Is my approach to diving with Nitrox logical?

[Nitrox_DiverNY]

You should not lie to or trick your computers. This will eventually or at least potentially cause confusion or worse. And computers do quit, now twice for me. I am old school enough that while I do now use two computers for some years now, I can run multi-level dives in my head (well, I used to could good enough).

Set your computers to the mix you are using. Set the GF and conservatism setting to your comfort level, use an adaptive or hard 5 minute safety stop (dare I mention a deep "pause"). Even as suggested use an additional self imposed wall (5 minutes before NDL or whatever).

Some computers do not have adjustable settings, the altitude has to be increased, I suggest if you have such a thing it might be time to visit your LDS or Amazon or something. But do not trick or lie to your computer, that is a bad pratice that leaves everybody guessing (you, your buddy, buddies, the DM). And, all dives should have a plan, Plan A, B etc. Does not have to be elaborate, it might be swim down and then swim along the wall for 30 minutes at maximum 80 feet, ascend to reef top at 40 feet and swim in the return direction and then swim up with X safety stop. So, when you are near 45 minutes and have not turned your dive yet and your computer is beeping low NDL, deco required or such, do you or do not you, what is right, what do you do? That is the question. Set the computers correctly please, dive your plan.

James

thanks for sharing, that was great info and I won't be lying to the computer, I can see now this is a safer practice. I will be definitely looking into GF and conservatism. Conflicting computers would be an very very confusing, and the idea self imposing wall is good. Thanks for the help

 

[Nitrox_DiverNY]

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.

I am using a Mares smart pro wrist computer

DCS, its just that one thing that really sticks in the back of my mind after surfacing from a dive, I probably have read to many horror stories about it. I don't think I am at increased risk

I dive in the Lauderdale bu the sea area off boats and the shore, and will be starting to try and dive frequently in NY where I live. I always wanted to check out that bridge in palm beach

 

[Nitrox_DiverNY]

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).

this is very interesting, so no matter the depth you always get the best mix possible by getting the pp02 right to 1.3 every time. I guess what I am thinking is that the reduced N2 loading, should reduce DCS but then again that is time at depth to.

how common is low pressure pulmonary toxicity with rebreather divers if they are using the ultra rich mixes?

thanks for sharing that was very interesting, I would love to get into rebreathe in the future

 

[Blackcrusader]

I am using a Mares smart pro wrist computer

Well do not assume your DC runs a GF software algorithm. My Suunto used the RGBM algorithms and not GF's relate to Bulhmann.
My Perdix uses the Bulhmann and gradient factors. So with the Perdix I get longer NDL times over the Suunto when I am doing 40 dives in 12 days. The Suunto was quite conservative and you would need to dive a lot shallower even is using the least conservative setting.

Perhaps you may want to reconsider your changing PPO2 1.4 to other settings if you do not fully understand the reasons for doing so.

Conservative settings on Mares computers

Does anyone happen to know what the P0 (standard setting / least conservative), P1 & P2 (most conservative) settings actually influence or refer to in the range of Mares dive computers? It would be helpful to understand if anyone can relate that to comparative gradient factors - I KNOW the...

scubaboard.com

 

[MaxBottomtime]

I guess what I am thinking is that the reduced N2 loading, should reduce DCS but then again that is time at depth too.

You are overthinking this as a new diver. DCS is not as common as you believe. There are hundreds of thousands of dives made every year, maybe more. Most are on air or 32%. If there were a lot of DCS cases, every training agency, dive resort, and commercial dive boat would have to re-write their policies. I know divers who began diving in the early 60s and dived deep air for decades without getting bent. I got certified in 1989 and have avoided the bends by making slower ascents than recommended. I strongly suggest you get in a lot of dives before considering rebreathers and/or high O2 concentrations for relatively shallow recreational dives.

 

[inquis]

how common is low pressure pulmonary toxicity with rebreather divers if they are using the ultra rich mixes?

