Riding GF99 instead of mandatory/safety stops

[kaylee_ann]

There is not a "supersaturation point". Tissues have the supersaturation values that they have. It's also not necessarily the tissue with highest tissue tension/pressure. (Faster tissues can handle greater tensions, according to Buhlmann.) There is a GF99 for each tissue compartment, and the controlling tissue is the one with highest GF99.

GF99 = fraction of the tissue pressure from ambient to the M-Value
GF99 = (P_tissue - P_ambient) / (M_value - P_ambient)
(P_xxx is the pressure of that whatever)

ahhh okay. yeah when I thought supersaturation point I thought of it as the GF-low point. your explanation makes sense- thanks

 

[L13]

I think there is a subtlety that's not coming across in your writing. Supersaturation is the amount over ambient (P_tissue - P_ambient). What is important is the supersaturation RELATIVE to how far the M-Value is over ambient. It's possible for a tissue with a lower supersaturation to control if it's M-Value is also low enough.

Good catch, I was sloppy and it might have confused people.

 

[L13]

GF99 = 0% --> saturation (P_tissue = P_ambient)

GF99 > 0% --> supersaturation(P_tissue > P_ambient), required for off gassing. The higher it is the faster the off gassing, but the higher the risk of DCS
EDIT: off gassing occurs even if GF99 < 0% if partial pressure of a particular inert gas in the tissue is > partial pressure of the gas inspired (ex. PP_tissue_N2 > PP_inspired_N2). Similarly, on gassing can occur even if GF99 > 0%, if the PP_inspired > PP_tissue for a particular inert gas(ex. PP_tissue_N2 < PP_inspired_N2).

GF99 = 100% --> supersaturation at the Buhlmann M-value limit for risk of DCS

GF99 = GF Low --> supersaturation at the more conservative limit for the first stop

GF99 = GF High --> supersaturation at the more conservative limit for surfacing

Each tissue has it's own numbers, the GF's displayed and used to determine the next stop is always the one for the most limiting tissue.

 

[J-Vo]

GF99 > 0% --> supersaturation(P_tissue > P_ambient), required for off gassing. The higher it is the faster the off gassing, but the higher the risk of DCS

Not quite. Off gassing can occur with GF99 < 0% if the inspired partial pressure of inert gas is < the tissue partial pressure.

 

[L13]

Off gassing can accure with GF99 < 0% if the in

Not quite. Off gassing can occur with GF99 < 0% if the inspired partial pressure of inert gas is < the tissue partial pressure.

You are correct. Darn, I'm getting sloppy! That is the whole point of high O2 % in deco gases. Will edit my post.

 

[EFX]

Not quite. Off gassing can occur with GF99 < 0% if the inspired partial pressure of inert gas is < the tissue partial pressure.

This can't be right. The GF99 is the % value between ambient pressure (which is 0%) and the m-value at that depth (100%) for the controlling tissue compartment (CTC). Off gassing can only occur when the CTC pressure is above the ambient pressure which is the inspired gas pressure. Since GF99 of 0% is equal to the ambient pressure no off gassing can occur below 0%. In fact, that region is for on gassing.

 

[L13]

This can't be right. The GF99 is the % value between ambient pressure (which is 0%) and the m-value at that depth (100%) for the controlling tissue compartment (CTC). Off gassing can only occur when the CTC pressure is above the ambient pressure which is the inspired gas pressure. Since GF99 of 0% is equal to the ambient pressure no off gassing can occur below 0%. In fact, that region is for on gassing.

On/Off gassing is driven by Partial Pressure difference.

GF99/bubble formation/DCS is driven by Total Pressure difference.

Therefore @J-Vo was correct in correcting me, and your statements above are incorrect.

 

[EFX]

Nope. You are badly mistaken. On and off gassing have to do with the inert gas pressure only. You even say so in your post above by your use of the term "partial". Flow of nitrogen in and out of the tissues is driven by the inert partial pressure of that gas. The quote below is from the Shearwater manual for the Perdix dive computer regarding GF99. They mention inert gas pressure.

"The gradient factor as a percentage (i.e. super-saturation percent gradient). 0% means the leading tissue super-saturation is equal to ambient pressure. Displays “On Gas” when tissue tension is ambient pressure.
Displays “On Gas” when tissue tension is less than the inspired inert gas pressure."

Just to be clear, the ambient pressure they mention above is not total pressure but is the inert partial pressure at the ambient depth. Inert gas pressure does NOT equal total gas pressure.

The following is an excerpt from Baker's paper "decolessons" where he talks about the Schreiner equation for calculating the inert tissue pressures:

/===================
In the first case (constant depth), the solution is:
P = Po + (Pi - Po)(1 - e^-kt)

This is the "Haldane" equation or the "instantaneous" equation. This same equation can also be written as:
P = Po + (Pi - Po)(1 - e^(-ln2t/half-time)) or
P = Po + (Pi - Po)(1 - e^(-0.693t/half-time)) or
P = Po + (Pi - Po)(1 - 2^(-t/half-time))

where:

P = compartment inert gas pressure (final)
Po = initial compartment inert gas pressure
Pi = inspired compartment inert gas pressure
t = time (of exposure or interval)
k = time constant (in this case, half©time constant)
e = base of natural logarithms
ln2 = natural logarithm of 2
/====================

Note that none of the variables above use total pressure.

 

[admikar]

No. I am right. On and off gassing have to do with the inert gas pressure only. You even say so in your post above. Flow of nitrogen in and out of the tissues is driven by the inert partial pressure of that gas. The quote below is from the Shearwater manual for the Perdix dive computer regarding GF99. They mention inert gas pressure.

"The gradient factor as a percentage (i.e. super-saturation percent gradient). 0% means the leading tissue super-saturation is equal to ambient pressure. Displays “On Gas” when tissue tension is ambient pressure.
Displays “On Gas” when tissue tension is less than the inspired inert gas pressure."

Just to be clear, the ambient pressure they mention above is not total pressure but is the inert partial pressure at the ambient depth.

And you can still have GF99<0% if you are breathing gas that has less inspired pressure for given gas at given ambient pressure, for example, if you are breathing pure O2 will bring GF99<0 for any compartment even at surface until equilibrium is achieved. GF99=0 at surface means that tissue percentage of N of all dissolved gas within is 79. Breathing pure O2 will drop that percentage, aka GF99<0.
Ongassing and offgasing is driven purely by difference in tissue pressure vs inspired gas pressure.

 

[L13]

No. I am right. On and off gassing have to do with the inert gas pressure only. You even say so in your post above. Flow of nitrogen in and out of the tissues is driven by the inert partial pressure of that gas.

exactly!

The quote below is from the Shearwater manual for the Perdix dive computer regarding GF99. They mention inert gas pressure.

GF99 = (P_tissue - P_ambient) / (M_value - P_ambient)
P_tissue is inert gas pressure only, but P_ambient is total pressure.
Think about it, if P_ambient was only inert pressure then GF99 would jump dramatically and instantly if you switched from 21% to 100% O2!

"The gradient factor as a percentage (i.e. super-saturation percent gradient). 0% means the leading tissue super-saturation is equal to ambient pressure. Displays “On Gas” when tissue tension is ambient pressure.
Displays “On Gas” when tissue tension is less than the inspired inert gas pressure."

Just to be clear, the ambient pressure they mention above is not total pressure but is the inert partial pressure at the ambient depth.

On/Off gassing is ambient inert partial pressure. GF99/bubble formation/DCS is total ambient pressure.