Question about Shearwater default GF low settings

[diversity]

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.

I realize this thread is a year and half old, but I liked the title and pertains to my question. There is so much written about the Perdix that I would need to read for another day in order to not be accused of not utilizing the search function.

I purchased a Perdix 2 in the fall of 2024, and noticed that even the lowest (low) conservativity GF (45/95) in recreational mode is still more conservative than the navy NDL tables I've used for 45 years. I also noticed that I can't seem to edit the "custom GF" parameter. My question is two fold....

1) What do I need to do in order to unlock/edit the Custom GF in rec mode, and
2)what would be appropriate GF values to closer mimic navy NDL tables?

I already got a very prompt reply back from Shearwater regarding another question, but they were a bit vague about unlock codes. They claim that there are 3, and are all "Paid". I interpreted this two ways, that they are all "pre-paid", or that they need to be "Paid for" in order to use them.

At this point, much of this algorithm topic is "way above my pay grade" so please dumb it down for me if going into detail.

I've only owned 4 or so computers in my lifetime, last one being a VR3, but this Perdix 2 is by far the best and most menu intuitive one yet!

Thanks,

Jim

 

[scubadada]

Hi @diversity

Your NDL is entirely dictated by the GF high, in your case, 95. This is considered liberal. The GF low only kicks in when you exceed NDL and have a decompression obligation. The GF low dictates your first deco stop. Custom GF is available in all dive modes. See the Perdix 2 manual pp 55 and 29.

It might be best if you learned a bit more about the Buhlmann ZH-L16C decompression algorithm with gradient factors before you further increase your GF high, attempting to mimic the Navy tables. The Perdix 2 offers several additional features that allow you to monitor your dive for safety.

 

[diversity]

Hi @diversity

Your NDL is entirely dictated by the GF high, in your case, 95. This is considered liberal. The GF low only kicks in when you exceed NDL and have a decompression obligation. The GF low dictates your first deco stop. Custom GF is available in all dive modes. See the Perdix 2 manual pp 55 and 29.

It might be best if you learned a bit more about the Buhlmann ZH-L16C decompression algorithm with gradient factors before you further increase your GF high, attempting to mimic the Navy tables. The Perdix 2 offers several additional features that allow you to monitor your dive for safety.

Thank you so much for that tutorial. Very helpful. Prior to checking back here, I did see what step I was missing in the system/Deco setup in order to edit the custom GF. But your explanation was very helpful. Much appreciated, and useful. I've already ran 2 Deco scenarios with various GF to prove what you said.

 

[LFMarm]

I realize this thread is a year and half old, but I liked the title and pertains to my question. There is so much written about the Perdix that I would need to read for another day in order to not be accused of not utilizing the search function.

I purchased a Perdix 2 in the fall of 2024, and noticed that even the lowest (low) conservativity GF (45/95) in recreational mode is still more conservative than the navy NDL tables I've used for 45 years. I also noticed that I can't seem to edit the "custom GF" parameter. My question is two fold....

1) What do I need to do in order to unlock/edit the Custom GF in rec mode, and
2)what would be appropriate GF values to closer mimic navy NDL tables?

I already got a very prompt reply back from Shearwater regarding another question, but they were a bit vague about unlock codes. They claim that there are 3, and are all "Paid". I interpreted this two ways, that they are all "pre-paid", or that they need to be "Paid for" in order to use them.

At this point, much of this algorithm topic is "way above my pay grade" so please dumb it down for me if going into detail.

I've only owned 4 or so computers in my lifetime, last one being a VR3, but this Perdix 2 is by far the best and most menu intuitive one yet!

Thanks,

Jim

Navy tables are not considered very conservative for modern standards of non-professional diving without a chamber in stand-by. Based on the research by Mitchel, Doolette etc and the gradients they personally use, I would recommend using more conservative gradients. Personally I dive 50/60 as a base and typically make it more conservative for high exposure dives. Lots of posts on this topic.

 

[dmaziuk]

Navy tables are not considered very conservative for modern standards of non-professional diving without a chamber in stand-by. Based on the research by Mitchel, Doolette etc and the gradients they personally use, I would recommend using more conservative gradients. Personally I dive 50/60 as a base and typically make it more conservative for high exposure dives. Lots of posts on this topic.

For NDL dives, high 90-s should be OK for a single one. On a 2-week Bonaire vacation I'd probably go high 80s as ZH-L will diverge from DSAT on repetitive dives, but generally speaking I'd try to mimic DSAT: it was designed and tested for NDL diving, and it has a lot of actual dives by PADI table users for its supporting evidence.

 

[lostsheep]

For NDL dives, high 90-s should be OK for a single one.

Is there any data to back this up?

Are you saying that we can tolerate higher gas tensions on an NDL dive vs a deco dive? What would the difference be?

 

[scubadada]

For NDL dives, high 90-s should be OK for a single one. On a 2-week Bonaire vacation I'd probably go high 80s as ZH-L will diverge from DSAT on repetitive dives, but generally speaking I'd try to mimic DSAT: it was designed and tested for NDL diving, and it has a lot of actual dives by PADI table users for its supporting evidence.

The easiest thing to do is to use SurfGF on the Shearwater to make sure your surfacing GF is no higher than your personal chosen value.

@dmaziuk makes a good point regarding Buhlmann and DSAT. Though a GF high of 95 generally matches DSAT reasonably well, the two algorithms behave differently. DSAT often gives more NDL, especially on first clean dive and deeper. Buhlmann often gives more NDL on repetitive dives, especially shallower. I gave two more extreme examples in a previous post

Thread 'DSAT vs GF high 95, examples'

Sep 28, 2021

I have written previously about my experience diving DSAT and Buhlmann ZH-L16C with a GF high of 95. I have an example of what I have previously reported. This dive is a first dive of the day on the Castor in Boynton Beach. It was relatively deep with a max depth of 107 feet and an avg depth of 73 feet. DSAT gave me a minimum NDL of 6 minutes. A GF high of 95 on my Teric gave me 1 min of deco at 10 feet. My surfacing GF was 79.

This is a dive later in the week. It is the 4th dive of the day, following 3 moderately deep and long dives. This was a...

• scubadada

 

[inquis]

What would the difference be?

Most tissue compartments are substantially lower than GFHigh for an NDL dive, so qualitatively less supersaturation across all compartments when surfacing. If you're someone who believes the probability of DCS to be proportional to the integrated supersaturation, that will also be significantly lower for an NDL dive.

Quantitatively, though, is the fact ZHL16C+GF x/95 gives very similar clean NDL times to DSAT, as was mentioned above. DSAT was designed/tested for NDL dives, and more details are available in the 1994 DSAT report.

 

[dmaziuk]

Is there any data to back this up?

Yes: it starts with 'D' and ends with 'SAT'. It's been around since 1994.
HTH

 

[lostsheep]

Most tissue compartments are substantially lower than GFHigh for an NDL dive, so qualitatively less supersaturation across all compartments when surfacing. If you're someone who believes the probability of DCS to be proportional to the integrated supersaturation, that will also be significantly lower for an NDL dive.

Quantitatively, though, is the fact ZHL16C+GF x/95 gives very similar clean NDL times to DSAT, as was mentioned above. DSAT was designed/tested for NDL dives, and more details are available in the 1994 DSAT report.

That is a good point, do we know if it actually increases likelihood of DCS? Your phrasing sounds like you are skeptical.

Yes: it starts with 'D' and ends with 'SAT'. It's been around since 1994.
HTH

Your reply is unnecessarily flippant. Asking for data or citations isn’t an attack, so relax.

I will read the DSAT paper.