Breathing & buoyancy control

[Richard Davis]

Hi,

A couple of niggling little questions.

When doing fin pivots I always seem to have to wait a second or two after breathing in a little before I start to rise. What's the physics behind this?

If I'm aiming for a horizontal hover rather than a pivot what am I doing differently ? I can do both, but I'm not sure what I'm doing differently between them.

Thanks

[PeteM]

When doing fin pivots I always seem to have to wait a second or two after breathing in a little before I start to rise. What's the physics behind this?

Inertia and hydrostatic drag. You are probably changing your buoyancy by a couple of Kg by breathing but that needs to accelerate 100Kg of person and kit against the inertia and water resistance, with so little force against that it aint gonna be fast.


If I'm aiming for a horizontal hover rather than a pivot what am I doing differently ? I can do both, but I'm not sure what I'm doing differently between them.

Nothing really, you are just off the bottom. Fin pivots are pretty useless in real life but are used as a teaching aid because it keeps you stable and gives you a reference point which a horizontal hover does not

[voltiana]

This is one of the issues I am having as well, sorting my bouyancy, when I am swimming around in the pool I seem to be ok, but the minute I have to o pivots I go hay wire and all over the place.

Any suggestions??? I currently swim with 3kg of weight due to being naturally boyant in water anyway,

Volty

[Nigel Hewitt]

This is one of the issues I am having as well, sorting my bouyancy, when I am swimming around in the pool I seem to be ok, but the minute I have to do pivots I go hay wire and all over the place.
Stick with it. Buoyancy control is pretty fundamental but from the sound of things you are most of the way there.

Try stopping breathing for a moment to see which way you are going. Lungs full you should be going up and empty going down. You want to be able to cope with putting something heavy down, say a 2Kg weight, and then going back and picking it up all in the hover but not finning. Then you can feel you are starting to have buoyancy hacked and dropping into the hold of a freighter, trimming all the way down and with a nice halt a meter or so from the bottom is easy.

[ChristianG]

Stick with it. Buoyancy control is pretty fundamental but from the sound of things you are most of the way there.

Try stopping breathing for a moment to see which way you are going. Lungs full you should be going up and empty going down. You want to be able to cope with putting something heavy down, say a 2Kg weight, and then going back and picking it up all in the hover but not finning. Then you can feel you are starting to have buoyancy hacked and dropping into the hold of a freighter, trimming all the way down and with a nice halt a meter or so from the bottom is easy.
Nigel - for shame - are you really asking someone to hold their breath underwater? To use their lungs as an additional method of buoyancy control?

Oh, OK then, carry on. :D

[ChristianG]

More seriously.

The trick is to supply just enough air to your buoyancy device so that you are completely weightless at half of whatever breath you consider normal when breathing underwater at the depth where you wish to level out. That means that when you breathe in you rise and when you breathe out you descend. It also means that the mean point in your depth, the +/- one, is where in an ideal world you would always like to be at, ie at half breath.

Unfortunately, because we're not fish, that doesn't ever happen to us SCUBA divers.

On a somewhat philosophical bent some say that we should take deep, slow breaths when diving, I happen (personal opinion) to disagree with that. I take the same types of breath, or so I think, as I do at this keyboard, which is to say that I sometimes even stop breathing (when I'm concentrating) but I quite certainly don't, ever, breathe "deep and slow" with conscious intent. Actually I doubt that I could do that for any significant period leave alone for the whole of a dive.

After many minutes at your own keyboard, having not thought about it, check how you are breathing. I'd wager that you'd say "hardly at all". That, IMNSHO, is what you should strive for when diving.

[STEVE MC]

Volty,
The other suggestion that I'd make to help you improve your buoyancy is occassionally stop swimming and try and remain still and in one position. If you sink when you stop finning your buoyancy was 'wrong' and your using the action of finning to hold a neutral position in the water.
Hope it helps
Steve

[voltiana]

Just got back from the pool session tonight and it does appear that I might have just started to crack the buoyancy issue, I've taken on board a lot of what has been suggested and managed to hover for a while and it felt realy good. My instructor was pleased that I managed to get there, next stage hopefully open water but who knows.....

