Riding GF99 instead of mandatory/safety stops

[LFMarm]

If your computer calculates GF99 relative to the phase of teh Moon in the House of Jupiter, your mileage will of course vary. But if you're calculating it relative to your current conservatism setting: GF Low..GF High, then at a stop your controlling tissue is at its M-value and its current overpressure is right on the line between GF Low and GF High and dropping. If you switch to a lower N2% gas, it will be dropping faster.

GF99 is not linked to GF Low and High.

 

[L13]

GF99 is not linked to GF Low and High.

Are you saying that GF99 is not calculated the same way that GF is calculated when keeping it at or below the GF low -- GF High line?

If it is completely unrelated to other GF statistics, why is is called a GF99 instead of something else?

I have always been under the impression that GF99 was the same as the GF calculated at the current time at the current ambient conditions. Is that wrong?

 

[dmaziuk]

GF99 is not linked to GF Low and High.

Like I said: if you're calculating your overpressure relative to your M-value line (adjusted to your chosen conservatism setting), then it means something. If you don't, it doesn't.

 

[LFMarm]

Like I said: if you're calculating your overpressure relative to your M-value line (adjusted to your chosen conservatism setting), then it means something. If you don't, it doesn't.

GF99 is calculated in reference to 99/99 — your choice of GFs is independent of it

 

[dmaziuk]

GF99 is calculated in reference to 99/99 — your choice of GFs is independent of it

FWIW I've always thought it would be about as useful if they calculated it relative to 99 surfing grapefruits, but if they're really doing what you say: that's even dumber than their "recreational" presets.

 

[inquis]

at a stop your controlling tissue is at its M-value and its current overpressure is right on the line between GF Low and GF High and dropping

GF99 is not linked to GF Low and High.

Not explicitly linked, but unless you screw up your deco stops, it will be as @dmaziuk said.

In other words, *your actions* are the link between GF99 and the GF Line. You "touch" the line -- make GF99 equal to the GF Line at some depth -- then wait a bit, then touch the line again at a 10 ft shallower depth. Repeat until you surface.

 

[LFMarm]

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.

Correct, safe depth in Shearwater is Ceil

 

[L13]

Correct, safe depth in Shearwater is Ceil

So if you are at the Ceil, is your GF99 = GF line between GF Low and High? or is it some other number? Does changing your deco gas change it?

I find it hard to believe that Shearwater named something GF99 that was unrelated to GF's.

 

[inquis]

Correct, safe depth in Shearwater is Ceil

At the risk of belaboring the point, if you ascend to the ceiling, your GF99 will equal the GF Line at that particular depth. In fact, that's how the ceiling is computed.

 

[L13]

At the risk of belaboring the point, if you ascend to the ceiling, your GF99 will equal the GF Line at that particular depth. In fact, that's how the ceiling is computed.

Are you sure? @LFMarm and @EFX claim that GF99 is calculated differently than other GF's. @EFX claims that the denominator for GF99 is (M-value - Inspired_Inert_preassue). Which doesn't make sense as compared to other GF's, but no one but me is stating different.