Shearwater Perdix Tissues Graph

[Wibble]

You're missing @admikar's point though: none of it will tell you which tissues would be controlling if you had to do an emergency ascent that got you over 100%, and thus: whether you'd likely walk away with some oxygen and skin bends, or need a chamber ride.

Does it matter which tissue compartment is oversaturated? The ceiling calculations are effectively setting a line for which no compartment can exceed the GF value (linear from GF-lo through GF-hi). Ascend further then you’ve pushed at least one compartment beyond the line and you risk a bend.

This is where SurfGF is probably more useful as it shows you the GF you’d hit if you’d ascend now. That starts off at many hundreds of percent at the end of your bottom phase (of a deep, long dive) and oh-so-slowly reduces as you decompress.

Thus if your SurfGF is 250% and you ascend, you could end up dead and bent.

If you’re well over time and SurfGF is 90%, you might choose to ascend and probably won’t be bent, although you’ve a greater chance than if it was 80%.

If out of gas then you can’t breathe water so your SurfGF is largely immaterial.

 

[dmaziuk]

Does it matter which tissue compartment is oversaturated? The ceiling calculations are effectively setting a line for which no compartment can exceed the GF value (linear from GF-lo through GF-hi). Ascend further then you’ve pushed at least one compartment beyond the line and you risk a bend.

Yes it matters. I don't have a quote handy but I do recall at least one study (DAN Europe?) associating recreational DCS cases with middle compartments. You normally expect the first stop aka ceiling to be controlled by the fast compartments and on the no-stop dive that stop is the surface. If both are true, then the $15 question is whether SurfGF displayed by your computer is computed from a fast compartment that is not likely to get you bent, and conversely: what about those middle compartments that are. The latter is what the heat map is about.

 

[KenGordon]

The tissue saturation graph is hilarious after a couple of hours under Swange pier at a ppO2 of 0.7.

 

[Wibble]

Yes it matters. I don't have a quote handy but I do recall at least one study (DAN Europe?) associating recreational DCS cases with middle compartments. You normally expect the first stop aka ceiling to be controlled by the fast compartments and on the no-stop dive that stop is the surface. If both are true, then the $15 question is whether SurfGF displayed by your computer is computed from a fast compartment that is not likely to get you bent, and conversely: what about those middle compartments that are. The latter is what the heat map is about.

Obviously if you’ve just dropped in and descended to, for example, 60m/200ft then your SurfGF will very quickly pass 100% as the fast compartments fill. The tissue graph will show this as a spike at the fast end. After five mins your ceiling will be at least 6m/20ft. However your TTS (time to surface) will be quite short — rule of thumb at that depth deco time is x2 — at about 10 mins and will start to clear as soon as you begin ascending as the fast compartments quickly off gas, probably only need to do 5mins at 6m/20ft by the time you get there.

Now run that 60m/200ft dive for an hour on the bottom. The slower compartments now begin to fill as shown in the tissue graph which fills towards the middle. SurfGF will probably be around 300% or more, your ceiling 30ish metres and the TTS at least 120mins with half at 6m/20ft. As you ascend through your stops the SurfGF will steadily drop along with the TTS and ceiling.

BTW on a rebreather if you're in a hurry to get out then keep your PPO2 (oxygen partial pressure) high as that will greatly affect the total deco time. If you're totally dans la merde, running down the oxygen clock and pushing the CNS with a high PPO2 may be an acceptable compromise rather than extending the dive; may mean a few days off of diving though.

The tissue graph doesn't actually tell you anything that the core parameters -- SurfGF, TTS, Ceiling -- do not reveal. If you're at 6m/20ft and you've an hour to go then it's definitely not being controlled by the fast compartments!

Not sure of the actual usefulness of the graph, which isn't actually calibrated per-se. It gives you something to look at during your decompression, although some meditation is probably of more use.

If you're doing deeper dives it's probably more useful to carry another computer set to an "emergency" GF such as 95:95. That will answer the question of "how quick can I get out of the water without getting bent" (actually "with an acceptable risk of not getting bent"). The miserable fact is that's not much faster than your standard gradient factors, maybe 10 mins over a long deep dive.

What does this mean...? Plan your dive. Carry sufficient gas for the risks you've analysed. Set your limits and don't exceed them. If you're on a rebreather you'll find your bailout is the limiting factor; if on open circuit then it'll be your backgas including any bottom stages. If on a rebreather keep your deco PPO2 higher for a faster exit.

Bottom line: Knowing that compartment 5 controlling your TTS means diddly squat in the grand scheme of things.

 

[Wibble]

You're missing @admikar's point though: none of it will tell you which tissues would be controlling if you had to do an emergency ascent that got you over 100%, and thus: whether you'd likely walk away with some oxygen and skin bends, or need a chamber ride.

This is simply summed up as the more TTS you've aborted with, the more chance your next stop is the pot.

 

[dmaziuk]

...
Bottom line: Knowing that compartment 5 controlling your TTS means diddly squat in the grand scheme of things.

Maybe, but it's rather ill-defined between "mandatory deco" and "no-stop" -- keeping in mind that I do think that BSAC way of teaching is better and the whole "NDL" concept is a bit of a misnomer, and all that -- the way the algorithms work, there is a disconnect there and as a programmer I hate not having a well-defined smooth transition from one state to the other. And so I personally would rather look at the "bigger" picture than a single proxy number that may or may not be representative on anything real: I'm just not entirely convinced that single number is "distilled" right.

 

[admikar]

I might be wrong in my understanding of all this, but since I follow numbers anyway, it shouldn't hurt either.

 

[beldridg]

This is simply summed up as the more TTS you've aborted with, the more chance your next stop is the pot.

I like to call this "exposure risk" and I personally measure it with SurfGF which is a simple "one number" indication of risk.

A dive to 60m / 200' for 1 minute might be safer than a dive to 30m / 100' for 30 minutes in terms of your risk exposure. Some people tend to think in one-dimension (depth) and not two (depth/time). To be explicit, I don't mean you.

As your depth increases and/or your time increases, the amount of "risk exposure" you have increases. It isn't linear and it isn't one-dimensional.

As you mention, and as I believe, the SurfGF gives a pretty good approximation of that risk.

- brett

 

[Wibble]

I believe it is a combination of SurfGF and TTS. Now need to go away and get the supporting evidence!

Intuitively a deep bounce dive will rack up a high SurfGF with the fast compartments loading up. This will result in a short TTS but a high SurfGF and can be viewed on the tissue graph with it skewed to the top on the Shearwater display.

A not as deep (but beyond recreational limits) but long bottom time dive could also rack up the same SurfGF but with a much longer TTS due to the medium compartments being loaded.

Which is more dangerous to break the SurfGF ceiling…? Intuitively it’s the longer dive. Don’t have any evidence to support that though!


I think you’ve now set this year's homework…. Note down these values for different dive profiles during the dive. At least it’s something to do whilst hanging around at deco