Is my approach to diving with Nitrox logical?

[scubadada]

Hi @Nitrox_DiverNY

I don't think you have shared with us what computer you are currently diving. Some deco algorithms are inherently more conservative than others

Why are you so concerned with DCS in no stop diving, do you think you are at increased risk?

Where did you dive in South Florida, in Palm Beach Co around 36% and around 32% are readily available. We generally dive 36% shallower, MOD 95 ft, and the 32% deeper, MOD 111 ft.

 

[rx7diver]

NAUI?

1.4 has been the recreational standard for at least a quarter century.

Not really, 1.5 was for a while.

My IANTD Recreational Nitrox cert is from Summer 1993. We used Rutkowski's "IANTD Enriched Air Nitrox Student Manual and Workbook" as our textbook. The threshold we learned for depth was PO2 <= 1.6 ATA. And in the case of thermal stress, or increased workload, or planned decompression dives using rich nitrox or pure oxygen, then the threshold for depth was PO2 <= 1.5 ATA.

We were doing extended range diving (usually using air as a bottom gas, and pure oxygen for deco at 20 ffw, and 5 min air breaks every 15 minutes or so) in the Great Lakes, so we set our bottom limit at PO2 <= 1.5 ATA (almost always). Bottom times were not terribly long, usually 25-30 minutes. 20 minutes if very deep. Max two dives per day.

rx7diver

 

[tursiops]

My IANTD Recreational Nitrox cert is from Summer 1993. We used Rutkowski's "IANTD Enriched Air Nitrox Student Manual and Workbook" as our textbook. The threshold we learned for depth was PO2 <= 1.6 ATA. And in the case of thermal stress, or increased workload, or planned decompression dives using rich nitrox or pure oxygen, then the threshold for depth was PO2 <= 1.5 ATA.

We were doing extended range diving (usually using air as a bottom gas, and pure oxygen for deco at 20 ffw, and 5 min air breaks every 15 minutes or so) in the Great Lakes, so we set our bottom limit at PO2 <= 1.5 ATA (almost always). Bottom times were not terribly long, usually 25-30 minutes. 20 minutes if very deep. Max two dives per day.

rx7diver

The 1.4 I mentioned was for recreational diving, not technical.

 

[DeepSeaExplorer]

When I was first Nitrox certified, it was 1.6. Then, they starting saying a “target” MOD of 1.4. Then it eventually became just 1.4. Some are even pushing to use 1.2. It’s still 1.6 for deco.

 

[doctormike]

The MOD of 40% at 1.4pp02 is around 82 ft, wouldn't using a 40% mix under that depth be a more conservative approach? so if your diving to a Max of 60 why not use 40%? I know DCS in rec is rare, but doesn't it make sense to use rich mixes when the dive allows to add in possible safety? why not be as safe as possible?

I'm not suggesting that you get a rebreather, but you do bring up an interesting point.

The deco advantage of closed circuit diving at moderate depths is exactly what you describe - you have the optimal mix for every depth, mixed on the fly, to minimize your N2 accumulation and maximize your NDLs. Instead of the FiO2 being constant (as it is in open circuit diving), the PPO2 is constant - typically around 1.3. Now this isn't done to reduce the already very low risk of DCS, but rather to minimize N2 loading, reducing the deco obligation.

If I'm diving with an open circuit diver at a depth where he or she is well above MOD for their mix, I will have a big deco advantage over them. On the other hand, if they are close to their MOD, they may actually have an advantage over me, since I run a PO2 of 1.3.

And getting back to your point... if, for example, I were diving at 33 feet (2 ATA), my rebreather would be mixing up the equivalent of EAN 65 if I had my PO2 setpoint set to 1.3 (in practice, the setpoint is usually lower at shallow depths).

 

[Blackcrusader]

im going to have to look into gradient factors and algorithms, that is all new to me but it sounds like a great solution for me. Thanks!

Firstly know your own DC. I changed from a Suunto to the Shearwater Perdix and spent several hours reading the manual and learning settings etc. I dive my Perdix in OC Tec mode not the OC rec mode. I also set to a GF45/95 as that is the least conservative setting.
LOW GF only applies to deco dives so that is not relevant for diving within NDL limits. I do a lot of diving on air with say 40 dives over 12 days of diving. Never had an issue with DCS in over 3 decades of diving including deco dives.

Then also you can upload your dive logs to divinglog.com or the manufacturers dive log program. I've done a lot of dives where I use nitrox 32% but use my DC in 21% mode.

Gradient Factors and recreational diving

I am looking at transitioning into a computer that uses Gradient Factors. I have superficially read some data on the theory and have a basic idea of theory. I currently use an Oceanic computer set to DSAT and run it pretty close with EAN. Last week in CZM I did 24 dives (26 hours) with most...

scubaboard.com

 

[Nitrox_DiverNY]

EACH dive computer out there, recreational types, allow you to change the conservatism factor from the default (default is usually the least conservative).

tomorrows goal will be Gradient factors research and tweaking with the computer, this is honestly something I have completely overlooked

 

[Nitrox_DiverNY]

You need not worry about gradient factors if you haven't done training to learn what this is all about. As a recreational diver, just user a higher conservatism setting in your dive computer. If the default is "0," just set the dive computer to "1" for example. You don't need to go crazy with setting the conservatism factor to very high limits to the point where you don't get any dive time.

interesting, so GF is not really a priority but the computers conservatism is. I will look into it, and read my computers manual but from what you describe the higher the number is more conservative?

 

[tursiops]

interesting, so GF is not really a priority but the computers conservatism is. I will look into it, and read my computers manual but from what you describe the higher the number is more conservative?

The lower GFs are more conservative. The numbers are percentages of a maximum. 100% means no conservatism; 70 means more conservatism than 85. However, "conservation factors" might mean anything. You cannot count on (for example) "2" being more conservative than "1". You have to RTFM.

 

[Nitrox_DiverNY]

One other option for increasing conservatism without lying to your computer, changing the conservatism settings, or leaning about gradient factors and getting a computer that uses them, is to set for yourself an artificial NDL cushion. Mine is 5 minutes; when the NDL gets down to that, I begin my ascent. If I'm swimming in toward shore on a gradual slope such that my NDL continues to go down as I slowly ascend, I come up off the bottom to keep the number at 5 or above. I treat 5 like it's zero, and in the event I fail and go below 5 into fake deco, I do a longer safety stop. You can set a higher number if you like. You can set an alarm for it, on some computers. That way, you always know where the line really is, but you always stay above it.

this was very helpful, I will indeed look into gf and the artificial NDL cushion is a great idea! thanks for sharing