On a NDL dive, which computers' NDLs are not affected by GFLo?

[dmaziuk]

I maintain that the first stop depth is determined by GFlow but of course as that approaches the surface the gradient factor slope diverges. So, admittedly, I had to play around a bit (including cranking up the ascent rate to 20m/min for the last bit). But I found a case that is influenced by GFlow: Do an air dive to 20m. Then with GFlow=30%, you have a maximal runtime of 49minutes while with GFlow=100% it becomes 50min. But I admit that's a small difference (I kept GFhigh=100%). Of course, the influence of GFhigh is much stronger since it is the part where the deco ceiling just goes away as you approach the surface and thus it is the near surface part of the ceiling.

Sooo... if you ran something ridiculous like 5/100 and made up a profile that gives you a deco stop at, say, 3 metres... Is it even possible to arrive at 3 m at 5% of base M-value and then come up to the surface at 100% of base M-value? Without spending negative time at deco stop?

Edit: or even at 9 m, for that matter (at 3 m you're so far above NLD you probably couldn't on-gas even with time running backwards)

Enquiring minds would like to know.

 

[stuartv]

The application of GFLo has been discussed a lot before. I still do not understand why anyone even thinks it SHOULD have relevance to your NDL.

GFHi means (in simple terms) "the percentage of the M-value I do not want to exceed when I arrive at the surface". At least, that is my understanding.

It seems to me to follow that NDL would be "the time remaining at my current depth, until I must proceed directly to the surface (at some preconfigured ascent rate) in order to avoid exceeding GFHi by staying longer and then doing a direct ascent." If the computer is programmed for an expected ascent rate of 30 ft/min and I leave the bottom exactly when NDL gets to 0, and ascend at exactly 30 ft/min directly to the surface, I should arrive there with my leading tissue compartment at precisely the percentage of its M-value that GFHi says.

Example: Suppose I use GF50/80. If I proceed to the surface when NDL==0 and arrive there with my leading tissue compartment at 80% of the M-value, then what difference does (or should) the GFLo value make? I don't have any reason to make any stops, so what role does GFLo play in an NDL dive?

My understanding of GFLo is that it is used to determine the first stop depth once it is known that there will be at least one mandatory stop. If I have to stop at 10' in order to avoid surfacing with a GFtissue of 81%, then I have a mandatory stop. In that case, my FIRST stop would be determined by calculating what depth I have to stop at in order to avoid exceeding 50% of the M-value of my leading compartment on the way up.

Once it is known that you have at least 1 mandatory stop, then you know that the last stop is at 10' (or whatever setting your computer is set for, for "Last Stop"). GFLo is used to determine the first stop (worked out in increments of 10', when diving Imperial units). From there, the difference between GFLo and GFHi is pro-rated, if you will, every 10' from the first stop to the last stop.

Example: You exceed your NDL (with GF50/80). You stay long enough at depth that your computer determines that your first stop must be at 30'. That means that you'll arrive at 30' with a GFtissue of 50% or something just slightly less. I.e. your GF for your 30' stop is 50%. From there, the rest is interpolated or pro-rated or whatever you want to call it. At 20', your GF will be 60%. At 10', it will be 70%. And, of course, your GF for surfacing is 80%.

So, you stop at 30' and stay until your computer determines that you can ascend to 20' without exceeding 60%. You ascend and arrive at 20' with a GFtissue of 60% of less. Your computer holds you there until it calculates that you can ascend to 10' without exceeding a GFtissue of 70%. You hang out, then ascend to 10'. Your computer holds you there until it calculates that you can ascend to the surface without exceeding 80%. When your computer says "deco clear", you can ascend immediately, at 30 ft/min, and arrive at the surface with a GF in your leading tissue compartment of 80%.

If you can ascend directly to the surface and arrive there without exceeding 80% of the M-value in any compartment, why would it matter whether your GFLo is 10 or 80? Why would that affect your NDL?

I think Shearwater does their calculations the "right" way, which factors in ascent time and, instead of looking at your current loading, it predicts what your inert gas tissue loading WILL be at each depth along the way. I think other planners and computers do things differently. I suspect that many of them only compare current loading to the GF settings. I.e. both computers may agree that you have a ceiling. One computer will then tell you you are have exceeded your NDL. In contrast, a Shearwater might determine that your ceiling will be gone by the time you ascend to it, so it says you have not exceeded your NDL.

Anyway, sorry for being so long-winded on something that is somewhat OT.

 

[dmaziuk]

Because once you've exceeded your 80% of the M-value, gf low suddenly kicks in, and your gf jumps from gf high to whatever comes out of the proportion from Eric Baker's "figure 4". So now you have a deco stop at 3 metres at 40% of the M-value, but you're already over 80% of the M-value at 0.3 atm above that stop. Can that happen?

 

[stuartv]

Because once you've exceeded your 80% of the M-value, gf low suddenly kicks in, and your gf jumps from gf high to whatever comes out of the proportion from Eric Baker's "figure 4". So now you have a deco stop at 3 metres at 40% of the M-value, but you're already over 80% of the M-value at 0.3 atm above that stop. Can that happen?

