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

[KenGordon]

Does it need to be either or, why not both?

Clearly if it protects from a fast ascent it should protect for a normal ascent. But if it protects for just a normal ascent and if fast ascent will bend a diver due to exceeding the limits at the surface then divers likely to suffer from fast ascents are not so well protected.

Obviously it is all points on a probability curve, but personally I’d want to define it as allowing a direct (fast) ascent to the surface. Going to the surface should never be required, but the point of no stop diving is to allow for mistakes.

 

[atdotde]

Obviously it is all points on a probability curve, but personally I’d want to define it as allowing a direct (fast) ascent to the surface. Going to the surface should never be required, but the point of no stop diving is to allow for mistakes.

I believe that popping like a cork is never a good option. If you want to protect yourself for that case your dives will either be very short or very shallow. 10m/min is both reasonable and can look painfully slow.

 

[huwporter]

I believe that popping like a cork is never a good option. If you want to protect yourself for that case your dives will either be very short or very shallow. 10m/min is both reasonable and can look painfully slow.

Have you not seen all those tv programmes and adverts featuring "divers" who swim straight up towards the surface full speed at the end of their dive? That is how we do it, you know!

 

[Jay]

Updating the list with Mares Genius being the latest addition. I'd like to get the SP HUD info, but I think it's pretty clear that GFLo basically does not affect NDL in a NDL dive.

GFLo basically* does not affect NDL on a NDL dive (source)
Shearwater
Dive Rite's discontinued Nitek - Q (post#2)
Deep 6 (post#93 and via PM)
Garmin Descent (from a Descent diver via PM)
TDC-3 (email and post#69)
Seabear H3 (the link in post#126)
Heinrichs Weikamp (post #4 of StuartV's link in post#126)
Dive Soft (post#122)
AV1 (post#123) <--?provisional (post#135)
Mares - Genius (from Mares tech info email)

GF Lo affects NDL on a NDL Dive
none yet

Unknown (yet) but do have custom GFs (i.e. easy to know if GFLo affects NDL)
Ratio/SEAC (Tech+ has custom GFs)
Scubapro - HUD (anyone got access to play with one?)
Suunto EON Steel <-- need to wait for 16C implementation later this year.

Unknown but doesn't have custom GFs (probably impossible to discover)
Liquivision Lynx



*basically means for all intents and purposes e.g: stops clear as you arrive at them, or, there is only a small unique window (e.g. a few meters to the surface) etc etc .... and rather than making a blanket statement which could be argued is technically incorrect ... leading to ambiguity of what the results practically mean for other divers that haven't read the whole thread in detail.

 

[NAM001]

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.

My Opinion again. but I think the gf 30/80 gfHi says the mnaximum GF that will be allowed to be obtained. That is fine until you leave the prescribed ascent rates. if all is fine on the ascent the max GF should coincidently happen at the surface. and it dies that because that setting of 80 set the NDL that would provide exposure that w9ill give a GF80 when hitting the surface if and only if you comply with the computers ascent rate of 30 ftm through out or on some computers possibly 60 fpm to 60 ft and 30fpm shallower than 60. In the latter case if you do 30 form bottom >60 ft, to the surface you just introduces another bit of safety buffer because you took longer that the computer expected you to take.

If in the case again IMO you have a puter that uses 30 fpm form say 100 ft and you loose ascent control and do say 40 fpm you will exceed your gf Hi value when you hit the surface you will hit is at say perhaps 20-30 ft instead of the surface and the puter will have to make you do perhaps a forced stop at say 30 ft till the tissues catch up. So as I see it you dont have to exceed NDL to go into deco. One could also look at it as you did exceed the ndl HAD THE CORRECT NDL ALUE BEEN GENERATED for an acscent rate of 40 instead of one for calculated 30 fpm.

Idont know what computers we have but if the ascent meter is any indication you may just have one that uses 60fpm below 60 ft and 30 fpm above 60 ft.

 

[stuartv]

If in the case again IMO you have a puter that uses 30 fpm form say 100 ft and you loose ascent control and do say 40 fpm you will exceed your gf Hi value when you hit the surface you will hit is at say perhaps 20-30 ft instead of the surface and the puter will have to make you do perhaps a forced stop at say 30 ft till the tissues catch up. So as I see it you dont have to exceed NDL to go into deco. One could also look at it as you did exceed the ndl HAD THE CORRECT NDL ALUE BEEN GENERATED for an acscent rate of 40 instead of one for calculated 30 fpm.

