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

[tursiops]

Is that the kind of people who do two such dives and then a third to lift the anchor?

Only once.

 

[scubadada]

Is that the kind of people who do two such dives and then a third to lift the anchor?

Only once.

Sad comments

 

[LandonL]

I created this thread to simply answer the question of if NDL is affected (on a NDL dive) by GFLo

Answer to your question below.

GFHi is the sole relevant GF input to determine NDL.

For all practical purposes... GFhigh sets the NDL, the GFlow sets the depth of the first stop. NDL is simply what percentage of the M-value you are willing to surface with, ie: the GFhigh.

Changing the GFlow will affect overall runtime, as more time at depth slowing the ascent will affect tissue saturation in various compartments, therefore you end up with a longer total run time, but NDL is set by the GFhigh.

GFlow comes into play once a dive passes the NDL.

@stuartv has given some great responses here to explain.

There is a very small window where the GFlow can affect an NDL, but its extremely marginal and really academic (ie: it makes about a 1 minute difference in the NDL due to the shift between the two values very near the surface).

See the article here, also linked earlier in this thread for a very good explanation.

There are also some good notes in the comments.

NDL and Gradient Factors – The Theoretical Diver


Cheers,
Landon

 

[huwporter]

I think it's becoming clear that GF Hi is responsible for NDL limits, but I just wanted to point out that GF Lo plays a part in overall dive mechanics. I have seen people refer to GF numbers like 95/95 and was wondering if GF pairs like these have any place outside of a theoretical discussion of NDLs.

Buhlmann himself recommended using 95% of his M-value limits in actual practice. This would be exactly analogous to "GF 95/95", i.e. "calculate all stops (including arrival at the surface) to not exceed 95% of the calculated maximum permissible supersaturation at any point."

using GF pairs with GF-Lo and GF-Hi different - e.g. 20/80 means "calculate the first stop to not exceed 20% of permissible supersaturation (M-value) at that point, and final arrival at surface to not exceed 80% of M-value at that point, and apportion any additional stops needed between those two points by equal sized steps between 20% and 80% depending on number of stops required.

Example, lets say we are diving 20/80, and we do a dive where the first stop is calculated to be at 18 metres. That means we have total seven stops (including the surface, and 3m), and the M-value

Stop depth - must not exceed % of M-value:
18m - 20%
15m - 30%
12m - 40%
9m - 50%
6m - 60%
3m - 70%
Surface - 80%

...That's it. The size of the % steps is dependent on the GFs in use, and the number of stops between the first stop and the surface. (This example was chosen to neatly give 7 steps between 20% and 80%, or 10% each step, for clarity.)

There is no mathematical reason why GFs need to be less than 100, or why GF-"Lo" needs to be lower than GF-"Hi". e.g. My Journey into UTD Ratio Deco. There is also no mathematical reason why the gradient factor steps need to be equal sizes, but that is how GFs are usually (effectively always?) programmed.

 

[NAM001]

I have tried to find a calculator of sorts to test the hi low question and have not found anything that will give me a ndl based on a gf lo hi input.

 

[NAM001]

I see how some are saying the gf lo can effect ndl but I dont think that anyone is considering that you are doing a controlled ascent at a determined rate in which the saturation level at the deep is no longer there because of the de gassing going on through the ascent. someone correct me..... I hit ndl and am 100% saturated I go up to 40 ft in a minute and say with a tissue with a 2 min factor you are no longer 100% saturated you are now perhaps 70%saturated with that decreasing saturation rate you will never exceed a gf lo of 50% to trigger a first stop at say 50 ft. and of course form 60 up you are doing 30 ft/sec so in the next minute you get to 30 ft and the saturation is what 45%saturated. Now compare that to a conservative setting of medium and that drops even more. I think the question of the gf lo triggering a stop even if gf hi is controlling the NDL assumes instant depth change from deep to 30 or 40 feet with out any off gassing time on the way. Perhaps it could happen, its hypothetical. The only answer I can envision is that until NDL reaches 0 gf lo is ignored and gf hi in the only one in play. If the compuiter used gf hi for both hi and low untill ndl is exceeded and then the gf lo is replaced bby the gf lo setting then the deco routine and direct to surface would be done the same. a 30/85 would be terated as 85/85 until ndl is exceeded then it turns to 30/85 to generate the deep stop. That process would prevent any possible chance that gf lo setting would trigger a stop. The high gf lo setting would insure direct non stop ascent to the surface. I would be interested in what would happen if it could happen that you got higher delta presure than the gf hi setting. Or is that ignored unless the depth is 0? I will guess the possibility that the calculations may be done with questionable results using say a 30/85 in these weird conditions but that software ignores the generated stop if it saw NDL not exceeded. I dont think that is the case because very high ascent rates would trigger a stop adn I have not heard of that from anyone. My thinking is that the gf hi means nothing but is a necessary component of having an interprelatable gf line with 2 ends to it. That the gf line means nothing unless NDL is exceeded.

 

[Jay]

My thinking is that the gf hi means nothing but is a necessary component of having an interprelatable gf line with 2 ends to it. That the gf line means nothing unless NDL is exceeded.

In the case of NDL diving, I think GFHi is an end of dive limit rather than acting one end of the adjusted m-value line. During the dive you're just compared to where you would be if you surfaced now (regular ascent). More or less like DSAT; as I've come to learn there is no DSAT m-line.

