Effect of slow compartments size in relation to NDL and DECO

[narke]

I think people are forgetting the role fat has in maintaining core temperature and thus efficient circulation and off-gassing.
I've seen plenty of skinny divers shivering and suffering from near hypothermia after deco dives even in relatively warm water. Just how effecient do you think their off gassing was? They were the coldest on theirdeco stops which is when you want good circulation to off gas.

The obese divers I've seen seem to be able to remain warm longer.

I think your comments should be taken in account. Along these lines another point would be hydration that would also play a significant role. Thank you.

 

[narke]

why? and why have I never been bent while diving identical dive profiles at 275lbs to one of my dive partners who is 110lbs?

so if inert gas is going in and going out at different volumetric rates as a function of half life, which you stated, why does the mass of the inert gas matter?

Your 3 questions:

1. Because I read it is found that divers that have more fat have a higher tendency to to incur in DCS especially for repetitive dives. I don't know why though, I would like to find out. One reason could be that people who are fatter could be so because of their life style, i.e. they are not training. Perhaps. Another could be related to the amounts of N2 they are accumulated dive after dive. And so on.

2. Dunno. You are not diving to Mo line? Who does?

3. There is no mass in the model. Let's ee what happens when add it?

 

[narke]

I revised the calculation. The result is the same, at the end of the dive the two divers have different quantity of N2.

How two divers on same dive end up with different N2?

Peter is 275lbs or 124kg and has Body Fat of 20% (BFP) with 25kg of fat and Paul has 35% BFP at 200lbs or 90kgs, is obese with 31.5 kg fat.

Concentration:

For each diver, how many grams of N2 are dissolved in in 1 liter of compartment F with Half Time 120 minutes (HT) at given constant temperature? Dive depth is 30 meters or 100ft.

Given KH of N2 in aqueous solution = 1600 atm/(mol/liter)
At equilibrium (saturation) the concentration is directly proportional to the partial pressure of N2 in the solution.

P=KHC
C=P/KH
C=4 atm / 1600 atm/(mol/L)
C=0.0025 mol/L

Amount in given size:

Convert to grams:
mass of one mole of N2=28g
g of N2 = mol N2 x 28
= 0.0025 x 28 g/mol
= 0,07g

1L of fat = 900g fat
1L of fat will hold 0,063g N2 when saturated at 30 meters or 100ft.

Peter: 900g fat tissue hold 0,063g N2, 2500g hold 1,75g
Paul: 900g fat tissue hold 0,063g N2, 3100g hold 2,17g

Once saturation is reached, it is reached, Peter's and Paul's compartment F won't hold more than what they can respectively.

So after 40 minutes of dive time, 30m-100ft, how much is the residue N2?

In this dive one whole HT period is not yet reached, the dive is 40 minutes so far, one half time is 120 minutes.

To build the table we take some values as anchor points:
F initial(g) time HT remains(g)
Peter 1600 120 120 800
Paul 2000 120 120 1000

Then:

120 minute compartment F
minutes %HT
40 20 <---
80 40
120 50
240 75
360 87.5
600 97,76
720 96.80
840 98.44
24h 100?
48h 100

Peter: saturation is 20% of 1.75g = 0.35g N2
Paul: saturation is 20% of 2.17g = 0.434g N2

Paul has more gas in his body as predicted. Different absolute N2 mass or volume between the divers before starting ascent.

Is this relevant after the dive?
Can become relevant for repetitive dives?

One could proceed to calculate the second diver's residual N2 dive after day by day after a weeks holiday.

I think the point is made.

A. Paul has more gas than Peter, which could lead to start bubbling before Peter would.
B. Peter and Paul's computers display the same residue N2, where it could be more accurate.

For "A," there are settings than can be adjusted (GF high, GF low) to mitigate the risk of bubbling,
there are no dive planning adjustments for surfacing M-values for "B.", in the current model.

Right now divers follow recommendations to be conservative.

Objections? Flaws?

Safe diving!

 

[tbone1004]

@narke if your logic was true, why would we not have seen it before? The quantity of gas isn't the issue, it's the saturation percentage of gas that is the issue. In both cases the saturation percentage is the same. While one may have more gas in him, it's because he took it in faster, ergo he can get rid of it faster.

The displayed residual nitrogen is a %, not a value. I'm not sure where you are missing that

 

[stuartv]

How two divers on same dive end up with different N2?

Peter: saturation is 20% of 1.75g = 0.35g N2
Paul: saturation is 20% of 2.17g = 0.434g N2

Paul has more gas in his body as predicted. Different absolute N2 mass or volume between the divers before starting ascent.

I think the point is made.

A. Paul has more gas than Peter, which could lead to start bubbling before Peter would.
B. Peter and Paul's computers display the same residue N2, where it could be more accurate.

You realize they start with different N2 as well, right?

A) Paul has more gas than Peter and more tissue that it's dissolved in. So what? They have the same risk of bubbling. If Paul bubbles, he'll have more bubbles than Peter - and they'll be distributed through more tissue.

B) Their computers are both saying 20% saturation. How could it be more accurate?

 

[Dr Simon Mitchell]

Objections? Flaws?

Hello,

It is obvious that a person with a higher body fat content will (all other factors being equal) end a dive with a greater absolute quantity of dissolved nitrogen, just the same as an adult will end a dive with a greater quantity of dissolved nitrogen than a child.

It is not the absolute amount of nitrogen that provokes bubbling; it is the tissue supersaturation. If the adipose tissue has been decompressed adequately there is no reason why bubble formation should be any more likely in a person with more fat than in someone with less fat.

With that said, the relevance of excess fat is almost certainly that if the adipose tissue does bubble, and you have more of it, then this might result in more bubbles entering the venous blood.

Simon M

 

[narke]

if the adipose tissue does bubble, and you have more of it, then this might result in more bubbles entering the venous blood

Thanks for this, makes sense.

 

[narke]

You realize they start with different N2 as well, right?

Yes they start with the same concentration but different quantity.

The culprit is that should they take the same time to reach new equilibrium at P change then they have different half-times (they start off with different quantities). I demonstrated that above.

The trouble is I am neither a chemist nor a physician so I don't know if this is really what is happening to a glass water vs a bucket of water and in two bodies with different adiposity (given same everything else of course).

 

[narke]

If the adipose tissue has been decompressed adequately

Would you know if an a body, due to whatever reason - say they way it is distributed, more adipose tissue would need more time to decompress, i.e. effects would not be due to to simple scaling up the quantity?

 

[narke]

The displayed residual nitrogen is a %, not a value

Yes I should have called it call %.