Conservatism

[Caversluis]

I had a dialog with a fellow diver on consevatism. He is an fierce advocate of eliminating risks, and consequently diving conservatively. He recommended to set my my Shearwater computer at high conservatism AND set max ppO2 at 1,2.

This made me wonder afterwards, how does each of this impact the risk of DCS? I am familiar with the concept of gradient factors as well as the principals of ppO2. My understanding is that.: 1) setting conservatism to high will reduce the gradient factors and consequently the NDL time 2) Setting ppO2 to 1,2 would reduce my max depth and thus my nitrogen and oxygen exposure. Is doing both appropriate or is it (far) too conservative?

A bit of background on my diving. I live in Denmark and dive 2-3 times a week al year around. I dive drysuit and doubles, at depths between 20 and 30 meters. Most of the time, I use Nitrox 32. Typical dive length is around 70 mins. I am Master Scuba Diver with (amongst others) deep and Nitrox speciality. I am 55 years old and in physically fit.

 

[steinbil]

Max ppO2 is not directly related to DCS, other than the efficiency of decompression. The concern is oxygen toxicity and possible convulsions. If you're diving Nitrox32 to 30m you're at 1.28ppO2, which is considered quite safe for the working part of the dive. I personally do the same all the time, and would not change it to 1.2ppO2. If you wanted you could dive air to 30m and have a lower ppO2, but you would get shorter NDLs.

Shearwaters "high conservatism" I think is GF35/75, which is not unreasonable. However I don't think just setting it to a random GF and calling it a day is the way to go. Everybody is different, and your personal risk of DCS on any single dive is different. As far as I know there are myriads of factors that come into play, like temperature on the dive, how much you've slept, your general condition, diet, and probably much more. So I would try to establish a baseline for myself, by evaluating how I feel after a dive. If I consistently feel tired and worn out after dives, I might consider lowering the GFhi. I have displayed SurfGF on my Perdix so I can also decide if I want to lower that number before the final ascent on an NDL dive, if I feel like the conditions are worse than usual.

The GFlo is not really applicable to NDL diving, but many argue that a GFlo of 35 is a bit low, and that it's better to skew the deco towards more time on shallow stops by setting GFlo to 40-50+

 

[Belzelbub]

I had a dialog with a fellow diver on consevatism. He is an fierce advocate of eliminating risks, and consequently diving conservatively. He recommended to set my my Shearwater computer at high conservatism AND set max ppO2 at 1,2.

1.2 seems a bit low. Depending on the dives you do, it may or may not be severely limiting.

This made me wonder afterwards, how does each of this impact the risk of DCS? I am familiar with the concept of gradient factors as well as the principals of ppO2. My understanding is that.: 1) setting conservatism to high will reduce the gradient factors and consequently the NDL time 2) Setting ppO2 to 1,2 would reduce my max depth and thus my nitrogen and oxygen exposure. Is doing both appropriate or is it (far) too conservative?

The low ppO2 may actually be counterproductive from a N2 standpoint. The 1.2 setting would have a lower MOD than the more typical 1.4. If that MOD is shallower than the planned depth, you may need to use a milder Nitrox mix, which would lead to more N2 absorption than before, which would mean your computer says you’ve reached NDL sooner.

Another option is to leave things as they are and use SurfGF as a buffer. I have my GFs set fairly liberal, but use SurfGF to determine when I’m OK surfacing. This might mean extending the safety stop a bit until that number drops. I also use adaptive safety stop, which will automatically extend the safety stop to 5 minutes based on dive parameters.

 

[100days-a-year]

1. Do we have any evidence 35/75 is safer than any other setting?
2. In an NDL dive does it really matter?

 

[scubadada]

Hi @Caversluis

The NDL for 32% at 100 feet (just over 30 meters) for a GF high of 95, 85, and 75, is 26 min, 22 min, and 17 min. For a single clean dive, risk of DCS using the SAUL recreational dive planner for these 3 dives is 0.066%, 1/1515, 0.008%, 1/12,500, and 0. One can easily argue that all of these dives are quite safe and run a very low risk of DCS.

SAUL Recreational Dive Planner | Modern Decompression

I run a GF high of 95. I also run SurfGF and have not surfaced with a GF greater than 80 since I bought my Teric in May 2019. Before that, I did an adaptive safety stop or padded my deco stop.

 

[jborg]

I run a GF high of 95. I also run SurfGF and have not surfaced with a GF greater than 80 since I bought my Teric in May 2019.

What does it mean for you to run a GF high of 95 if you make sure to not surface with more than 80?

 

[scubadada]

What does it mean for you to run a GF high of 95 if you make sure to not surface with more than 80?

I run my Teric at 80/95. My other computer is an Oceanic VT3 running DSAT, which is quite similar to a GF high of 95. I have about 2250 dives using DSAT, so am extremely familiar with it. I have about 1100 dives running Buhlmann ZH-L16C at a GF high of 95.

95% of my dives are no stop, the surfacing GF is already going to be less than 80 with a normal safety stop of 3 min. In fact, last time I looked, my average surfacing GF was in the low 50s. When I am closer to the NDL, I simply hang out in the shallows or increase my safety stop to lower the SurfGF. For my light deco dives, I do the same, hang out in the shallows after clearing deco or prolonging the stop to decrease the SurfGF. No more guess work of doing an adaptive safety stop or arbitrarily padding the deco stop.

DSAT and a GF high of 95 keeps me out of deco on aggressive dives, often prohibited on land based or liveaboard operators. I'm almost 70 years old and appreciate the extra margin that a surfacing GF of 80 or less might give me. In the past, I have surfaced many times closer to a GF of 95 and tolerated those perfectly well. Now, I simply have reset the risk tolerance.

 

[The Chairman]

Cold water requires more conservatism. Henry's law teaches us that gasses are more soluble at lower temps. If your core temperature is depressed, your saturated tissues like blood and spinal fluids will absorb and retain more gas. That just follows on down the line. People diss Suunto for their conservatism, but they were developed in the North Seas.

 

[formernuke]

For no deco the GFlo is irrelevant.

PPO2 will not really effect depending on how you dive. Doing the same dive time with higher (as long as 1.4 or under) is more conservative but if you go to NDL no matter what gas blend then really doesn't matter.

1.2 for NDL dives doesn't make sense. Seems like he is trying to avoid CNS, however hitting CNS using 1.4 doing NDL diving is very difficult.

For CCR I run PPO2 of 1.3 but the unit gives me the ability to do dives long and deep enough to hit CNS limits.

As for what to set your computer GF at, it is personal preference. I have mine at 50/70 but that is just me.

 

[Jim Lapenta]

We always ran our PO2 at 1.3 max for cold water due to the increased stress and workload. This was for Lake Erie where the bottom temps were often in the high 30s low 40s F.
We also had a guy we used to dive with tox at 1.35 on a cave dive that was our age (mid 40-early 50s at the time). If you have the time for deco and the gas, there is nothing wrong with running more conservative plans.
While it's fallen out of favor, I used VPM-B for 15 years of technical diving and it worked for me. And my students. All classes the dives were planned using +3 setting due to them being in 40-42 degree water temps with drysuits and doubles/sidemount and stages.
I ran it at +3 conservatism for all my dives as I got older and the last year I did any deep stuff ran it at + 4. It meant a little more time on deco but I was fine with that.
And I flatly refused to dive with people who used more aggressive numbers.
Some rebreather divers I know that are a little older and doing dives in cold water are setting their units to 1.2.