Descending

[grocottg]

Hi,

I am a OW diver just qualified. I am having some sticky points in my understanding of decent. I do my weight check and make sure with a normal breath held, i should remain at eye level on surface. My issue is understanding the point of exhale and you should then dive.. Of course i feel silly in asking as of course you dont just exhale and not inhale but this brings me to my silly point, if you inhale then you are likely to ascend...? I think i have missed something glaring obvious here..!!

[tony J]

Hi,

I am a OW diver just qualified. I am having some sticky points in my understanding of decent. I do my weight check and make sure with a normal breath held, i should remain at eye level on surface. My issue is understanding the point of exhale and you should then dive.. Of course i feel silly in asking as of course you dont just exhale and not inhale but this brings me to my silly point, if you inhale then you are likely to ascend...? I think i have missed something glaring obvious here..!!
I have a few comments

1. if I had my eye level at the surface before a dive within the normal range of breathing, then I would be underweighted. At the end of the dive (when I had used up the weight of air in my cyclinder ) I would not be able to stay underwater.

2. A weight check on the surface is always a false check. In a semi drysuit you probably have air trapped in the suit which will work its way out at the start of the dive. I have the same issues in my drysuit

3. In inexperienced diver is very often very anxious at the start of a dive so have their lungs over inflated, as they relax during the dive they us more of a normal range of breathing. This is very noticeable on try dives when I often have to weight the student at the start of the dive - then take a couple of kilos back after the first couple of minutes (the anxiety in an experienced diver is sometimes worrying about not being able to sink so it can be a negative feedback process). Before the days of drysuits, it was common to start a dive with a duck dive to get down.

4. In an over simpliied case : If you are neutral on the surface, then breathing out would make you descend (now you are negative), inhaling now returns you to neutral, so floating unserwater.

So doing what you suggested in your post works for many divers.

Tony

[grocottg]

This is where perhaps I am confusing myself. I dive with a 12L Cylinder which i suppose equates to 3kg of weight being used up at the end of the dive. I have been taught that diving a single 10 or 12L cylinder rig you should float eyes at water level holding a normal breath with the cylinder full and the BCD/suit empty.

You exhale to decend (negative) which is fine, understand that, but then inhale which increases your buoyancy (neutral). So decending water from surface should start with all air removed from BCD/dry suit and exhale which takes you down, then basically fin yourself down to x meters.

Have i confused myself ?

[BogSnorkeller]

As soon as you start descending, your buoyancy reduces. This is because as you descend all the air pockets on you are compressed by the increasing water pressure, and are therefore becoming denser (less buoyant.)

What air pockets? Well, you've got your lungs and sinuses for starters, but crucially, you've also got the air in your suit. Even in a wetsuit, the neoprene still contains millions of little air bubbles, which is why they are so 'floaty.'

So - your buoyancy reduces with depth. Therefore, once you've exhaled and descended a few metres, you can fill your lungs again but still be negatively buoyant. At this point, you start using your 'buoyancy compensator' to achieve neutral buoyancy.

[Richard Whitcombe]

The only accurate weight check is to see if you can stay neutral at about 2-3m with almost no air in jacket or suit with just about an empty tank.

I don't subscribe to the "neutral with 50 bar" idea - if your tank is empty for whatever reason you would then be unable to control the ascent.

Any check done with air in the tank involves a bit of fudge factor by adding weights to account for the gas breathed etc.

My way of starting a descent in the uk at least is to fully open suit valve, fully empty wing then exhale fully to get me down that first 1-2m which is always the hardest. Then roll shoulder vertically, squeeze elbow up to shoulder to try to force the auto dump to get rid of more air.

[tony J]

As soon as you start descending, your buoyancy reduces. This is because as you descend all the air pockets on you are compressed by the increasing water pressure, and are therefore becoming denser (less buoyant.)

What air pockets? Well, you've got your lungs and sinuses for starters, but crucially, you've also got the air in your suit. Even in a wetsuit, the neoprene still contains millions of little air bubbles, which is why they are so 'floaty.'

So - your buoyancy reduces with depth. Therefore, once you've exhaled and descended a few metres, you can fill your lungs again but still be negatively buoyant. At this point, you start using your 'buoyancy compensator' to achieve neutral buoyancy.