They are not using ultra rich mixes, in part to avoid toxicity issues. An OC diver may target 1.4 atm as the max ppO2, but will likely be shallower (with less ppO2). CCR dives are typically longer AND almost all of it is at the high setpoint so something lower than 1.4 is typically used. It's the combination of ppO2 AND time that matters.

 

[BlueTrin]

@Nitrox_DiverNY the gradient factors are just a % of distance you add for “conservatism” against the Buhlamn algorithm: there are two gradient factors GFLo and GFHi, if you set them to 100% you are diving the unmodified Bühlmann algorithm.

This picture below explains it, the ascent starts on the right. The M-value line is the ascent from using 100% Buhlman.

IMHO, although the question is interesting, you will probably be ok by just diving your computer algo for recreational depth. If you want to add some conservatism maybe try to not reach your NDL or do a longer safety stop?

 

[Graeme Fraser]

@Nitrox_DiverNY the gradient factors are just a % of distance you add for “conservatism” against the Buhlamn algorithm: there are two gradient factors GFLo and GFHi, if you set them to 100% you are diving the unmodified Bühlmann algorithm.

This picture below explains it, the ascent starts on the right. The M-value line is the ascent from using 100% Buhlman.

IMHO, although the question is interesting, you will probably be ok by just diving your computer algo for recreational depth. If you want to add some conservatism maybe try to not reach your NDL or do a longer safety stop?

View attachment 733466

I find this diagram from Deco for Divers useful when explaining ascents in relation to ambient pressure, M value and gradient factors.

At first it appears counterintuitive as we tend to think of a graphic ascent profile moving bottom left to top right, whereas this is depicted in reverse. Once you've got your head around that concept, it all makes sense.

 

[scubadada]

I am using a Mares smart pro wrist computer

DCS, its just that one thing that really sticks in the back of my mind after surfacing from a dive, I probably have read to many horror stories about it. I don't think I am at increased risk

I dive in the Lauderdale bu the sea area off boats and the shore, and will be starting to try and dive frequently in NY where I live. I always wanted to check out that bridge in palm beach

Hi @Nitrox_DiverNY

Thanks for answering my questions

Mares RGBM is one of the most conservative decompression algorithms. When you are close to your NDL, other divers with a more liberal decompression algorithm will still have considerable NDL time. Keep in mind that all dive computers are very safe, this simply represents the range in NDL values available. Have you dived with someone using DSAT or Buhlmann with a GF high of 95? You would likely signal that you are getting low on NDL while they had considerable time left. You have probably realized this but, as your Mares computer does not run Buhlmann ZH-L16C, you do not have gradient factors to adjust your decompression algorithm.

As others have pointed out, the rate of DCS is very low. I would not be overly concerned. Instead, you should follow all the diving guidelines you have been taught. Make your ascent at the correct rate, make your safety stop with a good controlled final ascent to the surface.

It appears that your diving in Florida has not yet exposed you to the common banked nitrox mixes of 32 and 36%

Best of luck in your diving

 

[doctormike]

this is very interesting, so no matter the depth you always get the best mix possible by getting the pp02 right to 1.3 every time. I guess what I am thinking is that the reduced N2 loading, should reduce DCS but then again that is time at depth to.

how common is low pressure pulmonary toxicity with rebreather divers if they are using the ultra rich mixes?

thanks for sharing that was very interesting, I would love to get into rebreathe in the future

Remember, it’s not that the CCR is giving you an “ultra rich mix”, although yes if you calculate the equivalent FiO2 at shallow depths it is much higher than any sort of recreational nitrox mix you would get at a dive shop for your back gas tank.

Pysiologically, FIO2 doesn’t matter, PPO2 does. So the O2 exposure is the same for a CCR diver running 1.3 at shallow depths and an OC diver a bit above their MOD.

Pulmonary oxygen toxicity is pretty rare, have never seen it, I think it’s more of a consideration for commercial divers. Others can correct me about that.

And remember, DCS risk is proportional to decompression stress, which is why the ascent is more important than the total N2 loading. When I got bent, it wasn’t a very long or deep dive, but rather a bad (multiple) ascent issue.