Using GF99 and SurGF

[Brett Hatch]

@iointerrupt point taken that the model is complicated (and just a model). So maybe something about the multiple compartments offgassing simultaneously is confusing me. But, in the example that you gave:

Purely in theory, if a middle compartment is leading, but off-gasses enough to make a fast compartment leading, the rate of change could be as the example shows, slower then faster.

Wouldn't we expect that the faster compartment would be offgassing, well, faster than the middle compartment? And so you could never have a situation where the diver is remaining at a constant depth, and first the middle compartment is controlling, and then a few minutes later the faster compartment is controlling? In my mind, this is a contradiction, but again, maybe I'm missing something obvious

 

[iointerrupt]

Oh I agree, during a safety stop, at a constant depth, it does seem unlikely to be possible as the example shows. Though I am open as well to someone showing us wrong.

I just wanted to point of SurfGF can have complex behavior, and avoid people thinking of it as a simple timer.

 

[scubadada]

Oh I agree, during a safety stop, at a constant depth, it does seem unlikely to be possible as the example shows. Though I am open as well to someone showing us wrong.

I just wanted to point of SurfGF can have complex behavior, and avoid people thinking of it as a simple timer.

You can always watch your tissue saturation graph if you would like. The compartments are already adjusted for their M-value line.

 

[EFX]

@iointerrupt point taken that the model is complicated (and just a model). So maybe something about the multiple compartments offgassing simultaneously is confusing me. But, in the example that you gave:

Wouldn't we expect that the faster compartment would be offgassing, well, faster than the middle compartment? And so you could never have a situation where the diver is remaining at a constant depth, and first the middle compartment is controlling, and then a few minutes later the faster compartment is controlling? In my mind, this is a contradiction, but again, maybe I'm missing something obvious

Multiple tissue compartments can offgass simultaneously. The controlling tissue compartment doesn't mean it is the only compartment to offgass but that it's calculated ceiling (yes, NDL dives have a ceiling) is the deepest or has the most positive ceiling. When I look at my spreadsheet that displays the TC pressures as the dive progresses (gets shallower), I never see the CTC go to a faster compartment.

 

[Brett Hatch]

Multiple tissue compartments can offgass simultaneously. The controlling tissue compartment doesn't mean it is the only compartment to offgass but that it's calculated ceiling (yes, NDL dives have a ceiling) is the deepest or has the most positive ceiling. When I look at my spreadsheet that displays the TC pressures as the dive progresses (gets shallower), I never see the CTC go to a faster compartment.

Thank you for your input, you've spent a lot more time staring at GF things than I have. If I understand the math correctly, it should be impossible for that case to happen. Because two different exponential functions can only cross at one point, and the faster tissues will always be ahead of the slower tissues. Of course, you'll be able to find many examples of the opposite, where first a faster tissue controls, and then a slower tissue controls.

 

[boulderjohn]

I assume you meant “punctuation” and not “punction”. Or maybe you were led to misuse?

Yep. More fumble fingered ineptitude. Corrected.

 

[EFX]

The Buhlmann ZHL-16C model uses 16 tissue compartments labeled 1 through 16; the fastest being 1 and the slowest 16. While the faster compartments on gas fast they also off gas fast. Compartments 1 and 2 generally clear on ascent. The slower compartments on and off gas the slowest. This suggests the medium compartments are going to control most of the dive. I've seen this in the spreadsheet where for the typical NDL dive compartments 7, 8, or 9 control the dive through ascent to the surface.

I enjoyed the article on using GF99 and SurGF to control the ascent and safety stop depth and time. I can see the value in GF99 as a real-time window into your inert gas loading. I especially appreciate the definition of a pressure gradient as a pressure difference between the tissues and ambient pressure. This pressure difference is what drives gas flow into and out of the tissues. The article states that to achieve more efficient offgassing it advises divers to do the safety stop at 10 ft where the pressure difference is greater. I would caution divers that while it will drop GF99 faster it will also start from a higher value than a safety stop at 15 ft.

 

[Lorenzoid]

You can always watch your tissue saturation graph if you would like. The compartments are already adjusted for their M-value line.

That graph is a lot more interesting to look at during a safety stop than some GF number.

 

[harvalen]

I think that using SurfGF to extend safety stop sounds bit backwards. Personally, I don't see any reason to surface with a GF higher than 70 for the dives I do when things go according to plan, so I set GFHigh to 70. I can then use SurfGF, or a second computer set to less conservative settings, to check that I can go directly to the surface in case the dive has to be aborted.

By doing this the computer will tell me how long I'll have to stay at 3 m (or 6 m or 4.5 m depending on last stop depth setting) to get down to 70 and I can ensure that the length of that stop is something I'm comfortable with, have enough gas for, and the boat operator won't be mad because I went past their max dive time. If I'm on a guided dive I can adjust my dive profile so I end up with a stop time of 3 min to match the safety stop of the rest of the group.

When using GFHigh of 85 it's pretty easy to get a nitrogen load high enough in the middle compartments to have to stay 10 min or more at 3 m to get down to 70, especially on the 2nd dive with a 1 hour surface interval. If it's hard to stay at 3 m for the safety stop then that can be quite a bit longer at 5 m or 6 m.

 

[stepfen]

Very useful info and lucky me I just bought my first shearwater (a second hand bargain peregrine) last week.

I still though don't understand something. Are there any references/resources discussing the "optimal" GF99 for recreational dives? In the article it is suggested that the optimal GF99 should be about half of GFHi, but assuming a GFHi of 80% why GF99 of say 40% is "better" than eg 30% or 20% or 5% or even 1%?? Yes 40% will be faster than 20% but why is faster better? (Tec dives where deco should be as efficient as possible to make long decos manageable is a different story).
A lot of times rec dives are not square profiles. Instead the max depth is reached relatively fast and then a slow ascent starts all the way to the end of the dive. By the time the surface is reached SurfGF can be around 50-60% or so while the GF99 might not have been more than 10% or so and even this for few seconds just before surfacing.
Is that a "bad" thing and if so why??
Thanks a lot