Question about Shearwater default GF low settings

[scubadada]

If I understand correctly (which is by no means certain), setting GF-Low should affect NDL times, either statically for planning or dynamically for reporting during the dive, namely when the corresponding (imaginary) tissue is the controlling one?

Or did I learn nothing in the 15 minutes I invested reading about gradient factors

There are practical fudges in place to ignore GF low for no stop dives. If you take a planner such as MultiDeco and play high low game to find the NDL (ie the longest bottom time for no stops n the ascent) you will find it is impacted by GF low. So for 30m 40/85 is 4 minutes while 85/85 is 12 minutes. For reference a Perdix at 40/85 gives 14 minutes. For widely different GF low/high pairs, once you have a celling based on GF low the change in instant GF as depth decreases is steep and so the ceiling gets shallower quickly and so never actually matters.

It is all about what you call "recreational dives", as on the other side of the Atlantic, the limit of recreational diving is 60m (CMAS 3* Diver)
That is why in Shearwater Rec mode, you can dive with up to 3 gases, including 99% Oxygen.


Indeed, from what I understood of gradient factors, for a no-deco dive, the GF low is impacting the NDL. If you have a small GF low, your NDL will be smaller at depth requesting you to ascend shallower to continue your dive, without mandatory stops.
And for a deco-dive with a short TTS, let's say 10/15 minutes, having a small GF low might make you start your decompression stops deeper than other divers of the group (first stop at 9m while others shall start at 6m, impacting on their decompression)

@tursiops is correct in his post. The GF low plays no role in your computer for no stop dives. The GF high controls the dive by ensuring that you will not exceed the set value, in any tissue compartment, as long as you make an ascent to the surface, before exceeding the NDL, at the designated rate. This topic has been discussed numerous times on SB and can be found with the search function. A specific thread polled users as to whether their computer took GF low into account on not stop dives, none did. There are no deco stops in a no stop dive.

However, @KenGordon is also correct. MultiDeco works differently than dive computers. MultiDeco does not calculate NDL, it only calculates deco. This topic has also been discussed numerous times on SB, I'm not sure I fully understand the issue. Some of the stops, can largely me mitigated by using a 1 sec stop time rather than a longer time, such as 1 min, in the configuration menu.

 

[dmaziuk]

Indeed, from what I understood of gradient factors, for a no-deco dive, the GF low is impacting the NDL. If you have a small GF low, your NDL will be smaller at depth requesting you to ascend shallower to continue your dive, without mandatory stops.

No: in the base model tissues tolerate greater overpressure at greater depth. On a no-stop dive this means your GF Hi: the surfacing overpressure, is always the lowest M-value and so it is controlling your no-stop time.

And for a deco-dive with a short TTS, let's say 10/15 minutes, having a small GF low might make you start your decompression stops deeper than other divers of the group (first stop at 9m while others shall start at 6m, impacting on their decompression)

Yes. What's worse is (I already posted it elsethread and it wasn't my calculation originally) that if you go over NDL on a 10/90, you could get a deco stop at 9 msw right away. For which you may not even have gas at that point if you didn't plan for it. Whereas the others diving 95/95, or e.g. DSAT, could still be in NDL.

This is not likely to happen with the common presets, though. (9 msw stop, not DSAT people still being in NDL -- that is quite likely.)

 

[MomZo]

Thanks for your inputs, but it actually doesn't answer my initial question: why GF low are set so small by default in Shearwater dive computers?

And again, Rec mode doesn't mean no-deco dive only, or Shearwater would not give us the option to define up to 3 gases in this mode.
Thus, Rec mode can definitely be used for deco dives when the user doesn't want to have a lot of information displayed at the same time on the screen as it is for Tec mode.

 

[dmaziuk]

Thanks for your inputs, but it actually doesn't answer my initial question: why GF low are set so small by default in Shearwater dive computers?

As @Valyngar says: probably for hysterical raisins. AKA "it sounded like a good idea at the time".

Check also: Deep-stops vs shallow-stops: an interesting read.

