Info Why are tables not taught in OW classes anymore?

[inquis]

When you get them, you can take your kid to a day of skiing at Vail resorts in Colorado. One day adult tickets go for about $160,

Ha, they broke the $200 mark a couple years ago. Currently $245 single day online (more at the window). 6 days is $1410. So yeah, your point is even more compelling these days.

 

[steinbil]

You've jumped from GUEs averaging the bottom depth (you are diving over a bottom with bumps between (say) 80 and 90 ft, so you call it 85 ft*) to depth averaging the entire dive, for example 90 ft is the max depth of the dive, and you do a kind of linear multi-level dive, which you now call 45 ft). THAT is quite wrong, will give the wrong assumed N2 uptake for the dive, and has been shown time and again that it is incorrect. The reason it is wrong is because depth averaging is a linear process, but the on-gassing of N2 is not. You on-gas more at depth than you would think from just simple extrapolations from shallower. You can convince yourself of this quickly with any table: look at the NDL for some depth; now look at the NDL for twice that depth; do you get half the NDL? No, not by a long shot.

I can't give you an example of a "multilevel dive that would be "allowed" by depth averaging the tables but that would overstep the limits of (i.e. lead to deco in) Buhlmann with a GF hi of 85, for example?" because there is no way to validly "depth-average" the tables. It is just flailing in the dark.

*By the way, you'd have to call this 90 ft on tables anyway, so what's the point?

You're misrepresenting what GUE teaches about depth averaging when using tables. I'm going to assume you don't know, and that it's not just a straw man, so I'll explain.

First, your example is wrong: GUE has never suggested averaging the entire dive. To use your example: If you have a continuous ascent profile from 90ft to the surface (as if you're swimming along a sloping bottom), you're supposed to average the max depth and half the max depth (which is where GUE min deco ascent would start on a square profile). In this case the NDL limit on 32% nitrox according to the GUE tables would be 60 minutes (depth average 67.5ft, using 70ft table), rather than 40 minutes (at 90ft).

The other profile GUE uses for depth averaging is a multilevel bottom phase with a vertical ascent:
Let's say your visiting a wreck lying on a slope between 100ft and 70ft. In this case, you could plan to spend some time on the deep part of the wreck at 100ft, and then spend the rest of the bottom time on the shallower part, or swim gradually from the deep part to the shallower part. You could then use the average depth of the bottom phase for using the tables. If you plan to spend equal time at the deep part and the shallow part, you would use 85ft as the average, so it would give you 40 minutes (90ft table) instead of 30 minutes.

As long as it's not a sawtooth profile, or a progressively deeper profile, this works quite well, and can be done on the fly if you can reset the average depth on your bottom timer, or estimate your average depth (obviously more risky and task loading).

I'm not trying to say that this is as flexible or efficient as using a computer for multilevel dives (which most GUE divers, including me, are doing). But as long as these profiles are safe and within the limits of a computer dive, it's misleading to say that it's flailing in the dark. This is why I'm genuinely interested to see if there are examples of dives planned in this way that would overstep the limits of a typical dive computer (Buhlmann), to learn from it.

 

[dmaziuk]

This is why I'm genuinely interested to see if there are examples of dives planned in this way that would overstep the limits of a typical dive computer (Buhlmann), to learn from it.

It's likely that if you overstay the NDL in the deep part of the dive a little, the ceiling will clear on ascent. It's easy enough to see in Subsurface by lowering GF High.

 

[steinbil]

It's likely that if you overstay the NDL in the deep part of the dive a little, the ceiling will clear on ascent. It's easy enough to see in Subsurface by lowering GF High.

I'm not familiar with the Subsurface interface. Does the green area indicate supersaturation/deco obligation? After just 12 minutes at around 30m? On air, I presume? Is GFhi75 really that conservative?

 

[dmaziuk]

I'm not familiar with the Subsurface interface. Does the green area indicate supersaturation/deco obligation? After just 12 minutes at around 30m? On air, I presume? Is GFhi75 really that conservative?

Yes to all, but that wasn't the first dive of the day.

(That's one of the tricky parts of writing an actual dive computer code for no-stop diving: when you get a ceiling, you don't want the user to see the scary zero on the NDL display if it will actually clear on ascent. But then you're assuming they won't blow their safe ascent rate, and if they do: it may not. )

 

[steinbil]

Yes to all, but that wasn't the first dive of the day.

(That's one of the tricky parts of writing an actual dive computer code for no-stop diving: when you get a ceiling, you don't want the user to see the scary zero on the NDL display if it will actually clear on ascent. But then you're assuming they won't blow their safe ascent rate, and if they do: it may not. )

I would need the profiles of the other dives that same day to see if they could be planned with tables if we're going to do any comparison.

It's not so tricky if you follow recreational guidelines of never entering a ceiling. The point of NDLs is to have direct access to the surface at any point in the dive. Your example would not be acceptable for a recreational dive as long as you base it on GFhi of 75. If it turns out that I could plan this same dive within NDLs on the table using averages, it would be a good example of a potential problem with that practice.

 

[scubadada]

Hi @dmaziuk

I assume your computer would give you a short shallow stop after you exceeded NDL. This a a profile from your computer fit to a conservative Buhlmann setting in Subsurface, correct? In the example below, my Teric gave me a minute at 10 ft for almost 5 min of dive time before it cleared. My Oceanic VT3 works the same way.

 

[dmaziuk]

If it turns out that I could plan this same dive within NDLs on the table using averages, it would be a good example of a potential problem with that practice.

It's only a problem if you do a CESA from the point where you have a ceiling. Which comes with a risk of bending the fast tissues if you go too fast, or blowing a lung if close your airway, so... meh. Don't run OOG and it won't be a problem.

 

[dmaziuk]

Hi @dmaziuk

I assume your computer gave you a short shallow stop after you exceeded NDL. Or was this a profile from your computer fit to a conservative Buhlmann setting? In the example below, my Teric gave me a minute at 10 ft for almost 5 min of dive time before it cleared. My Oceanic VT3 works the same way.

View attachment 772122

The latter: the profile's from a Cressi Leo that was not in deco at any point. The ceiling in the picture is calculated by Subsurface by lowering the GF High.

It wouldn't be any different at GH High 95 and more time at 30 msw: my point was that you can get transient ceilings that will clear on ascent, and planning for "average" rather than max depth is likely exploiting that.

 

[steinbil]

It's only a problem if you do a CESA from the point where you have a ceiling. Which comes with a risk of bending the fast tissues if you go too fast, or blowing a lung if close your airway, so... meh. Don't run OOG and it won't be a problem.

The point I was trying to make is that the recreational limits usually restricts divers from entering any type of overhead. I'm not advocating CESAs, but if you are unable to do a CESA to the surface it's a technical dive by definition (at least by most agency standards).