Gradient Factors For a Rookie

[inquis]

By way of explanation, I've augmented a graphic I found at Simply Scuba:

Bombing down to, say, 100 ft, you stay for 1 minute, which takes you to Point 1 with regard to tissue loading. You then ascend (trajectory shown in red), until you reach the point where that tissue loading (having slightly increased during the ascent) reaches ambient pressure. That anchors the right endpoint of the GF(1) line (shown in blue here using GF 30/90). You continue ascent WITHOUT crossing the GF(1) line -- therefore no stops required -- arriving at the surface with a tissue loading around 40% (eyeballing based on the left endpoint of the GF line at 90%). This is a dive well within no-stop limits.

If you stay until Point 2, the direct ascent (red arrows) would fall just shy of the blue GF(2) line, and the point of closest approach is at surface pressure. This is a dive just shy of the no-stop limit.

If you stay until Point 3, ascending (moving left in the graphic, rust colored arrow) hits the blue GF(3) line, indicating a stop must be made. Tissue tension reduces while you wait (trajectory moves down in the graphic). Thus, this is a decompression dive.

Hopefully it's clear that it's impossible to cross any GF line unless tissue loading is higher than GFHigh (left endpoint of all of the GF lines). If your trajectory surfaces below GFHigh (as from Point 1), GFLow values from 1 to 100 cannot reposition the line so that it is crossed. That's why GFLow has no bearing on whether stops are required. It is only AFTER it's been established that the line is crossed somewhere -- i.e., tissue loading is sufficiently above GFHigh -- that you must look to GFLow to see exactly where.

(Of course, each tissue compartment is playing this game. If none cross, then no stops are made. Again, the NDL limit occurs when the worst-case or "controlling" compartment just clips the GF Line at surface pressure which is exactly GFHigh.)

 

[dmaziuk]

Which bits of Baker’s writing or code supports this?

He has one paper on NDL that I know of, I'll have to look if there's any code there.

As you well know neither ZH-L nor Baker's mod were ever aimed at no-stop diving, so arguing about GF Lo for NDL is like arguing whether Earth disk is laying on the backs of three elephants or four. GF Lo applies when you're on a yes-stop dive, not if, is what deity intended.

 

[EFX]

Thanks @inquisit for posting the graph above. I'd like to point out something that is not readily apparent on the graph. For a deco dive involving decompression stops the GF (gradient factor) at the stops changes incrementally from GFLo to GFHi. At the first stop it is GFLo. At the last stop, at the surface, it is GFHi. Looking at the GF 30-90 (3) line on the graph and starting at point 3, the diver makes 3 deco stops before arriving at the surface. So, the GF will change in (90-30) / 3 increments or 24. The GF at the first stop is 30, the second is 54, the third is 78, and the surface is 90 (with the GF increment rounded off).

Another thing I want to point out is why does the m-value line increase in value from a shallow depth to a deeper maximum depth? The m-value line is called the super saturation limit line in the graph. Why doesn't it remain constant or decrease from a high value at shallow depth to a lower value at a deeper depth? The reason, I believe, is at a deeper depth there is a longer water column in which to off-gas and therefore the tissues can start with more inert gas at the beginning of the ascent. At the shallower depths there is not enough depth or time to sufficiently off-gas to a safe level before reaching the surface.

 

[dmaziuk]

Which bits of Baker’s writing or code supports this?

In Calculating the no-stop time he derives the NDL formula from Schreiner's equation -- so it's not the same thing -- but does say that "in a typical sports diving application" it only applies when ambient pressure is greater than M0 which in turn is greater than gas loading. It doesn't directly support the algorithm (as in sequence of steps), but it does support the "gas loading < M0" logic behind it.

(I'm not exactly sure what his "case B" means, I think it would apply to a fast TC "riding the line" during ascent.)

 

[dmaziuk]

Isn’t the idea that we need the gf low to stop deeper to prevent an adverse event? There is of course a trade off, but otherwise, we’d just rock the same lo and hi values.

Yes, but first you have to have a stop. When you plan a decompression dive, that's exactly how it works.

But OP said he has "no interest in decompression diving". GF Lo in NDL diving is a square peg in a round hole, you can do all sorts of fun number games with it, but it really doesn't mean squat. Until you overstay your NDL.

 

[Gandalf-the-Diver]

Hi,

I am a recreational diver with close to 300 dives. I recently purchased a Shearwater Peregrine mainly for the reasons listed here.

