Deco For Beginners Part Two

[Dave Crampton]

Fantasitc thread/post Mark,

I look forward to reading the next posts of yours.
Oh and can I print off the post to keep for the newbies. Its very clear and concise.

Dave C

[Mark Chase]

Originally Posted by dcrampt

Fantasitc thread/post Mark,

I look forward to reading the next posts of yours.
Oh and can I print off the post to keep for the newbies. Its very clear and concise.

Dave C

Help your self but remember I cant cover all aspects in such a short piece and it's only a general intorduction so I am sure you will advise them on further reading/reaserch.

ATB

Mark Chase

[Janos]

Chasey,

You said that:

"Fat will absorb nitrogen quickly so people with a higher body fat ratio will require greater decompression than skinny gits."

But if fat on-gasses quicker, then doesn't off-gas quicker, cancelling out the effect? Suppose two divers, one fat (Janos)and one thin (Fred) did a dive on (say) straight Buhlman. I would expect them to do the same stops, as although Janos is a bit fatter, Fred also has some fat in his body, and the ratio of Pint to Pamb*M is what forces you to stop, and this is independent of the amount of fat in your body? I would expect a similar logic to apply to other models.

However, I would expect that Janos, being exactly twice the weight of Fred, as exactly twice the chance of getting a bend, for a similar profile, just because he's rolling twice as many dice.

I'm not an expert on these things and am happy (and expecting!) to be put right.

Laters,
Janos

[And]

Hi

Thats not quite right, but its just a communication thing I think. Fat is a slow tissue. It has poor bloodflow (perfusion) and poor diffusion (transfer of gas). However, tissues can diffuse ntrogen from one tissue to another, and so if you have lots of fat then that fat will start to saturate earlier than a non fat person, as the faster tissues will diffuse the nitrogen to the fat tissues they are near to, does that make sense?

Fat is slow to release the nitrogen , slower than muscle say (poor diffusion) and therefore if their nitrogen ontake has started earlier then the ascent rate of the profile is wrong and bubbles can and are created. As the deco tables, profiles, whatever are programmed for mister average then the diffusion rates will not be accurate and fat people should ascend slower, or dive nitrox on air tables, or otherwise be more conservative,


or just not be fat, which works better



Andy

[Mark]

Originally Posted by And

or just not be fat, which works better

What if you are big boned

[Mark Chase]

Originally Posted by Janos

Chasey,

You said that:

"Fat will absorb nitrogen quickly so people with a higher body fat ratio will require greater decompression than skinny gits."

But if fat on-gasses quicker, then doesn't off-gas quicker, cancelling out the effect?

I'm not an expert on these things and am happy (and expecting!) to be put right.

Laters,
Janos

Sounds loguical dosent it but the answer was well covered by Andy post above. Wel spotted though, Nobody has picked up on the cold one yet, cold low blood flow so slower on gassing?? Unfortunatly not

ATB

Mark Chase

[wreckweasel]

Originally Posted by Mark Chase

Nobody has picked up on the cold one yet, cold low blood flow so slower on gassing?? Unfortunatly not

ATB

Mark Chase

Or even.... normal on gassing until you've been immersed a bit and chilled off.... So your deco efficiency drops, which you'd think would drive people to mild exercise on deco to keep up the circulation....

[Janos]

Still slightly confused I'm afraid. I understand how Nitrogen can be transferred from fast (blood) to slow (fat) tissues, but I would have thought that having more fat would lead to less saturation (same amount of Nitrogen in more tissue) then more?

Perhaps an analogy is cooking meat in an oven. The air in the oven is a fast tissue, and warms up quickly. The meat (mmm tasty meat) is heated by the air, but a bigger joint of meat (slow tissue) will take longer to cook (saturate) than a small one.

Ah, but if you reach the stage that both are cooked, and you open the oven door, (lots of cold air comes in = blood with low levels of dissolved nitrogen) then the bigger joint takes longer to cool down.

I seem to have answered my own question here. Is it that basically thin people have a higher blood to fat ration, and therefore off-gas quicker, then fat people. But this assumes a degree of saturation of the slow tissues?

Incidentally I am genuinely big boned. But unfortuantely I am a bit lardy as well. So I just take it nice and slow. For me 50% more deco time = 5 minutes tops so it's never really an issue. I just enjoy understanding what's going on in my body.

Mark

I always thought the cold thing was that you tended to be warm at the start of the dive (good blood flow to skin etc) but cold at the end of the dive (poor blood flow to skin) making your body non-symmetrical

---

Right. Back to digging foundations for the new shed. It's knackering work, but should make my tissues a bit faster.

Laters,
Janos

[Mark Chase]

Mark

I always thought the cold thing was that you tended to be warm at the start of the dive (good blood flow to skin etc) but cold at the end of the dive (poor blood flow to skin) making your body non-symmetrical

That’s about it. The blood will leave the extremities and blood flow will increase around vital organs. So ever decreasing of gassing to the extremities and ever increasing on gassing around the important bits. I believe cold also induces shallow breathing limiting gas transfer in the lower part of the lung where it works best.

ATB

Mark Chase

[Digger]

Nice article Mark. I'll print it off if that's ok, there are a lot of people asking questions at the moment that I can try and answer, but never quite hit the nail on the head, or end up in hours of discussions about this sort of thing.

Great job. We should do PADI high 5's I think. Or maybe some BSAC low 5's.