Riding GF99 instead of mandatory/safety stops

[L13]

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.

If that is true, then YOU ABSOLUTELY SOULD NOT USE GF99 to determine when to come shallower. Switching to pure O2 does not instantly reduce your risk of bubble formation/DCS. It does instantly increase your rate of off gassing.

I was under the impression that GF99 was the equivalent of instantaneous GF at current depth which depends only on current total ambient pressure, not inert ambient pressure. If it based on ambient inert partial pressure then it is only useful to determine off gas rate, not safe depth.

 

[L13]

@EFX, and @admikar : If you set your Shearwater to 20/80, does GF99 show "On Gas" when you reach your first deep stop? Your slow tissues are still on gassing while your fast tissues are off gassing. Does it display your most on gassing tissue, your most off gassing tissue or the on/off gassing of the limiting tissue? Does GF99 really jump instantly when you switch to a richer gas?

 

[admikar]

I never said GF99 drops instantly, just that is possible to get it under 0.

 

[admikar]

@EFX, and @admikar : If you set your Shearwater to 20/80, does GF99 show "On Gas" when you reach your first deep stop? Your slow tissues are still on gassing while your fast tissues are off gassing. Does it display your most on gassing tissue, your most off gassing tissue or the on/off gassing of the limiting tissue? Does GF99 really drop instantly when you switch to a richer gas?

In one of the previous posts you said that GF99=0% when tissue inert gas pressure=ambient inert gas pressure.
If you breathe pure O2 at surface for some time, what happens with your tissues when you go back to air?
Your tissues start ongasing towards .79 bar of Nitrogen, meaning GF99 is less than 0.

 

[L13]

In one of the previous posts you said that GF99=0% when tissue inert gas pressure=ambient inert gas pressure.

I was wrong if/when I said that, it should have been when tissue inert gas pressure=ambient total gas pressure. However, @EFX is making me question that, and therefore the usefulness of GF99.

If you breathe pure O2 at surface for some time, what happens with your tissues when you go back to air?
Your tissues start ongasing towards .79 bar of Nitrogen, meaning GF99 is less than 0.

GF99 would be "On Gassing" in that scenario regardless of whether ambient inert PP or total P was used.

 

[L13]

I never said GF99 drops instantly, just that is possible to get it under 0.

GF99 is the name for a Shearwater display. It is never negative, when it would be negative it displays "On Gassing".

 

[L13]

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.

This is exactly what I am saying: For tissue saturation calculations us ambient inert partial pressure of that gas. We are violently agreeing on that part!

BUT for GF calculations you use ambient total pressure! (none of your quoted section adresses this calculation)

You seem to be saying that:
GF99 = (P_tissue_inert - P_ambient_inert) / (M_value - P_ambient_inert)

If that were the case, we could instantly increase GF99 just by switching from breathing Air to breathing 100% O2. It would not be useful for determining bubble formation/DCS risk, since that does not instantly change with the gas change.

If you use:
GF99 = (P_tissue_inert - P_ambient_total) / (M_value - P_ambient_total)

Then GF99 is useful for determining your instantaneous bubble formation/DCS risk, also GF99 increasing would always mean off gassing increased, and GF99 decreasing would always mean off gassing decreased.

 

[J-Vo]

My understanding is that GF99 is tissue total pressure vs ambient total pressure. Otherwise it would be useless for determination of risk.

When I do 90 minutes on O2 for a big deco dive what is the GF99 of my 5 minute tissue when decompression clears? I'm still at 1.3 ATM, but the tissue effectively has no inert gas in it. GF99 of that tissue <0 , but it's also not on gassing because I'm breathing oxygen. Stay on O2 long enough and this would be the case for all tissues.

If this isn't the case, why do astronauts pre breath O2 before EVA?

 

[EFX]

If that were the case, we could instantly drop GF99 just by switching from breathing Air to breathing 100% O2.

Actually, it would be the opposite. GF99 would increase because the rate of off gassing would increase. For example: breath air (21% O2 + 79% N2) at 33 ft of sea water until the tissues are saturated, then switch to 100% O2. Let's neglect the MOD of the O2 for arguments sake. The total pressure at 33 ft using units of fsw and converting to absolute pressures is 33 + 33 = 66 fsw. The partial pressure of N2 at this depth is 0.79 x 66 = 52.14 fsw. Since we're saturated there can be no flow of N2 in or out of the tissues by definition. Do you agree that when we switch to 100% O2 those tissues will off gas? You should because that is why tech divers use 100% O2 at 20 ft to achieve accelerated decompression where the tissues are off gassing. We can write this difference of pressure (on the new gas) according to your equation as:

GF99 = (P_tissue_inert - P_ambient_total) / (M_value - P_ambient_total)
= (52.14 - 66) / (m-value - 66)

As you can see the value in the first expression will be negative making GF99 negative. I think in a previous post it was said that a negative GF99 meant the tissue was on gassing not off gassing. However, if we use my equation we get (on the new gas):

GF99 = (P_tissue_inert - P_ambient_inert) / (M_value - P_ambient_inert)
= (52.14 - 0) / (m_value - 0)

The value for GF99 remains positive and is higher than the previous calculated value which we said must be higher because we're on 100% O2.

 

[dmaziuk]

If that is true, then YOU ABSOLUTELY SOULD NOT USE GF99 to determine when to come shallower.

Why would you want to? You come shallower when your next ceiling hits your next stop depth. That may or may not have anything to do with supersaturation of the currently controlling tissue compartment.