Calculating air in the lungs

[gsclarke]

This is just a bit of interest for me to see if I've grasped some of the theory we learned on the Padi Course.

At 10 metres my lungful of air (lets say 5 litres) compresses to half and I take on more to replace it, also compressed to half. So on ascent I have to be able to clear 5 litres of air. ie my 5 litres would expand to 10 litres otherwise. Likewise at 20 metres I would have to clear about 10 litres as my lungs would compress to about 1/3 and have to replenish.

So I reckon at 3 metres I have about 1.5 litres of extra air to clear on ascent.

Does that sound right?

Thinking about it begs the question, how long does it take to compress the air on descent. For example, if I do lifeguarding stuff to fetch a 'body' up from 3 metres, I would assume I'm not down long enough for any compresion to take place. Or am I?

[PeteM]

This is just a bit of interest for me to see if I've grasped some of the theory we learned on the Padi Course.

At 10 metres my lungful of air (lets say 5 litres) compresses to half and I take on more to replace it, also compressed to half. So on ascent I have to be able to clear 5 litres of air. ie my 5 litres would expand to 10 litres otherwise. Likewise at 20 metres I would have to clear about 10 litres as my lungs would compress to about 1/3 and have to replenish.

So I reckon at 3 metres I have about 1.5 litres of extra air to clear on ascent.

Does that sound right?

Yes

Thinking about it begs the question, how long does it take to compress the air on descent. For example, if I do lifeguarding stuff to fetch a 'body' up from 3 metres, I would assume I'm not down long enough for any compresion to take place. Or am I?

No the compression and decompression is instantaneous. It is straight forward hydrostatic pressure which changes with depth.

You are getting confused tissue loading which is also known as decompression which is where nitrogen dissolves in the body tissues which does take time

[gsclarke]

No the compression and decompression is instantaneous. It is straight forward hydrostatic pressure which changes with depth.

That's interesting.

So does it therefore follow that assuming no change whilst underwater, any risk is exactly the same if I'm down for 1 minute as being down for 30 minutes?

I'm not about to base any decisions on your answer btw, this is purely an interesting subject for me. ;)

[PeteM]

That's interesting.

So does it therefore follow that assuming no change whilst underwater, any risk is exactly the same if I'm down for 1 minute as being down for 30 minutes?

I'm not about to base any decisions on your answer btw, this is purely an interesting subject for me. ;)

From the perspective of the risks imposed by the asthma yes.

From the perspective of the risks imposed by nitrogen loading no.

[gsclarke]

From the perspective of the risks imposed by the asthma yes.

From the perspective of the risks imposed by nitrogen loading no.

OK, I've a bit more understanding now. I'd grasped about time and nitrogen which is less relevant for me for a while since I'm some way off doing anything other than small shallow dives. I did enjoy all the theory on the dive tables and pressure groups though.

I think some of the questions I will be asking when I see the consultant are:


Is it really OK for me to dive in our local pool at 3 metres, which is something my GP would sign me off for?
If so, what is the difference between that and me diving at 3 metres in a very controlled open water situation?
With my current lung function does any risk appertain largely to any underwater change in my breathing eg an asthma attack?
Having taken challenge tests before to see at what point my breathing became a problem (which will no doubt be repeated), can we say the risk of an asthma attack is negligable?
Aside from the risk of an attack underwater, do my lungs function well enough to clear sufficient air at given depths eg 5 metre, 10 metres, 20 metres?

[tony J]

Is it really OK for me to dive in our local pool at 3 metres, which is something my GP would sign me off for?
If so, what is the difference between that and me diving at 3 metres in a very controlled open water situation?
With my current lung function does any risk appertain largely to any underwater change in my breathing eg an asthma attack?
Having taken challenge tests before to see at what point my breathing became a problem (which will no doubt be repeated), can we say the risk of an asthma attack is negligable?
Aside from the risk of an attack underwater, do my lungs function well enough to clear sufficient air at given depths eg 5 metre, 10 metres, 20 metres?

Difference between pool and confined open water = water temperature, stress, air quality.
If you suffer stress induced/cold induced asthma then I would not breathe compressed air.
How do we know if your lungs function well enough ?

I'm an ex asthmatic, and one thing I know is that you can not control the underwater environment.

If you are on the bordeline of being allowed, I'm sorry but I would advise you not to bother. I assume it can only get worse ? i.e. I grew out of child hood asthma, I assume you are growing into it ?. Lung function gets worse with time, so I assume you are on a hiding to nothing.

As far as letting your GP sign you off for a swimming pool, If unsure ask an expert, the GP is probably in no better position to advise you than us !

Good luck anyway.

Tony

ps My medical says 24 hours wheeze free and 48 hours medication free before I can dive. I haven't had an inhaler for 15 years hoowever.

[gsclarke]

Hi Tony

Very much appreciate your comments.

