same configuration, are you able to hold the safety stop? I have no idea how you are weighted ( typically someone with your dive history would more likely be overweighted ) but perhaps you haven't added enough lead to accounter for the bouyancy shift of your tank at the end of the dive.
It's also possible that you were stressed by this situation and holding more air in your lungs. Sometimes you can get by with less weight by just relaxing (when possible).
Adding weight tends to be counter-productive. Runaway ascents, are generally exacerbated by air-expansion in the shallows - more weight (beyond your actual requirements) just means more compensating air in the BCD... meaning a more exaggerated issued with air expansion/compression.
A diver needs sufficient weight to maintain neutral buoyancy in shallow water, with an empty BCD and a near-empty tank. If you're struggling to maintain a stop at the end of the dive - first examine your trim. Are you horizontal in the water? If not, then any fin propulsion is pushing you upwards. Is your BCD fully deflated? Visually and physically confirm that. Only then, with those issues confirmed, should you look to add some weight.
Some weighting info...I usually hover between 26-24lbs of lead with a 7mm john/jacket, hood/gloves/boots. Yeah, I'm a cold water wuss. Last night I used 26lbs, I've been using 24 for my last few dives but found my bouyancy hard to control near the end...I figured a bit more weight would help.
As for my trim, it's hard to tell, honestly. I *feel* like I'm nearly horizontal, but I think my head is just above my feet, so I'm inclined a bit. For me anyways, it's hard to tell exactly how my trim is.
"Tends" means usually but not always. I didn't claim that it was a cure all, just that it was a remote possibility and how to check to see if it applied. Also telling people what you wear isn't really a substitute for a neutral bouyancy check. As for trim, when horizontal, tuck your head down and look behind you. Can you see everything or is it blocked by your body?
Bolts 'thinks' he's vented. Maybe he is, maybe he isn't. (Including lungs...) It's kind of hard to say without diving with him. But yes, assuming your trim is good and your air is vented and you decide you need more weight, a couple lbs would be all it's likely to be. Don't go adding 5-10lbs.
Another trick would be to try doing your safety stop closer to 18-20 feet assuming the dive site allows it. See if you can hold it there. Maybe bring it up a foot each minute until you get to 15'.
Thanks chrpai! I'm pretty sure my lungs were vented, or in the process of it, because I clearly remembering going "Arrrrrgh!" in frustration into my reg... It was difficult to tell if my BCD was vented completely or not, I know I was pushing the vent button, and no air was coming out. I've noticed this in the past where it depends largely on the position of the hose relative to the vest, if I'm not mistaken.
It's a fairly common problem and unfortunately our human responses often contribute to it. When divers start ascending unintentionally, often the first response is to try to deflate the BCD, then realise that their body position doesn't allow air to escape, wriggle around getting the hips forward (which often involves small movements of the fins propelling them upwards), and taking a big breath in as some anxiety sets in. Especially in shallow water, it only takes a few seconds to be at the surface.
If you wear a weightbelt there are two opposing forces: the weightbelt trying to sink your body and the BCD trying to lift your body. Try to pay some attention to that lifting force. Just like driving a manual car, it takes some 'feel' to know when the clutch plates engage when you release your foot. When you feel it lift, breathe out. Properly breathe out- you'd be amazed just how much air you can release from your lungs. If an adult has about 4 quarts (for simplicity) of air in their lungs that they can breathe out, this equates to 4kg or 8-9lb which is a huge amount. By breathing out 4 quarts of air it's equivalent to adding an extra 8-9lb and vice-versa.
Thus breathing control is the first key to buoyancy control.
The other key component is being able to thrust your hips forward to become vertical (as opposed to using your fins), allowing air to rise to the shoulders before pressing the defllator button. Think Elvis the Pelvis or Stayin Alive. With very little air in the BCD you may even need to roll somewhat so that your left shoulder is elevated to the highest point possible. Normally you'll be anticipating that air/wetsuit will expand on ascent but if you're taken by surprise for some reason.... breathe out hard, get vertical and deflate some air.
At the end of your next dive (assuming you're wearing the same wetsuit/weight combination), on the surface try an experiment. Try deflating the BCD, breathing out and see if you descend. If you do and you're close to 50bar/700psi, you've probably got enough weight. If you sink easily, you've probably got too much. Try handing off a weight to your buddy/DM and trying again until you're descending a few feet under control. That is what weight you should be starting the dive with. The kicker is that after a 'normal' dive, your breathing is 'normally' relaxed and slow. Many beginner divers have difficulty getting in to this relaxed state of breathing before the dive, thus needing extra weight to descend, and starting the overweighted diver cycle of using more air etc.
Finally don't worry too much or overthink things too much. Know the techniques and then practice. One important thing is to stay shallow until you've really mastered your control. It's one thing to have an uncontrolled ascent after a single dive with a max depth of 20ft, it becomes that much more critical after deeper or repetitive dives. Again, don't worry- the learning curve is a steep one. You may consider a buoyancy class with an instructor- they're often the single most effective one off 'training' dives that you'll ever do.