Your Gradient Factors?

[scubadada]

That's one way of thinking about it. Another way of thinking about it is you are diving within 10% of the Buhlmann NDL limits--very conservative.

For NDL dives, the first 90 is not relevant, and the second is used fairly commonly.

Okay so a 10% error would put you over your deco ceiling, and possible dcs

You might want to take a look at the SAUL probabilistic deco algorithm SAUL Recreational Dive Planner | Modern Decompression The DCS risk of a 1st, clean dive to 60 feet on air for 54 minutes (GF high 99) is 0.308%, 1/325 dives.

 

[boulderjohn]

Quick history:

Albert Buhlmann worked for decades to develop his decompression algorithms, based on dissolved gas theory. A little after his death, the so-called bubble models (VPM, RGBM) came out, and these called for deeper deep stops on decompression dives than more traditional dissolved gas models. They became all the rage. They had decompression divers doing their first stops much deeper than other models. A man named Erik Baker developed what are called gradient factors as a way for people to fudge the Buhlmann algorithm to allow for deeper stops. It became common for people to use gradient factors like 30/85, meaning that you would do your first decompression stop at 30% of what Buhlmann called for, and you would surface at 85% of his safe M-value.

In the last decade, the trend toward deep first stops reversed as evidence suggested the depths people were using were too deep. Current thinking on decompression dives tends to use a much higher first GF number, but that is still notably deeper than Buhlmann. People are also tending to use lower numbers for the second GF as well. This indicates that people on decompression dives are convinced that pure Buhlmann is at least somewhat too aggressive, although they think people went too far on the deep stops earlier.

That is for decompression diving. For NDL diving, the deep stop is not a factor. For the GF high number, the lower the number, the shorter your NDL.

 

[Celt]

Gradient factors are essentially guardrails that your ascent profile is built between. Most people don't change them regularly. You really just have to figure out what works best for you, balancing the length of deco with how you feel after a dive. People do tend to mess with GF hi more frequently though, as it will extend your shallow stop time. It's common at altitude, in remote places, or anytime someone wants to pad the O2 a bit for whatever reason.

Okay so for someone like myself who’s getting on in the years and I was going to buy a computer it would pay me to get one where I could adjust the GF hi and extend those shallow stops, obviously the more user adjustable the more expensive.

 

[boulderjohn]

Okay so for someone like myself who’s getting on in the years and I was going to buy a computer it would pay me to get one where I could adjust the GF hi and extend those shallow stops, obviously the more user adjustable the more expensive.

I agree with this. There is also a new feature on some computers (Shearwater at least) called SurfGF. This tells you what your GF would be if you went to the surface right at that moment. For some people (including me) this is pretty much replacing the standard 3-minute safety stop.

Let's say I have a computer that I set to a GF high of 85. That guides my NDL, and if I stay within it and do my 3-minute safety stop, then I should finish somewhere beneath 85% of the Buhlman safe value. (Note the uncertainty.) If I set it at 85 and my computer also has a SurfGF reading, I can hang around under water until I like what I see there. 85? 80? 75? I can just watch those numbers drop until whatever I want.

This past winter I did a lot of relatively shallow reef divers breathing EANx 36, and I typically arrived at safety stop depth with my SurfGF values already less than 50. I hung around for the safety stop just to stay with my buddies, who would have had roughly the same SurfGF but did not have computers that would tell them that, so they did the old safety stop countdown.

 

[Celt]

I agree with this. There is also a new feature on some computers (Shearwater at least) called SurfGF. This tells you what your GF would be if you went to the surface right at that moment. For some people (including me) this is pretty much replacing the standard 3-minute safety stop.

Let's say I have a computer that I set to a GF high of 85. That guides my NDL, and if I stay within it and do my 3-minute safety stop, then I should finish somewhere beneath 85% of the Buhlman safe value. (Note the uncertainty.) If I set it at 85 and my computer also has a SurfGF reading, I can hang around under water until I like what I see there. 85? 80? 75? I can just watch those numbers drop until whatever I want.

This past winter I did a lot of relatively shallow reef divers breathing EANx 36, and I typically arrived at safety stop depth with my SurfGF values already less than 50. I hung around for the safety stop just to stay with my buddies, who would have had roughly the same SurfGF but did not have computers that would tell them that, so they did the old safety stop countdown.