Difference between pool and confined open water = water temperature, stress, air quality. If you suffer stress induced/cold induced asthma then I would not breathe compressed air. How do we know if your lungs function well enough ?
I'm hoping to soon prove that I don't have a response to cold or stress and to some extent my environment is something I can control within certain limits. OK it may mean I end up diving in 5 or 6 metres in sheltered bays abroad etc. (worst scenario) but that would be better than no diving at all in my book.

I assume it can only get worse ? i.e. I grew out of child hood asthma, I assume you are growing into it ?. Lung function gets worse with time, so I assume you are on a hiding to nothing.
Well this is where I think I am going to have problems because my asthma, if that is what it is, is not typical. I've been coughing for 30 years and the medical profession can't make their mind up if it is asthma or not. I've never had an asthma attack as such and when the nurse did the lung function test she commented that I had a remarkable ability to make a breath last forever. I just don't have the power to exhale forcibly.

My medical says 24 hours wheeze free and 48 hours medication free before I can dive. I haven't had an inhaler for 15 years hoowever.
Well if I got that I would have no problem at all. Never take inhalers and almost never wheeze, even then I can correct it with attention to breathing within minutes. Buteyko breathing techniques work well for me. Have you ever looked at it?

Just as an aside, when I decided I wanted to dive, something I've planned for ages, I organised a very structured approach to it so I could see what problems I may or may not have and obviously my cough was a major concern.

I found that I cough way less under the water, possibly because the air is cleaner. I did have a couple of coughing spasms right at the end of the week. By then I'd gained a bit of confidence so I dealt with the same way I would have at the surface by breathing slowly and breathing out more than I breath in. It was a minor event.

One of the things Buteyko teach is not gasping for air when you feel you need it and asthma aside, it's given me a great deal of confidence knowing that I am never out of control with my breathing.

I do feel more and more that even assuming I can get to 70% on the tests, I need to understand for myself what risk I may run, especially with it not being classic asthma.

And of course, one persons attitude to risk is very different from another.

Off-topic for a moment, I have various discussions with my GP about high cholesterol. There is a great deal of controversy over it. Cutting a long story short, mine was high initially, inside the normal range on medication and just outside normal with lifestyle and dietary changes and no medication.

However my good to bad ratios are better if I go against standard medical advice and follow the diet I believe best and take no medication.

The software most GPs use to analyse risk says that better than a 10% risk of having a heart attack in the next 10 years requires no mediation. Beyond that medicate.

My point is that I don't consider those good odds. Going against my standard GPs advice (with his blessing), I bought my odds down to a 6% risk and I'm still learning and working on it.

I don't want to turn this into a cholesterol discussion but I just use it to illustrate that people have different attitudes to risk. The hardest part is actually identifying and understanding what risk you are taking, especially with something like breathing difficulties.

At the end of the day, if it comes to pot luck and I can't quantify the risk to some degree, I guess I will have to play it safe and not dive. I hope it won't come to that but if it does, well at least I've had fun learning and discussing it all :(

[Dave (Simmo)]

I can control within certain limits. :(

i'd question that statment when diving - you never know what is going to happen next so your dealing with stress avoidance rather than stress elimination.

i'll give an example diving recently in blue water I rounded a rock and met a Moray Eel coming the other way! some big backward swimming occured on both our parts and my breathing rate shot up for a few moments as did my heart rate

would that our an equipment failure or loss cause you stress lets say getting your mask kicked off or your reg out of your mouth.

of course the more comfatable you are with the environment the less these are likely to stress you

Dave
Ps "met a Moray Eel coming the other way!" - my buddy who was above saw the whole thing from a few moments before we met, when it was obvious what was about to happen. Nearly drowned he was laughing so much

[gsclarke]

:D I know what you mean.

I think what I'm trying to say is that I can quantify whether my asthma is triggered by stress, cold, exercise etc. to a large extent by the tests that they do at the hospital and/or the diving medical.

If I were particularly susceptible (asthma-wise) then I probably wouldn't get the OK anyway.

Beyond that I can control things like depth, temperature, time spent standing round in full kit, arranging to have some-one dive with me eg an instructor that is aware of my potential limitations and so on.

It will probably come down to choices like a holiday where I can do a couple of dives rather than a boat holiday that's totally devoted to diving, or places like Egypt where I can reasonably organise some-one to carry my kit around and help me into the water with etc..

It's very limiting I know but I think it's a safe option to begin with. One of the golden rules with PADI and I assume BSAC too, is know your limitations.

For me it will be a question of taking time to be sure that I do know my limitations before I get too adventurous.

I know most people will want to warn me not to underestimate things, and rightly so. I hope I have the balance right and certainly listening to everyones views on this forum will give me the best chance I have of not putting myself or anyone else in uneccessary danger.

I just hope that Moray Eel has the kindness to wait until I have a bit more experience before putting me to the test ;)