Nitrox on boat with air refill

[Centrals]

Did you not follow the link? 18m limit etc. I'll paste it for you.


"Who should take this course?

Rebreathers used to be only for technical diving, but not any more. New Type R (recreational) rebreathers are lightweight, easy-to-transport and have sophisticated electronics to simplify their use. Why dive a rebreather? You get longer no stop limits, reduced gas consumption because you reuse most of your exhaled gas, and unmatched wildlife encounters because you don’t release annoying bubbles. The PADI Rebreather Diver course introduces you to rebreather diving to a maximum depth of 18 metres/60 feet and lets you experience things you never imagined possible as a scuba diver."

I did and this is my question:
What mix would they have use to get into CNS100% at 10m in a single dive of 3hrs?
The course only requires basic nitrox training ie. 40% max. Therefore at 10m and using 40% nitrox the PPO2 is 0.8. According to NOAA Single Exposure Table, at PPO2 of 0.8 the single exposure is 450mins.
3hrs of EAN65(PPO2 1.3) at 10m will get you CNS100%.
This is my understanding on the NOAA's table and am I wrong?

My knowledge on CCR is definitely out-date and thank you for the link.

 

[KenGordon]

You'd run a constant ppo2 of 1.3.

Typically there is air and 100% O2 available to the box to mix as a breathing gas. At the surface it will add O2 to get to a ppO2 of 0.7 and deeper to 1.3. Obviously you need to be below 3m to get to 1.3. Some switch gradually, some have a particular depth.

The fractional O2 pressure, which is the EAN percentage equivalent, changes with depth to maintain that 1.3 ppo2.

Thus the user of such a rebreather is not limited to 40%.

The 3h at 1.3 cns limit lines up with the 3h scrubber limit on many popular rebreathers as the effective dive time limiting factor.

CCR isn't recreational diving, is it?

I think that 'recreational' and 'technical' are marketing terms. In particular the typical SB use of 'recreational' to mean no deco and 'technical' to mean maybe deco does not match how diving is done in many places.

 

[Centrals]

You'd run a constant ppo2 of 1.3.

Typically there is air and 100% O2 available to the box to mix as a breathing gas. At the surface it will add O2 to get to a ppO2 of 0.7 and deeper to 1.3. Obviously you need to be below 3m to get to 1.3. Some switch gradually, some have a particular depth.

The fractional O2 pressure, which is the EAN percentage equivalent, changes with depth to maintain that 1.3 ppo2.

Thus the user of such a rebreather is not limited to 40%.

The 3h at 1.3 cns limit lines up with the 3h scrubber limit on many popular rebreathers as the effective dive time limiting factor.



I think that 'recreational' and 'technical' are marketing terms. In particular the typical SB use of 'recreational' to mean no deco and 'technical' to mean maybe deco does not match how diving is done in many places.

The pre-requisite for the course is OW, basic nitrox and 25 logged dives!
Are you suggesting that with this min qualification, the student is allowed to handle 100% O2? Even the Adv nitrox student can only handle 50%O2.

PADI pre-requisite for SCR(Atlantis/Dolphin) is only ow + nitrox + 10 logged dives, while it is Adv nitrox for IANTD!!!!

Breathing PPO2 of 1.3 at 10m will eat up your single exposure of 180mins but you have to breath EAN65 for the luxury. And that is for a OW diver with 25 logged dives.

---------- Post added November 13th, 2015 at 02:57 PM ----------

Centrals,

Please don't wait on my account! Have at it, if you wish. And, if you wish, show your result here. When I have time, I'll do likewise. Then we can compare our two results. And we'll get to see if my suspicions bear out. Now, won't that be fun??!!

Safe Diving,

rx7diver

Ken has kindly provided a solution: One dive of 3hrs in EAN65 at 10m on a rebreather.

 

[KenGordon]

The pre-requisite for the course is OW, basic nitrox and 25 logged dives!
Are you suggesting that with this min qualification, the student is allowed to handle 100% O2?.

Yes. A CCR needs 100% to do the blending.

I't expect the course content to cover proper handling of the gas. The type r does not allow manual adds I believe so they should not be able to directly tox so easily by blasting the loop full of o2 at depth.

So are we agreed, the OP should now get a rebreather? That way he can skip all the arguments over the best sort of BC, how to route his hoses and go directly to whether deeps stops are a good idea.

 

[Centrals]

So are we agreed, the OP should now get a rebreather? That way he can skip all the arguments over the best sort of BC, how to route his hoses and go directly to whether deeps stops are a good idea.

No, the OP didn't ask anything like that. He/she was concerned for not analyzing the tank after AIR top up on the left over from the previous nitrox dive.
The disagreement on CNS% toxicity is from someone else and unfortunately he/she hasn't came back with his/her calculation/plan yet.

The problem is how many breathers are happy to stay at 18m? It is on a very slippery slope.

Cheers.

 

[IVC]

So are we agreed, the OP should now get a rebreather? That way he can skip all the arguments over the best sort of BC, how to route his hoses and go directly to whether deeps stops are a good idea.

There are no stupid questions, only stupid answers.

The parameters were as follows: (1) Air over Nitrox, (2) No tables, (3) No sensor, (4) Mares Puck Pro basic DC. All the smarta@@ comments conveniently "forgot" at least one of the constraints above. If you have a need to be condescending, at least stay within the scope of the question and don't project what you *think* the question was.

