Re-Evaluating My GF

[dmaziuk]

As a whole the industry is moving away from bubble models, even RGBM hold outs like Suunto are introducing Buhlmann with gradient factors as options on some of their computers as consumers move away from those types of models.

The whole "tech" industry. For us vacation divers the "moving away" choice is the PPS line-up steadily losing the one actually tested vacation algorithm in favour of whatever "PZ+" is, and of course the Deep 6 $150 puck that is pretty perfect except for that one small thing.

I know, I know: or we can drop a grand on a shearwater and dive happily ever after.

 

[Celt]

my run time for a 30 min bottom time at 52m is 83min
52m/30min
42m/1min
32m/1min
22m/1min
15m/1min
12m/4min
9m/7min
6m/12min
3m/23min
back in the boat

 

[leadduck]

I don't think one can decide well on a GF without considering the deco gas and gas switches.

Two things are here in a trade off:
a) want to go shallow fast to avoid further on-gassing
b) want to stay deep to reduce over-saturation of tissues and DCS

Item (b) depends only on depth and is independent of the breathing gas, but item (a) depends on the breathing gas. The optimum trade-off therefore depends on the choice of deco gas. For deco with air optimum deco profiles will be shallower than for deco with EAN50.
The dive computer or planning tool does not calculate a trade off between items (a) and (b). The dive computer will show the allowed ceiling no matter whether ascending to that ceiling will speed up off-gassing significantly or not.

For example: when a diver switches from Tx 18/45 to EAN50 at 21m (70ft), then the helium off-gassing gradient jumps immediately to maximum. He can get rid of all helium in his tissues while staying at 21m breathing EAN50; ascending further will not speed up his rate of helium off-gassing. He may therefore stay at 21m for some 10min to clear the fast tissues of Helium before ascending further. Probably a good idea, it will protect the fast tissues but not extend total deco time significantly. Although his dive computer may show a 12m ceiling all the time, this is not the optimum decompression strategy but is just the allowed ceiling.

See also A Logical Application of Gradient Factors for OC Tech Divers

 

[J-Vo]

See also A Logical Application of Gradient Factors for OC Tech Divers

I read this a few weeks ago. I may be a beginner trimix diver, but as an engineer/scientist myself I don't like rules of thumb designed to avoid needing to know how things work. Especially when my life depends on it.

After reading Andy's article I attempted to apply his method to a typical weekend of diving for me (planning only). In order meet those standard my GFLow would range from somewhere around 10 all the way up to well above 100. These are not inconsequential adjustments to the deco model. His rule of thumb only applies to a very narrow range of normoxic diving and is not scalable beyond that.

 

[JonG1]

Strangely enough I tried the same and although I was twisting the concept a little, in terms of using it for the OC bailout from a CCR dive with modest enough BT prior to the bailout, I couldn't get the GFLo to a point that I would be comfortable with.

It may not be an appropriate approach for this application in fairness, but the first bailout gas is in effect back gas and I carry/plan with 2 deco gases in addition to the deep bailout but didn't really scale on MD for me.

 

[scubadada]

my run time for a 30 min bottom time at 52m is 83min
52m/30min
42m/1min
32m/1min
22m/1min
15m/1min
12m/4min
9m/7min
6m/12min
3m/23min
back in the boat

That seems aggressive, out of interest, what is the basis for your profile?

 

[leadduck]

I read this a few weeks ago. I may be a beginner trimix diver, but as an engineer/scientist myself I don't like rules of thumb designed to avoid needing to know how things work. Especially when my life depends on it.

After reading Andy's article I attempted to apply his method to a typical weekend of diving for me (planning only). In order meet those standard my GFLow would range from somewhere around 10 all the way up to well above 100. These are not inconsequential adjustments to the deco model. His rule of thumb only applies to a very narrow range of normoxic diving and is not scalable beyond that.

I don't agree with his method either and I don't suggest to use such a high GFlow only to avoid stops below 21m. But it's good to see that Andy thinks about the breathing gas in relation to the choice of GF, that's why I added the link.
It's also a hint to adjust in the opposite direction: if your deepest stop is shallower than 21m, that's not yet a reason to ascend immediately after the 21m gas switch.

What bottom times and depths did you try in your planning?

 

[rjack321]

my run time for a 30 min bottom time at 52m is 83min
52m/30min
42m/1min
32m/1min
22m/1min
15m/1min
12m/4min
9m/7min
6m/12min
3m/23min
back in the boat

and the gases/switches are?

That seems aggressive, out of interest, what is the basis for your profile?

what about this seems aggressive?
This plan is basically GFs 40ish/70 for 30mins at 52m with 50% and 100% for deco. ~40/70 is not typically called aggressive.
The 22 and 15m times are a bit short so I don't know how this profile was derived. But there's ample O2 time so it'll likely work

 

[scubadada]

and the gases/switches are?

what about this seems aggressive?
This plan is basically GFs 40ish/70 for 30mins at 52m with 50% and 100% for deco. ~40/70 is not typically called aggressive.
The 22 and 15m times are a bit short so I don't know how this profile was derived. But there's ample O2 time so it'll likely work

Sorry if I made bad assumptions, thought this was the NEDU profile, all on air. Given that, you could understand my question.

 

[J-Vo]

I don't agree with his method either and I don't suggest to use such a high GFlow only to avoid stops below 21m. But it's good to see that Andy thinks about the breathing gas in relation to the choice of GF, that's why I added the link.
It's also a hint to adjust in the opposite direction: if your deepest stop is shallower than 21m, that's not yet a reason to ascend immediately after the 21m gas switch.

What bottom times and depths did you try in your planning?

Our dives typically range between 100' and 275'. I don't really subscribe to the oxygen window theory that staying at a high PPO2 accelerates off gassing. I get that once you switch you can not off gas He any faster, but you still are also off gassing N2.