[hon]

Buoyancy control!! ARRR... something that I have never understood during my OD training a few months ago. I was shown how to do fin pivot and I thought I understood the mechanism - my lungs are just like an internal BCD. So every time I breathe in and out should affect my position in water as my buoyancy changes. However, I was also told that a good diver can maintain his/her position in water relatively motionless because he/she can control his/her buoyancy. If he/she is to have breathed normally, his/her lungs are also like a BCD and it doesn't matter whether he/she a novice or experienced diver, his/her lungs volume change the same and hence should go up and down the same.

In BSAC system there are cards awarded to those who take the buoyancy control test and the better you are, the less up and down movement you get. But I thought this is all to do with your lungs' size!!??! @@

In one of the OD pool exercises, I tried to "regulate" my breathing to see the effect of fin pivot/buoyancy. I managed to control my position with very little movement but the instructor said it was not the right way to do so. I wasn't holding my breath, just slowly and anticipating the movement and rhythm of movement and breathing. He asked me to breathe in and out slowly and fully. And this was when everything gone haywire. I was either pinned to the bottom or rising to the surface. I never understand this fin-pivot exercise. @@

[Gareth]

Buoyancy control!! ARRR... something that I have never understood during my OD training a few months ago. I was shown how to do fin pivot and I thought I understood the mechanism - my lungs are just like an internal BCD. So every time I breathe in and out should affect my position in water as my buoyancy changes. However, I was also told that a good diver can maintain his/her position in water relatively motionless because he/she can control his/her buoyancy. If he/she is to have breathed normally, his/her lungs are also like a BCD and it doesn't matter whether he/she a novice or experienced diver, his/her lungs volume change the same and hence should go up and down the same.

In BSAC system there are cards awarded to those who take the buoyancy control test and the better you are, the less up and down movement you get. But I thought this is all to do with your lungs' size!!??! @@

In one of the OD pool exercises, I tried to "regulate" my breathing to see the effect of fin pivot/buoyancy. I managed to control my position with very little movement but the instructor said it was not the right way to do so. I wasn't holding my breath, just slowly and anticipating the movement and rhythm of movement and breathing. He asked me to breathe in and out slowly and fully. And this was when everything gone haywire. I was either pinned to the bottom or rising to the surface. I never understand this fin-pivot exercise. @@

Its a combination of the two. The BCD is a coarse adjustment, the lungs the fine adjustment.
Ideally, you should be breating normally, like you are sitting at your desk, even, regular, reletively shallow breathing.
Also remember there is a lag between change & effect, which is also why, when breathing normally you don't continually go up & down, you should stay more or less where you are.
i.e. Breath out, slowly start to descend, the longer you leave it until you breath in again the faster you will start to descend. Breath in again, the rate of descent slows, stops & you start to ascend again, accelerating slowly.
The bigger the exhale & inhale & the longer the delay between inhaling & exhaling the more pronounced the effect.

Gareth

[ChristianG]

The other suggestion that I'd make to help you improve your buoyancy is occassionally stop swimming and try and remain still and in one position. If you sink when you stop finning your buoyancy was 'wrong' and your using the action of finning to hold a neutral position in the water.
That's a good trick Steve and actually what I do when I want to level out somewhere.

[Dave Whitlow]

That's a good trick Steve and actually what I do when I want to level out somewhere.
Yep, me too. It is also what I tell students to do if they tend to be negative and use finning and arm waving to compensate.

[Maria CM]

Yep, me too. It is also what I tell students to do if they tend to be negative and use finning and arm waving to compensate.

Good buoyancy comes when new divers start to realise that lots of finning and arm-waving is not considered 'cool'... then they learn good buoyancy very quickly;) :D

best wishes,

Maria