Question about Shearwater default GF low settings

[MomZo]

Hey guys.

I am wondering why @Shearwater chose to set such GF low values in the 3 default conservatism settings in recreational mode.

GF low values are set between 35 to 45, which implies to have a deep first stop while it is arguably accepted that deep stops are not recommended for Air/Nitrox dives.

I am wondering why default GF low are set as:
- LOW GF 45/95 instead of 95/95
- MEDIUM GF 40/85 instead of 85/85
- HIGH 35/75 instead of 75/75

Thanks in advance for your inputs.

 

[Valyngar]

That is indeed good question. Seems like leftover of old knowledge. Gf low should be raised to 55-70 value.

 

[MomZo]

Even 55~70 seems quite low for recreational Air/Nitrox dives.
I understand the benefit/obligation of starting deco stops early during Trimix dives, but for NDL dives, or small TTS dives, I really don't.

 

[Manwork]

Hi. I do not think a low GF of 40 will require deep stops in most of the recreational dives. Although I dive usually with GF 40/70 the Petrel and the Perdix (TEC OC) do not ask for it. I have done several dives in the range of 30 m in Bonaire (non decompressive) and also at the usual rec dives with air and never saw it on NDL dives.

 

[tursiops]

GF Low means nothing for an NDL dive. There are no deco stops, so there are no deep deco stops.
If you are doing a non-NDL dive, with planned deco, you shouldn't be in recreational mode; set it how you want it.
If you are doing a non-NDL dive, with unplanned deco, you probably shouldn't be in the water!

 

[d^2b]

GF Low means nothing for an NDL dive. There are no deco stops, so there are no deep deco stops.

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

 

[KenGordon]

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.

 

[Dsix36]

IIRC the defaults were changed to their current values after being onboard a live aboard with Dr. Neal Pollack and conducting enormous amounts of doppler bubble tests on divers over several days with various profiles. The results were extremely enlightening to many people.

 

[MomZo]

GF Low means nothing for an NDL dive. There are no deco stops, so there are no deep deco stops.
If you are doing a non-NDL dive, with planned deco, you shouldn't be in recreational mode; set it how you want it.
If you are doing a non-NDL dive, with unplanned deco, you probably shouldn't be in the water!

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.

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.

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)

 

[Boston Breakwater]

Hey guys.

I am wondering why @Shearwater chose to set such GF low values in the 3 default conservatism settings in recreational mode.

GF low values are set between 35 to 45, which implies to have a deep first stop while it is arguably accepted that deep stops are not recommended for Air/Nitrox dives.

I am wondering why default GF low are set as:
- LOW GF 45/95 instead of 95/95
- MEDIUM GF 40/85 instead of 85/85
- HIGH 35/75 instead of 75/75

Thanks in advance for your inputs.

Hello. Here is an excerpt from one of my post.
"I bought 2 of them for "Redundancy."
(One on each wrist.)
90% of my dives are solo.
They have different models and are definitely something to consider.
The have phenomenal customer service.
I had a problem/question last year, and when I called they put me on the phone with the guy that wrote the "Code." for this particular unit. REALLY?????????????????
Edit: Perdix (Non-A.I.)
You'd be hard pressed to get customer service like that, elsewhere.
I hope that is helpful?
Cheers."
Admittedly, I try and go right to the source, if possible.
Cheers.
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