Your Gradient Factors?

[scubadada]

Do you have data to support your statement that average depth works well for NDL or deco planning?

 

[CAPTAIN SINBAD]

Do have data to support your statement that average depth works well for NDL or deco planning?

If we perceive decompression risk in the context of Buhlmann gradient factors then average depth, as implemented by GUE/UTD does not generate a gradient factor that would be considered INSANE in technical diving. The most aggressive profile I would be able to generate are the extreme limits of UTD Technical 1 which fall between Buhlmann GF90 - 95.

There are plenty of technical divers who are using computers to generate the same GF-Hi of 90-95 so are they safer because they create the same risk using computers? For most of Tech-1 diving, depth averages and ratios should generate 30/75 which is more conservative than a lot of Gradient Factors people on this thread have confessed that they are using.

 

[elmo]

Do have data to support your statement that average depth works well for NDL or deco planning?

@tursiops gave us one datum, which in my opinion demonstrates average depth as a useful tool for NDL planning. He was less satisfied with only 80 minutes of bottom time for an NDL dive.

This misses the point entirely. We are not talking about the averate bottom depth, but rather the average dive depth.
Simple example using air, calculated on a PADI eRDPML.
100 ft, NDL, 20 mins, but ascend at 15 mins to 40 ft; new NDL is 83 minutes, but stay only 80 minutes. Nice 95-minute dive, plus some ascent time and a SS.
The average depth of that dive is 50 ft ((100x15 + 40x80) /95). A single dive to 50 ft has an NDL of 80 minutes.
You cannot validly plan or execute that two-level dive using average depth.

Please, try it yourself and come up with more data.

Let's assume you have some method to plan a no-decompression multi-level dive. It could be the PADI eRDP-ML, Wheel, or another organization's equivalent. Or it could be recreational dive planner software (e.g. Subsurface in Recreational planner mode), a phone app, or it might be built into your dive computer. Could you please humour me for a moment?
A) Plan a non-decompression dive with a square profile (e.g. all the bottom time spent at 100 ft) - no trimix, nothing beyond recreational limits - to determine your NDL for a particular depth
B) Plan a multi-level dive with the same average depth that would be allowed by the PADI eRDP-ML (deep-to-shallow - as I'm sure you learnt at some point), with the same average depth (e.g. first half of bottom time at 120 ft, second half of bottom time at 80 ft)

Can you say which plan gave you a longer NDL? I am willing to bet it was plan B. So, it you were planning a deep-to-shallow multi-level dive but planned for the NDL based on average depth using tables intended for a square profile, you would be on the safe side. Try it again - can you come up with a deep-to-shallow profile where that isn't true?

When planning a deep-to-shallow profile dive, it is not unreasonable to use the average bottom depth to calculate the NDL from a dive table. There are difficulties (e.g. how good are you at tracking your average depth during a dive) and by no means do you need to use it if you don't want to, but it isn't bogus, and it isn't a cult philosophy.

 

[scubadada]

If we perceive decompression risk in the context of Buhlmann gradient factors then average depth, as implemented by GUE/UTD does not generate a gradient factor that would be considered INSANE in technical diving. The most aggressive profile I would be able to generate are the extreme limits of UTD Technical 1 which fall between Buhlmann GF90 - 95.

There are plenty of technical divers who are using computers to generate the same GF-Hi of 90-95 so are they safer because they create the same risk using computers? For most of Tech-1 diving, depth averages and ratios should generate 30/75 which is more conservative than a lot of Gradient Factors people on this thread have confessed that they are using.

@tursiops gave us one datum, which in my opinion demonstrates average depth as a useful tool for NDL planning. He was less satisfied with only 80 minutes of bottom time for an NDL dive.

Please, try it yourself and come up with more data.

Sorry, so why don't you just use computers and the generated dive time. I don't understand the alternative calculation.

 

[CAPTAIN SINBAD]

Sorry, so why don't you just use computers and the generated dive time. I don't understand the alternative calculation.

I use computers all the time specially if I do no have standard gases. I have a Perdix and a Hollis TX-1. When I do have standard gases then the profile that a computer will generate is predictable. Do we need GPS when we drive to the same grocery store? No. Does it hurt if we have a GPS turned on when we drive the same grocery store? No.

 

[elmo]

Sorry, so why don't you just use computers and the generated dive time. I don't understand the alternative calculation.

No reason not to use dive computers. I use them, and I think they're great.

But, dive computers are reactive - they can't tell you about your dive before you've done it. For planning a dive, or adjusting a plan, they don't help much on their own. Most have just a square-profile built-in planner. Considering your average depth (or anticipated average depth when planning) can be a very useful tool.

 

[100days-a-year]

So I looked at an EDGE and it is 39 and 54 minutes for 70' and 60' respectively so 99 Buhlman for NDL. Nice to know, and a bit more conservative than some initially. Of course as you exceed NDL there's no guessing it's algorithm until it get dove parallel with a computer with an adjustable GF.

 

[Ayisha]

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.

Remember that a percentage of average depth is used, depending on whether it's for a MD/Deco dive.

Since that percentage determines the first stop and the ascent profile (shape of the dive), one could think of it as a method that can de-emphasize deep stops. The first stop is shallower, which back in the day would have been considered more aggressive. In the age of de-emphasizing deep stops, the ascent profile is purposely less conservative (first stop not as deep) and the first stop is more of a mid-stop.

Some of the participants are fixated on how it relates to bottom time, but the purpose of using average depth is also to shape the ascent profile.

 

[Ayisha]

@Storker I think the point is, once you've decided to discard the tables or use them incorrectly...

bold added

That's likely why no one has posted all the tables. If you're trained to use them, you have less likelihood of using them incorrectly. No one likely wants to be responsible for someone using something that they weren't trained for, and none of us, to my knowledge, owns the copyright to reproduce images.

People ask all the time for GUE/DIR formulas, tables or procedures under the guise of checking the math/safety.

If someone interested takes one of a few types of GUE one-year memberships, you can download course materials, class presentations, resources, etc.
I downloaded and read through all kinds of GUE courses through my annual membership that I will likely never take, just because I like to learn. Nothing is stopping anyone, except perhaps some money and time.

 

[tursiops]

bold added

That's likely why no one has posted all the tables. If you're trained to use them, you have less likelihood of using them incorrectly. No one likely wants to be responsible for someone using something that they weren't trained for, and none of us, to my knowledge, owns the copyright to reproduce images.

People ask all the time for GUE/DIR formulas, tables or procedures under the guise of checking the math/safety.

If someone interested takes one of a few types of GUE one-year memberships, you can download course materials, class presentations, resources, etc.
I downloaded and read through all kinds of GUE courses through my annual membership that I will likely never take, just because I like to learn. Nothing is stopping anyone, except perhaps some money and time.

OK, let me try it this way.

Of the openly available published dive tables, NONE use average depth of the dive in any way. There may be some who think they have discovered some cool way to use average dive depth, but they have no mathematical justification for it and no independent verification that it works. Same with thos that think they can do multilevel dives with a single-level dive table.