I disagree with the highlighted part - simply because the definition of NDL is No Deco Limit. By definition, you can't go into deco unless you exceed the No Deco Limit.

If you start up at NDL==0 and you go up at 40fpm, your computer should change from showing you have not exceeded your NDL to showing that you have a mandatory stop. I.e. by going up faster than the computer is programmed to expect, you have put yourself into deco. At that point, you SHOULD get a mandatory stop at 10' (or, optionally, 20' on a Shearwater). But, what you actually get really depends on the specific computer and how it is programmed. The mandatory stop could really be at any depth from 10' on down.

Regardless, yes, the NDL shown is based on an expected ascent rate. So, if you do something different, then the NDL is not going to be "correct".

 

[NAM001]

I disagree with the highlighted part - simply because the definition of NDL is No Deco Limit. By definition, you can't go into deco unless you exceed the No Deco Limit.

If you start up at NDL==0 and you go up at 40fpm, your computer should change from showing you have not exceeded your NDL to showing that you have a mandatory stop. I.e. by going up faster than the computer is programmed to expect, you have put yourself into deco. At that point, you SHOULD get a mandatory stop at 10' (or, optionally, 20' on a Shearwater). But, what you actually get really depends on the specific computer and how it is programmed. The mandatory stop could really be at any depth from 10' on down.

Regardless, yes, the NDL shown is based on an expected ascent rate. So, if you do something different, then the NDL is not going to be "correct".

You key point is that the computer compensates for the change in ascent rate. my position is that it can not because NDL is a computation that is forsight not hindsight and that a defined ascent rate is assumed in guestimating or calculating of NDL Ascent dies not happen until after NDL is calculated. You can do it I think in a dive planner and it uses a set rate. perhsps 30 fpm from depth to the surface. the NDL is good so long as you do not change any assumed parameters such as ascent rate. GF99 and surf GF will be live and based on reality. NDL is a prediction based on given constraints.

Being a pretty knowledgeable guy like you are do you know of a program I can use to derive ndls based on gf HI values

 

[stuartv]

You key point is that the computer compensates for hte change in ascent rate. mny position is that it cana not because NDL is a computation that is forsight not hindsight and that a defined ascent rate is used in guestimating or calculating NDL Ascent dies not happen until after NDL is calculated. You can do it I think is a dive planner and it uses a set rate. perhsps 30 fpm from depth to the surface. the NDL is good so long as you do not change any assumed parameters such as ascent rate. GF99 ND surf GF will be live and based on reality. NDL is a prediction based on given constraints.

Beig a pretty knowledgeable guy like you are do you know of a program I can use to derive ndls based on gf HI values

You are leaving out that the computer is continuously recalculating your NDL while you are in the water based on the tissue loading that it has recorded (via calculations, based on time, depth, and breathing gas) and your current depth. So, once you ascend 10', at 40 fpm, it has already recalculated your new NDL. That continuous, real-time calculation is why you can get to NDL==0, ascend at 40fpm, and have it pop up with a mandatory deco stop.

You can use Subsurface (free, runs on Windows, Mac, and Linux) to set GFs and let it tell you NDLs. It's not completely straightforward and obvious exactly how to to do that, but it's not too tough to figure out.

http://www.subsurface-divelog.org

 

[NAM001]

You are leaving out that the computer is continuously recalculating your NDL while you are in the water based on the tissue loading that it has recorded (via calculations, based on time, depth, and breathing gas) and your current depth. So, once you ascend 10', at 40 fpm, it has already recalculated your new NDL. That continuous, real-time calculation is why you can get to NDL==0, ascend at 40fpm, and have it pop up with a mandatory deco stop.

You can use Subsurface (free, runs on Windows, Mac, and Linux) to set GFs and let it tell you NDLs. It's not completely straightforward and obvious exactly how to to do that, but it's not too tough to figure out.

http://www.subsurface-divelog.org

thanks Ill try that. I may PM you on occasion regardint this stuff. I agree that the cmouter updates. BUT when ouare sitting at a table at home and you enter the depth fo hte dive and teh gf lo and hi it generates a NDL at that point. that NDL is based on assusmptions. flormnt hat point it is a clock that counts down unless you change the agivens such as gfHi setting.

 

[tursiops]

flormnt hat point it is a clock that counts down unless you change the agivens such as gfHi setting.

LOL. You keep saying this, and Stuart keeps pointing out it is incorrect.