GF high and NDL

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NAM001

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I have been reading as much as i can understand regarding GF's I have the most basic concepts down now except for one aspect. I understand how GF low sets the first stop but talk of GF high setting the NDL time evades me. So the question is how does GF's effect the NDL to where some puters say 25 min and others say 19 min. comments and links will both work for me.

thanks
 
I think this is how it works.

GFHi is the maximum allowable overpressure on surfacing, as a percentage of the difference between the ambient pressure line and the M value line.

So during the dive (at least for Shearwater computers), the computer is constantly calculating how long it will be - if you remain at the current depth and the current mix - before a direct ascent to the surface results in surfacing at GFHi. @tursiops posted this in the GFlo thread, along with a link to the Shearwater page:

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

So for example, you are diving 30/70. The computer determines that in 20 minutes, if you stay at the current depth without changing your breathing mix, and then ascend directly at a safe ascent rate (specified in the algorithm), you will surface with your leading compartment being at GF 70, and it will display NDL=20. If you switch to 40/85, then the computer determines that if you stay for 25 minutes and surface, your leading compartment will be at GF85. If you stay long enough so that a direct ascent will result in a GF higher than GFHi, then your computer will start generating deco stops.
 
I think this is how it works.

GFHi is the maximum allowable overpressure on surfacing, as a percentage of the difference between the ambient pressure line and the M value line.

So during the dive (at least for Shearwater computers), the computer is constantly calculating how long it will be - if you remain at the current depth and the current mix - before a direct ascent to the surface results in surfacing at GFHi. @tursiops posted this in the GFlo thread, along with a link to the Shearwater page:

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

So for example, you are diving 30/70. The computer determines that in 20 minutes, if you stay at the current depth without changing your breathing mix, and then ascend directly at a safe ascent rate (specified in the algorithm), you will surface with your leading compartment being at GF 70, and it will display NDL=20. If you switch to 40/85, then the computer determines that if you stay for 25 minutes and surface, your leading compartment will be at GF85. If you stay long enough so that a direct ascent will result in a GF higher than GFHi, then your computer will start generating deco stops.


Thanks I think I had most of that that figured out at one time but lost the logic of it all and could not find it again. So given what you have said. the shearwater will have a NDL of say 17 at 100" (dive table showing 20) because of no 3 min stop included in the algorithym. if there was a computable 6m stop time used the NDL at 100' would increase but a few more minutes as direct asent to the surface will be interupted for additional 6m off gassing that lowers the surface remaning tisssue pressures as opposed to going directly to the surface with no stop. If that is the case, then: It would be nice to select an option on the shearwater to consider a selectable time stop at 6m say min (((( 6m stop of x minutes parameter)))) range 3-6 min when calculating NDL. I havent been in the menues for a while but i do remember a settng dealing with shallowest stop of 10'

This discussion really clears up understanding of dive tables such as times shown assume a 3 minute stop or a 5 min stop under some conditions. I had originally assumed the 3 min was optional to further prevent DCS and not as a condition to the accuracy of the assigned NDLs.

thanks for your explanation .... it cleared a lot of stuff up for me. Correct me if I made some wrong assumptions.
 
What table considers 3 minutes worth of offgassing in it's determination of NDL's? Where did you get this information? Certainly the PADI ones do not, and I don't recall the IANTD ones doing so either. The instructions for RDP use (granted the only ones I have ready access to is an older PDF) state:

12. A safety stop for 3 to 5 minutes at 5 metres is recommended at the end of all dives. A safety stop for 3 minutes at 5 metres is required anytime you come within three pressure groups of a no decompression limit, and for any dive to a depth of 30 metres or deeper.

Nowhere does it say that a safety stop is taken into consideration when calculating NDL's.

Because safety stops are not REQUIRED in no-stop diving they are not factored into NDL calculations. Any REQUIRED stop becomes a decompression stop (hence the reason the instruction above is bullsh!t btw) If we were to pretend that DCS was a binary outcome (0 = No DCS, 1 = DCS), and NDL is the line at which a stop is required or you WILL get bent, calculating NDL based on a diver doing a stop would mean that the NDL is not actually the NO-DECOMPRESSION limit. Because you are required to do decompression otherwise you enter the binary state of 1 = DCS.

Also, comparing any random table to Buhlmann with GF's is comparing apples to oranges, so you can't make assumptions like, "it would be a longer NDL on the Shearwater if they factored in a 3 minute safety stop." By its very nature, offgassing on ascent means that your no decompression limit will increase as you ascend in the water column. That's why computers go back to saying things like, "NDL = 99" when you're at your safety stop.

You're thinking about the NDL thing incorrectly, and you're comparing things that aren't comparable. It's not valid to assume that by adding a safety stop, your NDL time will be increased, and it's also not valid to assume that by ignoring a safety stop, your NDL time will decrease. That's just not how those things are calculated. If you are relying on a stop to dictate your NDL, you're not actually calculating your NDL, you're calculating a required decompression stop at W depth for X minutes for a profile where you've stayed Y minutes beyond Z depth. That it may align with safety stop recommendations is not evidence of correlation.
 
