So, who is going to market an inexpensive rec computer running Buhlmann with GF?

[dmaziuk]

Not sure what you mean by "curve": it's typically drawn as straight line. In theory you basically have a 45-degree M-value line. GF Hi line has the same starting point at depth and simply moves the "surface" point away from M-value line. But on an actual no-stop dive you can either swim up to the surface without going over the M-value in any of your tissue compartments, or you can not. So there isn't really a "curve" until you get right up to the NDL.

I think heat maps are about the best way to visualize it, subsurface has them, but again: on a no-stop dive you need to go to 100'+ to get some colours in there... So if you don't have any of those logged, your heat maps may not look very, uhm, interesting.

 

[ericBSoCal]

OK, So GF just adjusts the offset and slope of the M line, expressed as a % of the distance between the ambient pressure line and the m line. It's still linear. If gf Hi and gf Lo are different, then the slope of the line changes as well

Would GF Lo influence the NDL time for a given profile? For instance, in multi deco a dive to 100 ft with 7:20 of bottom time at ZHLB + GF 70/70 will not incur a deco obligation. The same profile with GF of 30/70 will get you 3 deco stops for a total of 2:12. Presumably because a TC exceeded the more conservative saturation value.

 

[tmassey]

With those kind of questions, I think this paper will help: http://tecvault.t101.ro/Clearing up the confusion about the deepstops.pdf

 

[Jay]

In theory you basically have a 45-degree M-value line.

I think you might mean the ambient line is 45deg ... not the M-value line.

OK, So GF just adjusts the offset and slope of the M line, expressed as a % of the distance between the ambient pressure line and the m line. It's still linear. If gf Hi and gf Lo are different, then the slope of the line changes as well

Correct ... but ...

Would GF Lo influence the NDL time for a given profile? For instance, in multi deco a dive to 100 ft with 7:20 of bottom time at ZHLB + GF 70/70 will not incur a deco obligation. The same profile with GF of 30/70 will get you 3 deco stops for a total of 2:12. Presumably because a TC exceeded the more conservative saturation value.

Gf Lo doesn't affect NDL because of the way it's implemented* in a NDL dive. GF Lo only is taken into account when one enters deco. *From other discussions on SB this is true for Shearwater; IDK re other dive software / computers - depends on implementation.

Here's my reading list I had saved - defaulted to open at the page with main info - I've not checked that each deals with both GF Lo AND NDL.. time might be either or etc.

Gradient Factors and recreational diving (see post #10)
Dive computers... SO many choices! (and p.19)
Ceiling/Deco obligation in recreational diving
Effect of slow compartments size in relation to NDL and DECO
Why plan decompression with a Gf (lo)?
Getting First Deco Stop with GF-Lo

 

[dmaziuk]

I think you might mean the ambient line is 45deg ... not the M-value line.

Yes I did, thanks. Should've looked at the picture again first. E.g. Gradient Factors | Dive Rite

Would GF Lo influence the NDL time for a given profile? For instance, in multi deco a dive to 100 ft with 7:20 of bottom time at ZHLB + GF 70/70 will not incur a deco obligation. The same profile with GF of 30/70 will get you 3 deco stops for a total of 2:12. Presumably because a TC exceeded the more conservative saturation value.

There's two ways you can check the NDL:

1. calculate direct ascent to the surface and see if you violate GF Hi. If not, you're in "NDL time".

2. Calculate the depth of first deco stop based on GF Low. If it's at or above the surface, you're in "NDL time".

Theoretically I don't see why there can't be a profile/set of GFs where one would put you in deco while the other still thinks you're in NDL. I never tried running the numbers though. Rumour has it that typical implementations do #1 and if that says you're in deco: then #2. I have not seen any computer code myself. (That is, I looked at OSTC code a while back but I don't recall now if I even paid attention to this bit.)

 

[ericBSoCal]

Thanks for all the links and explanations. So it's implementation dependent. The ratio/seac planner might give an indication of which approach they've taken. I'll try running plan mode with different GF settings to see if that affects NDL.

You're correct, the OSTC code doesn't look at GF low (source)

 

[scubadada]

...Would GF Lo influence the NDL time for a given profile? For instance, in multi deco a dive to 100 ft with 7:20 of bottom time at ZHLB + GF 70/70 will not incur a deco obligation. The same profile with GF of 30/70 will get you 3 deco stops for a total of 2:12. Presumably because a TC exceeded the more conservative saturation value.

MultiDeco does take GF lo into account in every plan. Shearwater and my discontinued Dive Rite NItek Q do not use GF lo until you exceed NDL and are in deco. I can't vouch for the other computers that run Buhlmann, Divesoft, Garmin, Divecomputer.eu, Heinrichs Weikamp. Perhaps owners of these computers could let us know, easy to determine using planner and different GF lo values.

See post 19 Shearwater Gradient Factors=DSAT?

 

[ericBSoCal]

Can confirm seac / ratio doesn't change ndl based on gf low.

 

[C Dub]

Seems to me the computer Scubadada is looking for already exists........a used Petrel 1 or Petrel 2.

Does everything you want, reliable and very durable, and can be purchased for $300-$500.

 

[atdotde]

I would say: the deepest stop is only determined by GFlow as by definition it applies at the depth of the first stop. GFhigh is irrelevant for that as it applies at the surface and thereby at intermediate stops, stronger for the shallower ones. At least that’s what we do for the subsurface planner/ceiling calculation. But as long as you are doing NDL dives, considering gradient factors is pretty much overkill. You could slso add conservatism by only staying dome percentage of your NDL. They are only relevant if you want do shape the profile of your deco stops which you don’t want to do.