Riding GF99 instead of mandatory/safety stops

[L13]

Actually, it would be the opposite.

You are right about that, I had the direction of the step change wrong, and eddited.

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.

So you are under the impressing that there is a step change in GF when we change % O2?

 

[L13]

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.

Your next ceiling is directly related to the supersaturation of the currently controlling tissue compartment. The entire thread is about using a continuous rather than step change in depth as the ceiling changes by maintaining GF99 = your chosen GF all the way to the surface.

But, if GF99 is calculated differently than GF (inert pressure ambient instead of total pressure ambient), then it CAN'T be used this way.

 

[dmaziuk]

Your next ceiling is directly related to the supersaturation of the currently controlling tissue compartment.

No. Specifically, "currently controlling" is not a given.

 

[EFX]

You are right about that, I had the direction of the step change wrong, and eddited.


So you are under the impressing that there is a step change in GF when we change % O2?

No. It won't be a step change because the new value of inert gas in the tissues is not just defined by the difference but by the half-time of the controlling tissue compartment according to Schreiner's equation.

 

[dmaziuk]

... It feels like the best decompression procedure would be to maintain an ascend rate that keeps GF99 = GLow all the way to the safety stop depth and then wait until SurfGF = GfHigh

There's only one point in the ascent that has anything to do with GF Low and that's the first decompression stop. From then on it's "GF current" that lies on the line from GF Low to GF High.

E.g. if you were diving 30/70, maintain the ascent rate as you propose above, and arrive at the safety stop at GF99 = 30, the way you can get your GF99 to 70 is if Earth loses most of its atmosphere while you're doing your 3/3.

 

[L13]

No. It won't be a step change because the new value of inert gas in the tissues is not just defined by the difference but by the half-time of the controlling tissue compartment according to Schreiner's equation.

You keep confounding Schreiner's equation which you are correct is based on the partial pressures of each individual gas, and the GF equation which depends on the total pressures(total inert for tissues, and total ambient for ambient).

using your equations on a switch from air to 100%:
GF99 = (P_tissue_inert - P_ambient_inert) / (M_value - P_ambient_inert)
There would be a step change in GF99 because P_ambient_inert would step from:

P_ambient_inert = .79*Depth​

to:

P_ambient_inert = 0​

​

But as you point out P_tissue_inert would change slowly following Schreiner's equation, and M_value would be constant with constant depth. The result would be a step change in GF with the step change in P_ambient_inert.

 

[EFX]

No. It won't be a step change because the new value of inert gas in the tissues is not just defined by the difference but by the half-time of the controlling tissue compartment according to Schreiner's equation.

The more I think about it there would be a step change because the inert gas at ambient pressure is changing instantly at the switch from 52.14 to 0 fsw.

 

[EFX]

The entire thread is about using a continuous rather than step change in depth as the ceiling changes by maintaining GF99 = your chosen GF all the way to the surface.

But, if GF99 is calculated differently than GF (inert pressure ambient instead of total pressure ambient), then it CAN'T be used this way.

What do you mean by "your chosen GF"? GF99 is calculated the same way that GFLo and GFHi are using the inert gas pressures. If we determine the first stop according to GFLo and follow a continuous ascent to the surface arriving at GFHi, the computer would need to calculate a current GF based on the current depth interpolating between GFLo and GFHi. As you continuously ascend you would need to keep your GF99 at or just below the current GF. So, instead of following a constantly changing depth we are following a constantly changing GF based on depth.

 

[L13]

What do you mean by "your chosen GF"? GF99 is calculated the same way that GFLo and GFHi are using the inert gas pressures. If we determine the first stop according to GFLo and follow a continuous ascent to the surface arriving at GFHi, the computer would need to calculate a current GF based on the current depth interpolating between GFLo and GFHi. As you continuously ascend you would need to keep your GF99 at or just below the current GF. So, instead of following a constantly changing depth we are following a constantly changing GF based on depth.

GFLo and GFHi are calculated on inert tissue pressure and total ambient pressure.

Think about it: You arrive at your first stop with GF = GFLo, If you immediately switch to a depth appropriate deco gas, does GF step up above your GFLo limit? No, your rate of degassing goes up instantly, but your GF goes down slowly.

All other GF's are calculated on total ambient pressure. I always assumed GF99 was the same, but you are trying to convince me different.

 

[EFX]

I think you're confusing GF with GF99. As Dmaziuk has already pointed out GF increases from GFLo to GFHi. It never decreases. You keep saying total ambient pressure. The total ambient pressure in the inspired gas includes both N2 (and possibly He) and O2. O2 does not contribute to DCS. It is consumed by the body. All the equations I've worked with use inert gas pressure. Go read Baker's paper "decolessons" if you don't believe me. I'll gladly change my viewpoint if you can find a document that says otherwise.