Now that I have it I am wondering how I should choose which GF to use. The Peregrine comes with three conservatism presets: Low (45/95), Med (40/85), and High (35/75) for GF Hi and Low, as well as custom ones. I have never gone into deco before, based on my previous computer, which ran a "Modified Haldanean Algorithm" with 12 compartments based on DSAT. I am in petty good shape for my age, but I am in my mid-60s (a Beatles song from Sergeant Pepper hits the nail on the head)

Also I have become quite interested in decompression theory lately and am currently reading Powell's Deco for Divers and have read Erik Baker's 2 articles about M-Values and Deep Stops. While I conceptually understand most of it, I still feel like I'm drinking through a fire hose while it slowly sinks into my slow tissue compartments.

Of relevance also is I really have no interest in decompression diving; I am predominately a vacation diver that may do a land based trip and a liveaboard once a year or so. I'm AOW and NX certified.

So...back to my question. @rongoodman states in post #4 of this thread:



Is that pretty much it? Or should I do something else? I guess based on how I am feeling before/after a particular dive I could always adjust it up or down?

Thanks. After you have indulged me here I may hit you up for SurfGF and GF99 questions.

I too, have a Peregrine as my primary computer. I have been diving with it since Sept, 2020, when I got my OW. For a large part of my diving, my air consumption ruled how long my dives were. I left my Peregrine on Med, and just went diving.
Then my SAC rate got better, and soon, NDL was telling me when I had to come up. The 1st time I had my Peregrine flash red lettering, Deco stop, I went $#!*, and started up without hesitation. (I did not panic as I was slinging a 30cu pony, so I was not concerned about OOA issues) Around 40', the Deco obligation went away, and I had to do a 5 min safety stop. Interesting I thought, the computer adapts to what is going on. ( I now have a vibration alert set at 3min to NDL ) For quite a few dives, I would play with my NDL to the point my ascents, started to all stay at 1-2 min to NDL, a couple of times, letting it go into Deco. Climbing up when it does, I find the Peregrine, comes out of Deco obligation, and back to a 5 min SS. I should point out, that when I do get the Deco notice, I am not hanging around at depth, but rather making my way up. I reset the GF from Med, to Low, which has been giving me more time, down lower. The NDL times I now find, when I have been swimming near NDL, that with slight raises in depth, NDL will go from 1min NDL, to 4-5min on a very small rise.
I had around 190 dives on my Peregrine, before I reset the GF to Low. As far as the custom gradient factors that can be used; I do not have the training to even consider playing with anything more than: low, med, high. Having said that, AN/DP is one course I hope to do in the future, currently PADI AOW and SDI Solo, 265 dives.

 

[dflaher]

Around 40', the Deco obligation went away, and I had to do a 5 min safety stop. Interesting I thought, the computer adapts to what is going on.

Thanks for sharing your experiences. I believe that the Peregrine's safety stop can be set to 3 min, 5 min, or Adaptive. When it's set to Adaptive it determines your SS based on profile. Of course this is from my reading the manual, as I have not dived it yet. Good idea to enable the NDL warning.

 

[Gandalf-the-Diver]

Thanks for sharing your experiences. I believe that the Peregrine's safety stop can be set to 3 min, 5 min, or Adaptive. When it's set to Adaptive it determines your SS based on profile. Of course this is from my reading the manual, as I have not dived it yet. Good idea to enable the NDL warning.

I set mine to adaptive from the word go.

 

[inquis]

So, the GF will change in (90-30) / 3 increments or 24. The GF at the first stop is 30, the second is 54, the third is 78, and the surface is 90 (with the GF increment rounded off).

On this point specifically, I suspect the deco stops depicted in that graph were notional. Practically speaking, the various implementations wait/offgas until the next shallower multiple of 10 ft or 3 meters is on the "good" side of the GF line. The change in %loading would depend on the slope of the GF line (i.e., the GF values themselves), but the pressure reductions are consistent at 0.3 atm per stop.

 

[inquis]

why does the m-value line increase in value from a shallow depth to a deeper maximum depth?

My understanding is that empirically, Haldane found the limit was a multiplicative factor of ambient pressure. I think originally about 2x, later backed off a bit, but still some factor of ambient. With ambient pressure on the horizontal axis, the M-Value line would always increase (for positive multiplicative factor).