Your Gradient Factors?

[CAPTAIN SINBAD]

UTD uses average depth to calculate the entire decompression profile. Everything is based on average depth and mixed gases.

 

[tursiops]

Which gradient factor is this? I am unaware of PADI or DSAT but Minimum Decompression Limits tend to fall into Buhlman 30/70 ish.

DSAT is not a GF model.

 

[CAPTAIN SINBAD]

DSAT is not a GF model.

Ya I thought so. I do not have deco planner with me but I dont think Buhlmann with 30/70 or 35/70 will permit a dive like that. It will bring you up within 60 minutes or so which is where Buhlmann 35/75 should be.

 

[kensuf]

How did you arrive at this conclusion pls, obvious signs of stress, Doppler etc?

Also any difference in thinking depending on profile, so 4 hours could accumulate over a really long dive at shallowish depth, versus a deep "bounce"?

Sorry about the delay in responding. I've been teaching for the last week non-stop and spent 16 hours diving (plus countless hours lecturing, in-water debriefs, etc) and little time on forums.

In short, on several dives where the computer said I only need 1-3 minutes of deco, I didn't feel "quite right" when I would surface, but if I added 5 I felt fine. I have also blown off 30 minutes of mandatory deco (according to my computer) on a 7 hour decompression (9 hour run-time) and was absolutely fine, on that one my computer was saying I still had a 1' or 2' ceiling and it would take 30 minutes to clear.

 

[PfcAJ]

Avg depth works just fine for calculating your deco.

 

[Storker]

My view of tables and actual depth comes from pre-computer days diving in Chicago, New England, Europe, using many kinds of tables. ALL of which clearly specify actual depth, not some sort of average depth.

We all know the drawbacks of using tables and max depth only.

But intuitively (no, I haven't done the math. Anyone who wants to is free to whip me with a wet noodle for that), using average depth can't be right. If we were in a steady state situation, then maybe. But we aren't. We're in a dynamic situation. We're not sat divers, we're bounce divers ongassing and offgassing. And both ongassing and offgassing depend on Fick's law, which again depends on Henry's law. Show me the math that proves it, and I'll concede that average depth is a good variable. Until then, color me sceptical.

 

[tursiops]

Avg depth works just fine for calculating your deco.

So you disagree with Navy, NAUI, PADI, DCIEM, BSAC, and the Buhlman deco tables, all of which use max depth, not average depth?

 

[tursiops]

UTD uses average depth to calculate the entire decompression profile. Everything is based on average depth and mixed gases.

Can you show one of the tables?

 

[Storker]

Can you show one of the tables?

Tables are an extremely simplified representation of reality. All tables I've seen assume a square profile, which, IME, is pretty far from reality (unless you go on a boat dive on a wreck or a flat reef, which I personally have almost never done. In that case, when depth changes are nearly insignificant, average depth might be a relevant parameter).

On a more or less triangular profile dive (which basically describes most of my dives), average depth doesn't mean crap except for SAC/RMV. Bühlmann works with a certain number of theoretical compartments, each with it's own half-time. And whether a compartment is on- or offgassing depends on Fick's law. While a triangular profile dive may be roughly approximated by a two- or three-level theoretical dive using this approach to multilevel dives, the best approximation is made by computer algorithms considering both on- and offgassing while tracking the diver's depth profile. I'd like to posit that that math is too complicated for mental calcs. Particularly under water.

 

[CAPTAIN SINBAD]

Can you show one of the tables?

You mean UTD Average depth table? I have posted those a few times in a different thread which I will try to pull but I can reproduce those from memory.

Standard Gas: Air
Depth 0 - 60 feet

10 - 100 mins
20 - 90 mins
30 - 80 mins
40 - 70 mins
50 - 60 mins
60 - 50 mins

Standard Gas for dives below 60: Nitrox 32
Depth 0 - 100

60 - 60 mins
70 - 45 mins
80 - 40 mins
90 - 35 mins
100 - 30 mins

These depths are averages instead of max depth for square profiles and these limits are meant to be followed by UTD ascent schedule. This demands that you make a 1 minute stops every 10 feet starting from 50 feet of your average depth so a dive to 100 for 30 mins on nitrox would generate 5, one minute stops @ 50, 40, 30, 20, 10. On Buhlmann this should get you out at 35/85 ish?

MODS: Can we please split this into a different thread on "Using average depth for deco calculations?" This way we can keep this thread focused on peoples gradient factors and channel responses and skepticisms of average depth in their appropriate category? Thanks.