So now we have SW, Deep6, Dive Rite all agreeing that for all intents and purposes it's GFHi that affects NDL on a NDL dive, and I suspect once we get info on the other computers we'll be able to say that with some certainty

 

[stuartv]

I see how some are saying the gf lo can effect ndl but I dont think that anyone is considering that you are doing a controlled ascent at a determined rate in which the saturation level at the deep is no longer there because of the de gassing going on through the ascent. someone correct me..... I hit ndl and am 100% saturated I go up to 40 ft in a minute and say with a tissue with a 2 min factor you are no longer 100% saturated you are now perhaps 70%saturated with that decreasing saturation rate you will never exceed a gf lo of 50% to trigger a first stop at say 50 ft. and of course form 60 up you are doing 30 ft/sec so in the next minute you get to 30 ft and the saturation is what 45%saturated.

I think you have a small misconception here. Let's say we're talking about a 5-minute compartment.

After 6 half-lives in any compartment, that compartment is >98% saturated. So, in a 5-minute compartment, after 30 minutes that compartment is essentially saturated (presuming you have stayed at a constant depth the whole time).

If, at that point, you were to descend, your compartment would no longer be saturated, but it would equalize to ambient fairly quickly and you'd be back to saturated.

So, imagine that you are now at some depth and have been there for >30 minutes. That 5-minute compartment is saturated. You begin to ascend. No matter what speed you ascend, you are now supersaturated. Which means you are off-gassing. If you ascent to the surface without stopping (no matter how quickly or slowly), you will be supersaturated the whole way. Meaning, you will be at 100% saturation the whole way.

What the Buhlmann (ZHL-16B/C w/GF) algorithm says is that there is a supersaturation ratio during your ascent.

Let's say you were breathing air and you were at 100' for 30 minutes. 100' is 4 ATA. For ease of math, let's pretend air is 80% Nitrogen (instead of ~79%). After 30 minutes at 100', your 5-minute compartment is saturated, so your tissue tension of Nitrogen is 80% (the fraction of N2 in your inspired gas) of 4 ATA, or 3.2 ATA (approximately - this is a little bit simplified).

If you ascended instantaneously to 33', where the ambient pressure would be 2 ATA, you would have tissue tension of 3.2 ATA of Nitrogen and the inspired partial pressure of Nitrogen (i.e. the ppN2 in the gas you're breathing) would be 80% of 2, or 1.6 ATA.

That ratio of 3.2 : 1.6 is the over-pressurization gradient (in your 5-minute compartment). So, at that moment that you arrive at 33', that compartment has an over-pressurization gradient of 2.0. The Buhlmann algorithm is accompanied by a set of M-values. Those M-values are the Maximum over-pressurization gradient for each compartment. I don't know what the M-value is for a 5-minute compartment, so let's just say it is 4.0. Then, that would mean that, after your instantaneous ascent from 100' to 33', your OPG is 2.0 and the M-value is 4.0. So, your current gradient factor for that tissue compartment would be 50% - i.e. GF50. In other words, at that moment, your 5-minute compartment is 50% of the way to its M-value.

If you ascended at 30' per minute, instead of instantaneously, yes you would off-gas some from that compartment, so when you arrived at 33', your tissue tension of N2 would no longer be 3.2. The math to calculate what it would actually be is what is called the Schreiner Equation, which is part of any implementation of the Buhlmann algorithm.

Regardless, where your post was off is that as you ascend, your saturation level does not drop, ever.* That saturation level only decreases when you are descending (and even then, it only decreases if you descend fast enough to overcome the rate of on-gassing that is trying to keep you saturated).

When you are breathing air during the dive, that means you started with a body that was saturated on the surface. So, it has 0.8 ATA of Nitrogen in your tissues. As you breathe air and descend, the ambient pressure increases and the pressure of Nitrogen in the air you're breathing increases correspondingly. If you drop to 33', your body has 0.8 ATA of N2 and the air you're breathing has 1.6 ATA of N2. So, you are not saturated and you are on-gassing. If you stay there for 30 minutes, that compartment will be saturated. I.e. your tissues will have 1.6 ATA of N2 - the same as the air you're breathing. As soon as you start to ascend after that, you will go from being saturated to being supersaturated. Your body will have 1.6 ATA of N2 and the air you're breathing will be less than 1.6 ATA of N2. It will drop from 1.6 to 0.8 when you get back to the surface. Your body will gradually follow suit until it changes from supersaturation back to saturated with 0.8 ATA of N2.


* Note: That statement applies to if you are breathing air during your dive. Your level of saturation is always relative to the inspired gas (i.e. the gas you're breathing). So, any time you change breathing gases, your saturation level instantly changes up or down, depending on what you were breathing, for how long, your dive profile up to that point, and what you changed to.

 

[dmaziuk]

There is also no mathematical reason why the steps need to be equal sizes, but that is how GFs are usually (effectively always?) programmed.

It seems the reason they are of equal 3 msw sizes is the combination of "big enough", "not too big" and "we've always done it this way".

There's a physical reason the stop must be at some X bar above ambient pressure: to get the off-gassing going effectively. Obviously, it can't be so large it takes you over the M-value. E.g. Buhlmann says (on p.41 of Decompression 1984 English edition) :

For the sport diver it is difficult to carry out decompression at steps less than 2-3 m in the water, corresponding to differences of pressure of 0.2-0.3 bar. The internationally used interval of 3 m between steps has proved reliable.

 

[dmaziuk]

That the gf line means nothing unless NDL is exceeded.

Not only this is correct, I don't believe "NDL", "no stop", or "no decompression" is mentioned even once in the entire Erik Baker's "Clearing up the confusion about 'Deep Stops'", where he proposed and defined the GF system.

No-stop diving was not a part of the problem statement in the first place.