There were some great answers in this thread, from theoretical to practical, and I made sure I thanked and acknowledged the ones that were a learning experience. As a result, I now have an O2 sensor and a set of tables so I can quickly get the ballpark numbers for a range of mixes. That's the most important part to me - knowing that I'm staying within limits of the accepted models.

Then there are posts like yours. The final irony in your condescending tone towards gear and setup is that you didn't pioneer either of the two, so you yourself had to learn that from somebody by asking (gasp) what you consider "stupid questions."

---------- Post added November 13th, 2015 at 09:42 AM ----------

No, the OP didn't ask anything like that. He/she was concerned for not analyzing the tank after AIR top up on the left over from the previous nitrox dive.

Exactly - thanks.

What I've learned from the majority of posters is that CNS toxicity due to O2 exposure at recreational depth requires times that significantly exceed NDL times for exposure to N2. Simple concept, but not something that can be determined through some ad hoc calculation. Instead, it's a table lookup.

 

[JohnnyC]

As a result, I now have an O2 sensor and a set of tables so I can quickly get the ballpark numbers for a range of mixes. That's the most important part to me - knowing that I'm staying within limits of the accepted models.

Having an O2 analyzer and tables is never a bad idea. Just make sure you know how to use both as well as calibrate your analyzer, and how to tell when the cell is dying. Second, the crux of the issue is that you are equating "knowing" with having a precise measurement to reference against. In reality, "knowing" doesn't need to be precise at all given you have enough information that you can create a dive plan that remains safe, is even more conservative than your original dive, and relies on simple choices that do not require precision. You have the one variable you need.

What I've learned from the majority of posters is that CNS toxicity due to O2 exposure at recreational depth requires times that significantly exceed NDL times for exposure to N2. Simple concept, but not something that can be determined through some ad hoc calculation. Instead, it's a table lookup.

It's actually quite the opposite, in fact you DON'T need tables to make an informed determination. You don't need to determine the extent of which it exceeds NDL, only whether it exceeds or doesn't exceed. We know that since it must SIGNIFICANTLY exceed in order to be an issue, this it is no issue at all. You don't need a table to prove that. As long as you are diving within NDL's, you are safe by a WIDE margin. You will also be far away from the gas requirements needed to execute a dive to that length of time.

Realistically, the lesson you should take from this is that in the recreational sense, a procedure taking into consideration all of the variables you mentioned is quite easy, requires no calculation nor table use. In fact, all of those variables are extraneous save your initial O2 percentage. I understand the desire to be precise, but at the level involved, there really is no pressing need. You will be far away from any of the risks specifically associated with breathing enriched air.

Here's why.....
By setting your computer for air you are being conservative as your mix realistically has several percentage points of O2 higher, thus taking advantage of the extended NDL time that enriched air potentially offers. The amount above air doesn't matter, it is of no concern.

By using the MOD of the original mix, you are being conservative as your actual MOD is deeper than indicated. You will stay above this artificial floor, thus staying above your actual, but unknown MOD. The amount below the original doesn't matter, it is of no concern.

The end result is that you are padding your conservatism on two fronts, and because of the depth, time, and gas volume, you are at no risk for either oxtox due to exceeding your MOD, CNS tox due to exceeding your total exposure, and less risk of DCS as you are actually breathing a mix with a lower inert gas content.

All of these are determined simply by knowing your original mix, and knowing that it's being topped with air. Is it more conservative than if you had all of the exact data? Yep. Is that an issue? Nope. Certainly not to me. The difference is negligible in the grand scheme of a diving career.

 

[Freewillow]

Well said. this is exactly the essence of what I said on a much earlier page. But I did not get a thank you nor a like from IVC . So I am NOT happy, unless he did not understand it at this stage .

@IVC.

I really love your argument.

The "problem" comes from the fact that the boat is making a refill with air in a tank that has a nitrox left over.

Now consider this.

1. Good practice demand that the second dive is a shallower dive that the first one. Correct?

2. The O2 content of the tank will be anywhere between the original Nitrox mix % and air. Right, you do not need a calculator to find this out

So the advise on the first page of this subject was: put your computor on air and do not worry about the nitrox content.

As long as your max depth is lower than in the first dive and your NDL is calculated on air, there is NO PROBLEM.


I hope that you got it, this time .

 

[IVC]

...CNS tox due to exceeding your total exposure, ...

This is the only one that I still don't see how you can know/estimate unless you already know the relationship between CNS and NDL time limits.

The rest of your post is exactly along the lines of how I was thinking on the boat: keep original MOD, use air for NDL - conservative on both ends. The problem was I had no idea how to estimate or figure out the effect of O2 on CNS since "air" is a conservative setting for NDL, but aggressive setting for CNS. My gut feeling was that it was "close enough" and it wouldn't be a problem, yet I had a bad taste in my mouth about already having to use "gut feeling" on my first post-OW dive - it shows lack of planning. Also, I was looking at little "CNS bar" on my DC and it was non-zero, so I wasn't sure how fast it was "filling up," especially relative to the "NDL bar."

I'm glad I started this thread because I learned a thing or three, even if I got some scorn in the process.

---------- Post added November 13th, 2015 at 11:50 AM ----------

But I did not get a thank you nor a like from IVC . So I am NOT happy, ...

Here, I gave you a "like." I couldn't live with myself knowing I hurt someone's feelings on the Internet. Sheesh, you Belgians are demanding...

 

[Freewillow]

No relation between NDL and CNS. CNS is related to PPO2 and time, NDL is a function of depth, time and Nitrogen content of your gas.