If my depth varies +- 2m when breathing in and out, what's going wrong?
This week, when diving mostly with people more experienced than myself, I noticed something about my buoyancy compared to theirs. I'm wondering how to fix it.
Before setting off, we all selected what seemed an appropriate amount of weights. Everyone (possibly after a small change) was able to achieve eye-level buoyancy at the surface after breathing out, so I think we all had about the right amount of weight.
When we'd reach our diving depth, and had put the right amount of air into our BCDs, we all seemed to be floating about level.
Around then, and later when swimming along at that depth, I noticed that the more experienced divers were pretty much staying exactly at that depth, not really varying at all. However, I found myself rising anything up to 2m higher as I breathed in slowly, and falling to up to 1.5m below them as I breathed out. My average buoyancy was the same as theirs, but it varied a lot more with my breath than for them.
What do I need to do / change to have my buoyancy change less at constant depth as I breathe in and out?
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Moving vertically +-2m is a lot. An awful lot. It shouldn't be more than a few cms. This sounds like there's something else going on.
Good buoyancy requires lots of practice to master. The best place to practice is 15cm above a platform at around 6 metres. You should be still; fins not moving; and not moving forwards.
Your breathing should be slightly heavier than 'normal', meaning that you need to take slightly larger breaths to clear your 'dead spaces' such as your windpipe, etc. This doesn't mean great gasping breaths, just "normal" breaths.
Your buoyancy should be set that when you breathe in, then half out, then hold, you should stay vertically still. A big breath in should mean you'll rise, a big exhale should mean you'll sink. Normal breaths mean you'll be more-or-less stable vertically.
If you move up more than about 50cm, you'll probably need to adjust your Buoyancy Control Device (BCD) to reset to a stable platform.
The most important issue is to be still in the water. If you're swimming forward, your attitude (trim angle) will have a great effect on your buoyancy, particularly if you're not flat in the water -- many people mask poor buoyancy skills by swimming. As soon as they stop, they sink or float.
Buoyancy and trim are probably the two hardest skills to master in SCUBA diving. Once sorted, diving is so much more pleasurable as you can effortlessly hold your position in the water for all sorts of reasons; from taking photos through watching the wildlife, and holding your deco stops if you move into technical diving. Spending time diving in a swimming pool is a good winter passtime.
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You don't mention if you are adding/removing air from your BC while doing this. It sounds like you may be overweighed and compensating by having too much air in your BC. As you breath in you displace more water and rise a little. This causes the air in your BC to expand lifting you higher and magnifying the effect. Same happens exhaling. You descent, the air compresses and you descend more. This is known as "yo-yoing" in reference to the popular child's toy that goes up and down.
It's hard to say for sure without seeing you so either consider taking a class such as peak performance buoyancy, diving a lot while working on your weighting, buoyancy and trim skills and/or finding a mentor to help you with this.
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Firstly, it's very nice to see someone do the eye float thing :) But this should be done on an near empty cylinder (50 bar). Else you are perfectly weighted with about 2kg of air in your cylinder (single cylinger) and at the end of your dive you are 2kg lighter, and therefore might struggle with safety stops, ascent rates, etc. (2kg is just an estimate; depends on cylinder volume and pressure. Air weights about 1.2466km/m3). Also remember the eye level buoyancy check states - float at eye level while holding a normal breath, then as you breathe out you should descend.
On to the actual question. By regulating how much air you breathe in and out, you can regulate how much you rise and fall with each breath. This is easy to practice in shallow dives.
Set your buoyancy to neutral at, say, 5m and then swim in a straight line, keeping to that depth by regulating your breathing. Then once you get that right, adjust your depth with your breath only between 4m and 6m.
It is a fun exercise and will improve air usage and buoyancy.
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I think this means you are varying the volume of your body too much or holding a full breath too long. Try holding your chest in a more consistent position as you breathe.
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