I would not use the fact that the suit compresses as a reason to be underweighted. At the end of the dive one slowly acends, so at 3 metres one does not want to discover that one has no more air to dump (something that is nice when freeediver, byt dangerous when scuba diving)

The air pockets in the lungs and sinuses ? irrelevant I beleive as both are of constant volume (one reinflates the lungs and the sinus are in a solid bone)

Tony

[BogSnorkeller]

I would not use the fact that the suit compresses as a reason to be underweighted. At the end of the dive one slowly acends, so at 3 metres one does not want to discover that one has no more air to dump (something that is nice when freeediver, byt dangerous when scuba diving)

The air pockets in the lungs and sinuses ? irrelevant I beleive as both are of constant volume (one reinflates the lungs and the sinus are in a solid bone)

Tony

Tony - read the original post. The guy is asking a fairly basic question which can be paraphrased as follows: "if my surface buoyancy is so finely balanced that I can descend just by breathing out, why is it that during the dive I don't shoot back up to the surface when I breathe in again?"

My answer was slightly dumbed-down, but hopefully explains the principle - "as you get deeper, you get denser." You've read things into my post which just aren't there - I'm not giving any advice on weighting, and certainly not advocating that anyone be underweighted.

(And you're getting confused with your volumes! Yes, our lungs and sinuses do remain at fairly constant dimensions during diving. However, the volume of gas contained within the lungs and sinuses changes because we are filling them with pressurised gas. I.e. as you get deeper, the weight of gas in your lungs/sinuses increases. It's easy to get confused about this stuff, but you'll just have to trust me on this one - the weight of gas in your lungs/sinuses gets heavier as you go deeper.)

[tony J]

Tony - read the original post. The guy is asking a fairly basic question which can be paraphrased as follows: "if my surface buoyancy is so finely balanced that I can descend just by breathing out, why is it that during the dive I don't shoot back up to the surface when I breathe in again?"

My answer was slightly dumbed-down, but hopefully explains the principle - "as you get deeper, you get denser." You've read things into my post which just aren't there - I'm not giving any advice on weighting, and certainly not advocating that anyone be underweighted.
Indeed unecessarily dumbed down (and confusing to refer to sinus's as air pockets when they are rigid)

you also wrote

So - your buoyancy reduces with depth. Therefore, once you've exhaled and descended a few metres, you can fill your lungs again but still be negatively buoyant. At this point, you start using your 'buoyancy compensator' to achieve neutral buoyancy.


And I apologise for misinterpretting this as advice.

We do not want the original poster to think what he wrote in his post is good practice, it is very poor practice (and potentially dangerous) to be weighted neutrally with full tanks and no where in the training scheme do we suggest it.

Tony

[BogSnorkeller]

Indeed unecessarily dumbed down (and confusing to refer to sinus's as air pockets when they are rigid)


Sinuses are air pockets. The fact that they are partially walled by a semi-rigid material (bone) is entirely irrelevant because they are also open to the ambient pressure. As you descend, the gas they contain is compressed. Would you also argue that lungs aren't air pockets just because they are surrounded by rigid ribs?

Unnecessarily dumbed down? I'm beginning to think I should have stuck to crayon drawings...

[Richard Whitcombe]

What is the total volume of your average sinus passage? Im willing to bet its not a lot and insignificant when thinking of things such as buoyancy.

[BogSnorkeller]

What is the total volume of your average sinus passage? Im willing to bet its not a lot and insignificant when thinking of things such as buoyancy.


What is the total volume of a single bubble of air in a neoprene wetsuit?

Pretty small.

Does this mean it's insignificant to buoyancy?

Yes.

Does this mean we can't use it when explaining buoyancy changes?

No.

(The original poster just needed to know about a basic law of diving physics. I gave him a simple but accurate description of that law. It doesn't matter whether I used sinuses, widgets, or plastic bottles to illustrate my point - the original poster now knows why he doesn't rocket back to the surface when he breathes in.)

[Richard Whitcombe]

What is the total volume of a single bubble of air in a neoprene wetsuit?

Pretty small.

Does this mean it's insignificant to buoyancy?

Yes.

However, when you have possibly a few million small bubbles in the material at the start of a day it all adds up and has an effect. You only have 1 sinus network.

