What is the deepest you have been on pure O2?

[shoredivr]

That "Deep Stops Increases DCS" is one of the most informative threads on current and leading edge decompression theory I've had the pleasure to read. Long but worth it.
Interesting in terms of deco software manufacturers as well.

 

[Kevrumbo]

Kevrumbo, thanks for sharing your experiences. On hindsight, in future deco dives, would you still run with GF of 30/85 or would you go with a more conservative GF like 35/75 or something else?

I would still run initially with GF 30/85, but also having the Petrel Dive Computer's on-the-fly realtime option of resetting GF-Hi to a more conservative value like 70 (i.e. Extending O2 Deco time at 6m). . .

Utilizing the Petrel Computer's GF99 Function. . .

Originally Posted by Kevrumbo
What I like about the Petrel is that you can electively adjust the GFHi (surfacing Gradient Factor) real time during the dive if you want to extend your safety stop time (or O2 deco time if using Oxygen); For example, if you start with 45/95 GF's (Tech Mode), you can at your option change the "95" to a lower more conservative surfacing value like "60" during the dive if you wish; the Petrel will recalculate & update the Tissue Loading Bar Graph, CNS and reflect the new extended Time to Surface & Deco Time/Depth displays as needed.

Originally Posted by uncfnp
Yes, I actually used part of that article in my discussion. I hate to add yet another link but for those readers interested in this discussion specifically as it relates to the Petrel, there is another function they might find interesting, the GF99...

[Conversely, the GF99 value can also be used to add conservatism. For example, if you knew that you had abeen exposed to factors that increase decompression sickness risk such as exercising during the bottom time or being colder on deco than the bottom, then you could use this value to extend shallow stops. Of course, you could always just add a few minutes to your last stop, but this gives a quantitative measure to correlate with “how you feel” after the dive.]

https://www.shearwater.com/shearwate...ay-value-mean/


Another instance where you might want to decrease the GF99 during a dive to a lower more conservative surfacing value is when doing repetitive deco dives over several consecutive days of a week or more --especially if using deep bottom mixes with high FN2 percentages like Air or Nitrox which can significantly load slow tissues with excess residual inert Nitrogen. By Day 4, I would start adding more O2 deco profile time to washout this residual N2 at 6meters/20feet by reducing the Petrel's GF99 value from 85 to 70 or 60 --and sometimes even lower to 50 or 40 -- finishing with a very slow ascent to the surface (0.5m/min or 1.6 foot/min), and monitoring for signs & symptoms of slow tissue type I DCS. BE ADVISED THAT THE CNS OX-TOX VALUE AS TRACKED BY THE PETREL COMPUTER MAY BE UNCOMFORTABLY HIGH.

Better yet --to safely Off-Gas further those N2 saturated slow tissues (the Petrel Tissue Bar Graph clearly tracks & indicates that very nicely!) and to reset the CNS to zero as well-- take a Day-Off after three consecutive days of Deep Air bottom gas diving with 50% and Oxygen decompression gases. . .
​

 

[Divetech99]

Thanks for sharing Kevin. Very informative and enlightening. Two follow up questions:

1. If the intent is to extend shallow O2 stops beyond what the Petrel 30/85 calls for, why not just configure the Petrel to 30/80 or 30/75? With the on-the-fly option to switch hi-GF to 85 in a real time basis if the situation calls for it.

2. I take it that you still see the value in deep stops that's why your lo-GF is still at 30. Will setting lo-GF to 40 or 35 be less beneficial in your assessment?

Thanks again for sharing.

 

[Kevrumbo]

Thanks for sharing Kevin. Very informative and enlightening. Two follow up questions:

1. If the intent is to extend shallow O2 stops beyond what the Petrel 30/85 calls for, why not just configure the Petrel to 30/80 or 30/75? With the on-the-fly option to switch hi-GF to 85 in a real time basis if the situation calls for it.

2. I take it that you still see the value in deep stops that's why your lo-GF is still at 30. Will setting lo-GF to 40 or 35 be less beneficial in your assessment?