Right, was just going to say something like that but Johnny beat me to it, and much more accurately! The standard analogy for understanding DCS risk is measuring with a micrometer and cutting with an axe. The other one is a bright line through a gray area. Risk of DCS is not binary (just like the risk of ox tox, another area where we attempt to put strict rules on wide physiological bell curves)

There have been a lot of threads discussing the reason for safety stops and why they were developed, but it seems to mainly be that (1) for a diver that may have added to his decompression stress by doing things like sawtooth profile or fast ascents that weren't necessarily penalized by simpler computers, this just adds a bit of safety to compensate for these unknown numbers, and (2) by telling all new divers that they have to stop for 3 minutes at 15 feet, it makes it less likely that they will just shoot to the surface when they decide the dive is over. Sort of like a speed bump. Again, that reduces decompression stress.
 
What table considers 3 minutes worth of offgassing in it's determination of NDL's? Where did you get this information? Certainly the PADI ones do not, and I don't recall the IANTD ones doing so either. The instructions for RDP use (granted the only ones I have ready access to is an older PDF) state:

12. A safety stop for 3 to 5 minutes at 5 metres is recommended at the end of all dives. A safety stop for 3 minutes at 5 metres is required anytime you come within three pressure groups of a no decompression limit, and for any dive to a depth of 30 metres or deeper.

Nowhere does it say that a safety stop is taken into consideration when calculating NDL's.

Because safety stops are not REQUIRED in no-stop diving they are not factored into NDL calculations. Any REQUIRED stop becomes a decompression stop (hence the reason the instruction above is bullsh!t btw) If we were to pretend that DCS was a binary outcome (0 = No DCS, 1 = DCS), and NDL is the line at which a stop is required or you WILL get bent, calculating NDL based on a diver doing a stop would mean that the NDL is not actually the NO-DECOMPRESSION limit. Because you are required to do decompression otherwise you enter the binary state of 1 = DCS.

Also, comparing any random table to Buhlmann with GF's is comparing apples to oranges, so you can't make assumptions like, "it would be a longer NDL on the Shearwater if they factored in a 3 minute safety stop." By its very nature, offgassing on ascent means that your no decompression limit will increase as you ascend in the water column. That's why computers go back to saying things like, "NDL = 99" when you're at your safety stop.

You're thinking about the NDL thing incorrectly, and you're comparing things that aren't comparable. It's not valid to assume that by adding a safety stop, your NDL time will be increased, and it's also not valid to assume that by ignoring a safety stop, your NDL time will decrease. That's just not how those things are calculated. If you are relying on a stop to dictate your NDL, you're not actually calculating your NDL, you're calculating a required decompression stop at W depth for X minutes for a profile where you've stayed Y minutes beyond Z depth. That it may align with safety stop recommendations is not evidence of correlation.

I believe i was reading a post where it was said that shearwaters were much more conservative because the NDL computation did not include safety stops and as such the lack of safety stop time reduced the calculated NDL's. As the tables go they have rules for safety stops of 3 minutes and extending to 5 minutes as I believe assumptions on the NDL's given. I will have to find a table to rechek this. That along with table use definitions of Bottom time etc., as it is applied to teh table use.

to be honest i have never heard of them being manditory but strongly advised. this is the portion that i got form the other post that shearwater does not incorporate in the NDL calculation. that they calculate the pure straight non stop ascent to the surface ndl. perhaps that varies in tech vs rec mode. I dont even recall if my shearwater even calls for a safety stop on the display like some computers do. I always stop for e safety stop but that is moot regarding the NDL calc.

from your post below.....;

12. A safety stop for 3 to 5 minutes at 5 metres is recommended at the end of all dives. A safety stop for 3 minutes at 5 metres is required anytime you come within three pressure groups of a no decompression limit, and for any dive to a depth of 30 metres or deeper.
 
I believe i was reading a post where it was said that shearwaters were much more conservative because the NDL computation did not include safety stops and as such the lack of safety stop time reduced the calculated NDL's. As the tables go they have rules for safety stops of 3 minutes and extending to 5 minutes as I believe assumptions on the NDL's given. I will have to find a table to rechek this. That along with table use definitions of Bottom time etc., as it is applied to teh table use.
No, NDL is direct ascent to the surface at the "normal" ascent rate of the computer or table. Shearwaters can be quite conservative or very liberal depending on what you set your GFhi at, 95 generally considered liberal, 85 moderate, 75 conservative, all +/-
 
No, NDL is direct ascent to the surface at the "normal" ascent rate of the computer or table. Shearwaters can be quite conservative or very liberal depending on what you set your GFhi at, 95 generally considered liberal, 85 moderate, 75 conservative, all +/-
My understanding is it agrees with you so far in that it computes NDL assuming direct ascent to the surface with no stops safety or other. That any added stops like a 3-5 min safety stop just further reduces teh chances of unexpected DCS event.
 
This is out of the petrel manual

6.1. Safety Stop A safety stop is an optional stop added to all dives before surfacing. Safety stops can be set to fixed times of 3, 4, or 5 minutes, set to adapt based on dive conditions, or turned off completely. See System SetupDeco Setup for options.

do these setting or are these settings taken in consideration in NDL calculations ?
 
https://www.shearwater.com/products/peregrine/

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