[Vic]

We do not want the original poster to think what he wrote in his post is good practice, it is very poor practice (and potentially dangerous) to be weighted neutrally with full tanks and no where in the training scheme do we suggest it.

Remember that the OP claimed to be "recently OW qualified" - so he's trained by PADI.

When I did PADI OW, the weight check was to float at eye-level with half a breath - but this was with *empty* tanks. And as the OP didn't mention full tanks, it's a fair assumption that this is what he meant.

Vic.

[BogSnorkeller]

However, when you have possibly a few million small bubbles in the material at the start of a day it all adds up and has an effect. You only have 1 sinus network.

Richard, let's recap.

This post is about someone who couldn't figure out why he didn't remain at constant buoyancy when he descended.

I've explained the reason to him by telling him about what happens to air pockets when they are 'squashed.'

By way of illustration, I mentioned the three main locations of 'squashable' air pockets - lungs, sinuses and suit, but stressed that the suit was the crucial one.

Why are you now making a pedantic point about how sinuses do not contain much air? I already know this - I have sinuses of my own.

[Richard Whitcombe]

Because id say they're totally irrelevant.

Lungs yes - they could have up to 5l or so buoyancy initially.

Sinus however id venture are far too small to have any real world effect what so ever on buoyancy.

[BogSnorkeller]

Because id say they're totally irrelevant.

Lungs yes - they could have up to 5l or so buoyancy initially.

Sinus however id venture are far too small to have any real world effect what so ever on buoyancy.

Richard. Do me a favour. Go back through this thread and find any statement by myself, or anyone else, that indicates that sinuses have a substantial effect on buoyancy.

Couldn't find any? No? Right, well that's because I merely mentioned sinuses because they are one of the three locations of 'squashable' air on a diver.

Yes, I agree with you that there is far more 'squashable' air in your suit or in your lungs. That's obvious.

[MattS]

I don't subscribe to the "neutral with 50 bar" idea - if your tank is empty for whatever reason you would then be unable to control the ascent.If your tank has 50 bar, and you can descend from the surface you should be able to hold a stop at 6m with virtually no air in the cylinder due to the compression at 6m reducing overall buoyancy. It works for me as they say.

[Tony Dwyer]

Sinuses are air pockets. The fact that they are partially walled by a semi-rigid material (bone) is entirely irrelevant because they are also open to the ambient pressure. As you descend, the gas they contain is compressed. Would you also argue that lungs aren't air pockets just because they are surrounded by rigid ribs?


The sinuses are air pockets, but their volume does not (should not) change when diving. The pressure of the air in them goes up and down, but the volume (and hence the buoyancy) remains constant.

This does depend on the sinuses being clear and for air to flow freely in and out as one breathes. if this is not the case, on becomes a candidate for the red filled mask syndrome.
Not pretty, but not usually an injury that one would worry about too much. I've seen a number of people suffer from it over the years. Usually because they dived too soon after a Rhino virus infection.

[BogSnorkeller]

The sinuses are air pockets, but their volume does not (should not) change when diving. The pressure of the air in them goes up and down, but the volume (and hence the buoyancy) remains constant.

This does depend on the sinuses being clear and for air to flow freely in and out as one breathes. if this is not the case, on becomes a candidate for the red filled mask syndrome.
Not pretty, but not usually an injury that one would worry about too much. I've seen a number of people suffer from it over the years. Usually because they dived too soon after a Rhino virus infection.

Okay, okay - I've thought about the above long and hard and... you and Richard are right, and I am wrong. I shouldn't have (and wish I hadn't) mentioned sinuses as they are unlikely to cause any significant change in displacement.

To be fair to me though, I just threw the sinuses in as examples of air pockets. I did point out that compression of the suit was the crucial factor in creating negative buoyancy on the descent.

[Michael Purcell]

To be fair to me though, I just threw the sinuses in as examples of air pockets. I did point out that compression of the suit was the crucial factor in creating negative buoyancy on the descent.

As does BSAC training...

[grocottg]

Thanks guys for all the responses. Yes it does explain and thanks B.S for the dumbed down explination on air pockets. Unfortunately the science behind decending doesnt really get explained in better detail (for me), so where most just accept that what happens, i felt something was missing.

I found a great article about Buoyancy Skills http://www.crabbsac.org.uk/articles/A003.htm