Thanks again for sharing.

I use GF's 30/85 on the Petrel as the minimum contingency deco profile to follow at best & to complete if possible, should I have to surface quickly because of some emergent condition (i.g. Medical problem; loss of deco gas; large Great White Shark bumping me etc.). Also the GF-Hi initial setting of 85 on the Petrel yields total intermediate deco stop times on Nitrox50 starting at 21m similar to the stop time profile totals that I manually calculated pre-dive using the Ratio Deco Method as a Back-up Decompression Profile Schedule.

In other words, I'll complete the nominal profile for Nitrox50 intermediate deco stops for GF85, then go to 6m depth O2 gas switch AND THEN reset the surfacing GF to 70 or 60 to extend the Oxygen deco stop time as needed. . .

Deep Stops with high Helium mixes IMO are still beneficial; you don't want that smaller faster monatomic He gas particle with its greater diffusivity to start micro-bubbling out of Fast Tissue (neuro type II DCS), or out of venous blood returning to the Lungs Alveolar Capillary Bed ("chokes" type II Pulmonary DCS).

 

[Divetech99]

I use GF's 30/85 on the Petrel as the minimum contingency deco profile to follow at best & to complete if possible, should I have to surface quickly because of some emergent condition (i.g. Medical problem; loss of deco gas; large Great White Shark bumping me etc.). Also the GF-Hi initial setting of 85 on the Petrel yields total intermediate deco stop times on Nitrox50 starting at 21m similar to the stop time profile totals that I manually calculated pre-dive using the Ratio Deco Method as a Back-up Decompression Profile Schedule.

In other words, I'll complete the nominal profile for Nitrox50 intermediate deco stops for GF85, then go to 6m depth O2 gas switch AND THEN reset the surfacing GF to 70 or 60 to extend the Oxygen deco stop time as needed. . .

Got it. Makes sense. Thanks.

 

[kensuf]

I would still run initially with GF 30/85, but also having the Petrel Dive Computer's on-the-fly realtime option of resetting GF-Hi to a more conservative value like 70 (i.e. Extending O2 Deco time at 6m). . .

I do the reverse. I'll start the dive with something like 30/75 and then adjust upwards to 30/85 or 30/90. It's strictly a mental thing, I like to be able to see a "worst case" TTS number when I get back to deco.

 

[Doppler]

Got it. Makes sense. Thanks.

Can you explain the science behind it for me then please?

 

[Divetech99]

Can you explain the science behind it for me then please?

Hi Doppler, I understood Kevin as saying he starts off with a more liberal 30/85 setting to get out faster in the event of a contingent necessity. Then he adjust to a more conservative surfacing GF of 70 or 60 to extend his O2 stop as needed. It appears to be a sound strategy. The wildcard could be the O2 exposure but that depends on the dive profile.

I'm not sure I understand your question. Can you clarify?

 

[Remy B.]

Kevin, just for my understanding, as I'm not a tec diver, and I'm trying to follow you, but it is kind of confusing to me, as well I have other questions.

If you start with deeper than normal stops to avoid rapid expansion of the bubbles in your fast tissues, and you start your dive with less conservative profiles but then you switch to conservatives at 6m plus you extend your Deco time, why you just don't go conservative for the complete profile of the dive ? have you experienced type 1 events doing so or worse by following VPM or Buhlmann or RGBM profiles without changing them ?

What about your daily O2 CNS limits, and your OTU for multiple deco dives during the week, are you not getting to close to the limits or exceeding them ?

Is part of your dive planing to change the Deco stops profiles or you just change them at your convenience ?

 

[Doppler]

Hi Doppler, I understood Kevin as saying he starts off with a more liberal 30/85 setting to get out faster in the event of a contingent necessity. Then he adjust to a more conservative surfacing GF of 70 or 60 to extend his O2 stop as needed. It appears to be a sound strategy. The wildcard could be the O2 exposure but that depends on the dive profile.

I'm not sure I understand your question. Can you clarify?

It's OK, I asked a couple of diving doctors I know. No need to clarify...