That’s a handy feature, I’m starting to see why some cost more than others but it’s hard to know what features you need and what you don’t. I’m going to do a bit of studying. Thanks for the info.

 

[helodriver87]

That’s a handy feature, I’m starting to see why some cost more than others but it’s hard to know what features you need and what you don’t. I’m going to do a bit of studying. Thanks for the info.

One thing I like to do is set GF high as aggressively as I'm comfortable with (typically 85 on OC, 90 on CCR) because I know I can surface there and probably feel fine. I prefer to do the final ascent from 20 pretty slow, so I usually end up with a surfGF of 75, but if time/comfort/safety is a factor, I want my deco timer to be counting down to the most aggressive non emergency GF high I'm willing to accept routinely. In an emergency, I'm gonna weigh the current situation against surfGF and decide how much I'm willing to risk it. It's a super useful tool.

 

[Pressurehead]

You might want to take a look at the SAUL probabilistic deco algorithm SAUL Recreational Dive Planner | Modern Decompression The DCS risk of a 1st, clean dive to 60 feet on air for 54 minutes (GF high 99) is 0.308%, 1/325 dives.

Thank you for that link.

 

[dmaziuk]

Thank you for that link.

Keep in mind that a) his probabilities are based on DCIEM model that is supposed to be the most conservative of them all and b) SAUL is for no-stop dives only.

Here is another take on it:
Setting Gradient Factors based on published probability of DCS – The Theoretical Diver
Fraedrich follow-up – The Theoretical Diver

 

[Pressurehead]

Keep in mind that a) his probabilities are based on DCIEM model that is supposed to be the most conservative of them all and b) SAUL is for no-stop dives only.

Here is another take on it:
Setting Gradient Factors based on published probability of DCS – The Theoretical Diver
Fraedrich follow-up – The Theoretical Diver

And thank you for those links also.
I like to read everything I can on the subject, and the knowledge base is amazing.
And I thought I had "a fair handle on it" but now know my [supposed ] knowledge is basic at best.

 

[Blackcrusader]

I agree with this. There is also a new feature on some computers (Shearwater at least) called SurfGF. This tells you what your GF would be if you went to the surface right at that moment. For some people (including me) this is pretty much replacing the standard 3-minute safety stop.

Let's say I have a computer that I set to a GF high of 85. That guides my NDL, and if I stay within it and do my 3-minute safety stop, then I should finish somewhere beneath 85% of the Buhlman safe value. (Note the uncertainty.) If I set it at 85 and my computer also has a SurfGF reading, I can hang around under water until I like what I see there. 85? 80? 75? I can just watch those numbers drop until whatever I want.

This past winter I did a lot of relatively shallow reef divers breathing EANx 36, and I typically arrived at safety stop depth with my SurfGF values already less than 50. I hung around for the safety stop just to stay with my buddies, who would have had roughly the same SurfGF but did not have computers that would tell them that, so they did the old safety stop countdown.

Thanks for your insight. This thread just started as what GF you are using. It has delved into going away from the question to all sorts of "what if" and discourse on DECO dives. Well not everyone is doing planned DECO dives. Pretty much I am not doing DECO dives as I am doing vacation dives at padi run dive centers. My Shearwater is setup as OC Tec mode. Why? Well I don't need a babysitter and I also changed my safety stop to 5 minutes at 6m. Most of my dives actually end up with my Shearwater show CLEAR plus ten minutes or more as I often end the dive on reefs where I can look for some marine life to watch or take photo's of. My GF is set as 45/95 and over a period of consecutive days gives me longer time to NDL than my Suunto Viper would ever allow. In fact the Suunto ends up keeping you very shallow after a few days as a more conservative setup.

The other thing is that I may end up diving with others using different dive computers which will give them less time to NDL and I have to remind them to dive to their own dive computer and pay attention. I had one dive buddy who didn't and her Suunto had her do a 25 minute stop at 5m. We were on a second morning dive and she lost track of her dive computer I have no idea if that she must have exceeded NDL ( I had not )

So if I planned a DECO dive I would also ask my dive partners what GF setting they have for the dive. I prefer everyone is on the same GF for that but hey that's just me. Good thing with the Shearwater is that if you do exceed NDL you would often clear the need for a deep stop by simply ascending to a shallower depth for some time.