31 replies to this topic

[mvillanueva]

Been reading about saturation diving and I have tons of questions.

1. I assume under some scenarios when scientists and commercial folks do saturation diving, they work at some depth (D1), at a given pressure (P1), for some period of time (T i), then return to the surface (D0).

When they return to the surface for T i they are kept at P1 so they can easily return to D1 to do more work. The reason for this model is to avoid the lengthy time of decompression (DCT) in going back and forth between D0 and D1.

I am assuming that is one model of saturation diving. Is the model as I have outlined it? Or am I missing a key element of saturation diving?

2. Is it the the fact that the longer you spend working at P1, (either at D1 or at surface D0), the longer your DCT? The question is rooted in the no free lunch concept. While saturation diving can provide immediate time release from DCT while being kept at P1 (whether at depth or at the surface), it seems to me at some point you have to pay the piper. Is that time debt linear?

3. I want to know what type of relationship exists between these three variables: DCT, and Ti at P1. Is this relationship linear? Is it curvelinear?

4. At some pressure P1, it would seem to me that there has to be some limit to the amount of nitrogen the body can absorb, thus providing (at least in theory) an upper limit to the amount of time required to decompress. Or is it the fact that the body continually absorbs nitrogen ad infintum at depth?

[SquattingRadishDM]

OK, the uptake and elimination of ANY gas (apart from O2 as it is motabolised) takes the exponential form.

Imagine water pouring from a hole of a certain diameter in the bottom of a water butt. Lets say it takes 5 minutes for the butt to be half full. This is called its "halftime" - the amount of time for it to increase/decrease by 50%. It will then take another 5 minutes to get to 25% full. And another 5 minutes to get to 12.5% and then 5 minutes later it will get to 6.25%. In this example this is because as time goes on there is a lesser weight of water pushing down, forcing the water out the whole.

In this anology the water butt would never empty (this is where it falls down) but for practical purposes we say that after 6 halftimes something is either saturated or de-saturated. This model works for gas uptake/release in the body tissues or radio-active decay and the graph produced is exponential. ie curving 90 degrees from vertical to horizontal.

Still with me?

Now different tissues have different halftimes, ie they absorb nitrogen (or watever ur blend of gas) at different rates. This is because of differences in blood flow and a number of other factors Im not sure about, ur brain gets way more blood than ur joints (thats why DCS pain is more common in joints). These halftimes (rate of gas absorbtion remember!) can vary greatly, Buhlmann used halftimes ranging from 4 to 635 minutes in his model used in many dives computers and tables. This would imply that it would take 6*635 minutes or 63.5 hours for every tissue in the body to become COMPLETELY saturated, and hence 63.5 hours to then be de-saturated.

I am not sure if this answers all ur questions but hope it goes someway to explaining things.

I suggest you get hold of a book called "The Decompression Matrix" by Dr Bob Cole who once worked for Suunto. Its a usefull read and has lots of practical advise for divers, I highly recommend it.

After that post I FEEL like I've done a saturation dive! Phew

-Jack

[triggerfish]

geez, michael, lemme get out my physiology books again....!!!!


there is a section on saturation decompression in the bove and davis book you ordered (in the chapter on mixed gas diving) that explains things simply. it pretty much says it's a linear relationship. bear in mind that it'd never be perfectly linear (as mathematics would suggest!!) due to the numerous variables provided by the human body.

but you're absolutely right about the "no free lunch' thing. if you spent 6 hrs at 30 feet, you'd still need to decompress*


*yes, all you dive gurus, i know that's simplified. i'm just making a point.

[mvillanueva]

Ok. That makes one linear and one non-linear vote.

Trigger -- unfortunately, that book is not here yet. Been wading through the NOAA tome which arrived yesterday.

SR: I think the pain in the joints is due to pain receptors in the joints. The brain does not have pain receptors --- well, not counting emotional ones.

[triggerfish]

the joint pain is caused by the rapid expansion of a "closed" space (the joint). the distension of the joint stimulates the nerve fibers and shoots 'em up to the control desk.
while the brain doesn't hurt (like the awake cranis i did this week?!), it's awfully damn sensitive to ischemia...which can be caused by the interruption in blood flow secondary to a bubble.

i think i'd rather be in pain!!

[Walter]

Actually, Jack did an excellent job of explaining the concept as far as he went.

A couple of points.

In this anology the water butt would never empty (this is where it falls down)

Actually it doesn't fall down at all. After 1 half time the compartment (tissue is a misleading term in decompression theory as the compartments don't really coorespond to particular tissues, hense the change in terminlogy about 15 - 20 years ago) is 50% full (on up take, 50% empty if off gassing). After 2 half times it's 75% full, 87.5 after 3, 93.75 after 4, 96.875 after 5 and 98.4375 after 6 half times. We can extend it to 7 half times or beyond, but at this point there's less than a 1% change in saturation. This is considered insignificant. For that reason, your compartments are considered full (or clear) after 6 half times. No breakdown in theory.

Now to cover a point in the original post not yet answered.

When they return to the surface for T i they are kept at P1 so they can easily return to D1 to do more work.

Close. The diver would actually be kept at P1 not so they can easily return to D1, but so they don't have to decompress when returning to the surface for T i.

Once you are saturated, the additional time you spend at depth does not add to your final decompression time. You are correct in your reasoning in # 4, in fact, that's why it's called saturation diving. You are saturated with N2 at the pressure.

One point you haven't mentioned that is interesting concerning saturation diving. Let's say you are a saturation diver living in a habitat at 50 ft. When your time is over and you return to the surface, you'll have a lengthy decompression to undergo, but while you're at depth, you can work at deeper depths, returning not to the surface, but to your saturation depth. You will have tables with NDLs (No Decompression Limits) to allow you to return safely to your saturation depth with no decompression required.

Excellent question. Was everything covered?

[triggerfish]

thanks, yoda.

i thought it was interesting that there are different ways to get you back to "sealevel"pressure. the b&d book describes decompressing at depth for the appropriate amount of time in a habitat in which pressure can be controlled (like a chamber underwater). or, divers are brought up in a "bell" to decompress topside...

[Walter]

Same theory is involved in either method.

[triggerfish]

it's also interesting to see xrays of hips/shoulders in saturation divers.

makes you think twice about doing it for a living, regardless of the money...

[mvillanueva]

it's also interesting to see xrays of hips/shoulders in saturation divers.

makes you think twice about doing it for a living, regardless of the money...

What do you see? Can you describe it?

[Walter]

BINGO! It also one of the reasons people question the wisdom of children diving.

[mvillanueva]

One point you haven't mentioned that is interesting concerning saturation diving. Let's say you are a saturation diver living in a habitat at 50 ft. When your time is over and you return to the surface, you'll have a lengthy decompression to undergo, but while you're at depth, you can work at deeper depths, returning not to the surface, but to your saturation depth. You will have tables with NDLs (No Decompression Limits) to allow you to return safely to your saturation depth with no decompression required.

Excellent question. Was everything covered?

That was going to be my next question, but I did not want to ask it until got the graphical relationship between the variables sqaured away in my head.

Yeah... I got a few more, but I need to sort out these formulas I am reading in my head first.

[Walter]

The ends of the bones die.

[mvillanueva]

The ends of the bones die.

Is that statment supposition, observed fact, or an assumption based on belief?

[Walter]

Fact. You can read the details on page 3-29 of the NOAA Diving Manual. You might find it on an internet search. Look under Aseptic Bone Necrosis or Dysbaric Osteonecrosis.