 

[MomZo]

Here is what I recall from reading your link:
"For Bühlmann ZHL-16C, GF low settings lower than 55 led to first stops that were considered to be too deep. The study mentioned that a GF Low of 70 was also acceptable, so it explicitly approved of a range of 55-70 but did not specify an upper limit. It also said a GF high of 70 or below was within the preferred US Navy limits."

 

[Miyaru]

First of all I totally agree with @tursiops that GF-low plays no role in NDL diving.

The reason for the default 30/70 setting goes back several years. Dr. Neal Pollock published the results of a study in 2015, which looked into gradient factors. Watch the BSAC Conference of 2016, Dr. Pollock touches the subject 40 minutes into the video.
That's around the time that VPM turned out to be not-so-good-after-all, and GFs were a safer addition to Bühlmann. A specific agency favored 15/85 and this study advised against 15/85 as too aggressive, and proposed 30/70 as a starting point to determine your personal gradient factor.
Soon after, this advice was programmed in the Shearwater firmware as a default setting.

Increasing the GF-low to 50 or higher, is just another illustration of how the decompression knowledge is evolving, while still nobody has all the answers.
Shearwater will probably change this default once study results with validated data have been published.

 

[scubadada]

...Shearwater will probably change this default once study results with validated data have been published.

What studies are these? We would all like to see them. There was a recent thread previewing some of the results from one recent study, a podcast by Mark Powell,

 

[Miyaru]

What studies are these? We would all like to see them. There was a recent thread previewing some of the results from one recent study, a podcast by Mark Powell,

About one hour into the podcast, @Mark Powell shows some really interesting results from that study with DAN, at the same time he says that the number of divers was small, the results are not significant enough to draw any conclusions, but it does indicate which direction to go with the next study.
So as far as I know, no other studies have been done and the DAN results haven't been published yet. But it will be interesting!

 

[MomZo]

For information, I contacted Shearwater by mail and here is their answer.
Looks like these values come from the past and they didn't dare changing them until a new study proves there are better GF couples.

May 25, 2020, 4:00:58 PM PDT

Hi Quentin,
Thank you for contacting us.
You did touch on part of the answer. As decompression science and knowledges changes and expands we've made customizable gf factors available in Rec mode as well as Tec mode. The standard gf factors have changed upon consideration from time to time. However, this is not something we want to be changing on divers frequently in firmware updates, etc. At present default settings are meant to reflect some tried and tested industry norms both in line with decompression theory and how it is put into use through various agencies and other manufacturers so that divers can easily choose some settings similar to how they have been diving or their buddies, etc may be diving.

There tends to be a trend in the industry away from both bubble models and deeper stops in general. We absolutely encourage divers to research ongoing decompression theory. We don't make any recommendations regarding GF settings beyond the defaults as there isn't enough scientific validation to definitively say that one setting is better than another. Everyone has a different susceptibility to DCS. So, each diver is responsible for choosing their own settings based on their study of the available source material.
If you would like to learn more about how the ZHL16-C decompression algorithm works, I recommend Deeper into Diving by John Lippmann and Deco for Divers by Mark Powell. These are both great resources.
Best,

Ness Greenough
Customer Care Manager

 

[Caversluis]

I know this thread is a bit old, but still wanted to make this comment for those who are interested.

This video explains in great detail why there are two gradient factors, and what the implications are for deco dives.


In short: the GF low determines the risk at depth, whereas the GF high determines the risk at the shallower part of the ascend. In mathematical terms: they are both used to modify the M-line. The GF high determines a point at the lower part of the M-line. The GF low determines a point at the higher end of the M-line. If your GF-low is lower than GF-high, you modify the slope of the M-line.

Ofcourse this is a bit of a simplification. The video explains that a lower GF-low will result in earlier and deeper stops. This reduces the risk for the faster tissues, but at the same time increases the risk for the slower tissues (as they will keep on-gassing).

The video makes a good point on treatment options for DCS caused by fast tissues vs treatment options for slow tissues. See the video for the explanation.