Presuming you're responding to my post...

You don't exceed 80%. It is determined that on a direct ascent, you WOULD exceed 80% before you get to the surface.

Also, let's remember, when you are at depth (any depth, for any length of time), you are at 0%. Or, negative, I suppose, if you want to look at on-gassing that way. You can only achieve a positive GF by ascending from your deepest depth of the dive (and even then you might not immediately, if you weren't there long enough).

You only have a positive percentage of the M-value when there is a pressure gradient between your tissues and the partial pressure of your inspired gas (with the pressure in your tissues being higher than the other).

Anyway, I'm not really sure what "your gf jumps from..." means. The GF of your leading compartment doesn't ever do any jumping. As you ascend, it goes up gradually (presuming a normal ascent rate), starting from 0.

Assume you use GF10/100, and you get to the point where a direct ascent would result in surfacing at 101% of the M-value. Now you have a stop at 10'. At that point, the computer is then going to calculate what depth you have to stop at to avoid exceeding 10% of the M-value. I think it depends on the implementor to decide whether to factor ascent time into that specific calculation or not. Regardless, most likely, in this scenario the actual result will be that, as you ascend you will never quite catch up to the GFLo and intermediate GF values and, from the diver's perspective, they will perform a direct ascent to 10', with any possible deeper stops clearing before the diver even gets there.

 

[Jay]

I still do not understand why anyone even thinks it SHOULD have relevance to your NDL.

One reason why GFLo might affect NDL is that, like GFHi, they both work create an Adjusted M-Value line which is closer to the M-Limit line (slope if merely one GF is changed, a shift if both are changed in the same direction, a pivot if in opposite direction). That has the effect of decreasing your NDL. That naturally depends on the implementation.

The implementation question is an interesting one, but my simple question / reason for this OP is merely does it (GFLo) have an effect or not on NDL.

Let's find divers with those computers that can be easily checked using Custom GF and fixing GFHi, altering GFLow.

 

[rsingler]

Here is an example with MultiDeco using the @rsingler method with 1 second stop times, NDL being the longest time requiring only 1 second stop at 10 feet. An extreme example, comparing 100/100 to 5/100. My Dive Rite Nitek Q gives the same NDL at a given GFhi, regardless of the GFlo.
View attachment 499669View attachment 499670

Here is another example, using a 10 second stop time, with NDL being the longest time with no stop. Again, 100/100 compared to 5/100
View attachment 499671View attachment 499672

I just realized that Multi-Deco is NOT adding stops in my method of looking for an ersatz NDL dive. As your printout shows perfectly, all those "stops" that appeared beginning at 60' when we decreased GF Lo are really just standard ascents with a minimum 1sec stop at each level. It must be a function of the computation methodology, but these are not added deco stops from decreasing GF Lo. These are still NDL dives, and GF Hi is the driver.
I confirm what @tursiops found for Shearwater. But I can't explain what TDC-3 might be doing that makes GF Lo relevant. Can someone post a profile?

 

[atdotde]

I admit, in reality the influence of GFlow on NDL is at best marginal. But I thought it was worthwhile thinking about this and thus wrote a blog post about this: NDL and Gradient Factors – The Theoretical Diver (featuring my confusion).

 

[scubadada]

Hi @rsingler

In my second example in post 17, the run time of 5/100 is marginally longer than 100/100. Perhaps this is due to the 10 sec, rather than 1 sec stop time. I'll have to go back and look at it again. At the moment, I am getting ready to board a plane to Florida, for a week of diving The GFlo on my Nitek Q will not affect my NDL

Good diving, Craig

 

[rsingler]

Hi @rsingler

In my second example in post 17, the run time of 5/100 is marginally longer than 100/100. Perhaps this is due to the 10 sec, rather than 1 sec stop time. I'll have to go back and look at it again. At the moment, I am getting ready to board a plane to Florida, for a week of diving The GFlo on my Nitek Q will not affect my NDL

Good diving, Craig

Yes, I think you're right. The marginal increase in time was due to the minimum stop time you defined. I don't know what's triggering that, but I don't think it's a real effect of GF Lo.

Safe diving, Craig!
I'm jealous.

 

[dmaziuk]

Anyway, I'm not really sure what "your gf jumps from..." means. The GF of your leading compartment doesn't ever do any jumping. As you ascend, it goes up gradually (presuming a normal ascent rate), starting from 0.

There is no GF in your any compartment. There is current gas loading Pinsp at or below ambient pressure Pamb. As you ascend your ambient pressure goes down, causing the difference between Pamb and Pinsp to go up. Buhlmann's formula defines the maximum tolerable difference as "safe ascent ceiling".

With GFs, when you are on a no-stop dive, that ceiling is calculated by Buhlmann's formula times GF high. When there is a ceiling, it is calculated by Buhlmann's formula times GF low (at first deco stop). That's the "jump". When you hit 0 NDL, that's when the ceiling appears.