Air break on rebreather, how to proceed?

[tbone1004]

I was told the opposite by two instructors. I was thrown off that early in deco on helium it was harder to keep my p02 up and was told it was due to offgassing. Specifically helium coming out quickly. I also discussed with an instructor that you can see when offgasing slows as deco progresses. I’m fairly new to rebreathers so someone else can comment and tell me if I’m wrong or not. Possible I misunderstood my instructors’ points but I dont think so.

that is what I have been told by several instructors and what I have experienced on longer decos as well.

 

[wedivebc]

Pretty sure the volume of gas is insufficient to alter the loop composition, but I may be wrong.

I've seen some figures on it somewhere from someone cleverer than me!

The volume of inert gas retained in tissues during deco is roughly 1 litre. So depending on the rebreather it can represent 10% to 20% of the rebreather volume. That is more than sufficient to alter the PPO2 noticeably

 

[rjack321]

How do you make air brakes on a rebreather? Do you switch to OC air? Do you change to low setpoint? Which setpoint? How long do you make air brakes, how long do you stay on high oxygen?

Theorie about deep stops, deco and air brakes seem to have changed. So I wonder what's state of the art.

I do air breaks on really long decos now. 2+ hours. Not for CNS reasons but because I get pretty mucus filled and OC gas is dry and help me blow out the snot

 

[rddvet]

I do air breaks on really long decos now. 2+ hours. Not for CNS reasons but because I get pretty mucus filled and OC gas is dry and help me blow out the snot

that was one of the surprisingly important minor details I learned in cave crossover. I suffer from horrible allergies and my sinus are always funky. Nobody ever told me how much cc diving can screw with your sinuses. I was finding on dives like Ginnie and little river that I’d have a ton of congestion build up as I ascended from the greater depths. During the class I even started getting a reverse from ascending through Olsen twice. I never thought to jump to oc and clear all that crap out. Luckily Ken gave me the hint and it’s made a huge difference.

 

[Dsix36]

that was one of the surprisingly important minor details I learned in cave crossover. I suffer from horrible allergies and my sinus are always funky. Nobody ever told me how much cc diving can screw with your sinuses. I was finding on dives like Ginnie and little river that I’d have a ton of congestion build up as I ascended from the greater depths. During the class I even started getting a reverse from ascending through Olsen twice. I never thought to jump to oc and clear all that crap out. Luckily Ken gave me the hint and it’s made a huge difference.

Care to share that hint?

 

[rddvet]

Care to share that hint?

Just to switch over to bailout for a few minutes. It worked surprisingly well. Since the pressure differential is different since it's not all a continuous loop all that pressure and gunk in my sinuses started moving free and I could snot it out. I started getting a bad sinus squeeze on a dive as well and about 30 seconds of OC breathing fixed it. It's a very simple idea, but just one that didn't come as common sense to me. Obviously it's not going to correct everything, but it's really helped when I feel sinus pressure building. I was getting to the point on deco that I couldn't vent out of my nose. Now it's a non-issue.
BTW, Florida and allergies sucks

 

[tbone1004]

@rddvet when you switch over to OC, learn to blow your nose without removing your mask. You can flip the nose pocket up and out of the way so you can blow your nose out easily on your way up.

 

[kensuf]

Yeah I frequently chunk out a big bag of mucous that I swear is the size of a baseball.

 

[jstrang]

I think air breaks are foolish in CCR. Hbot, Long deco in OC ok, but that does not equal it being good for CCR.

personally I just run my dive at 1.2 or lower if it is going to be longer than 4-5 hours. Except in the frigid GL where I’ll run it at 1.3 - but I’m not in the water for more than 90 minutes then.

My reason for 1.2 has less to do with the risk of OT - the reason to do an air break and more to do with myopia. My eyes get messed for a couple of weeks after more than a week of diving daily if I push the SP up to 1.3 for more than about 3-4 hours a day.

The risk of OT is not linear but exponential- the greater the O2 the greater the risk therefore I don’t really see a good reason to keep flushing with O2 and then letting it drift down. Except to check that the cells are linear up to 1.6 - but you only need to do that once a dive beginning or end you choose.

I am reassured that at 1.2 I have 4 hours per day diving as per the NOAA tables. Which might not mean much.

The one thing I think we should remind people is O2 is not like narcosis where you can build up a tolerance. O2 is unpredictable both hypoxia and hyperoxia- one day no issues the next - not so good. I recall a study which I read in med school (years ago) where the British HB doc put recruits in his 2atm chamber and just waited to see when they toxed. Average was 20minutes. Longest was - he didn’t tox after 8!! Hours he went home, shortest was 8 minutes.
The other issue is you can’t stop OT once the cascade is in motion, you just have to ride it out - which in the water may not be survivable.

wish I was diving - supposed to be on the Big O this week.

 

[grantctobin]

I think air breaks are foolish in CCR. Hbot, Long deco in OC ok, but that does not equal it being good for CCR.

personally I just run my dive at 1.2 or lower if it is going to be longer than 4-5 hours. Except in the frigid GL where I’ll run it at 1.3 - but I’m not in the water for more than 90 minutes then.

My reason for 1.2 has less to do with the risk of OT - the reason to do an air break and more to do with myopia. My eyes get messed for a couple of weeks after more than a week of diving daily if I push the SP up to 1.3 for more than about 3-4 hours a day.

The risk of OT is not linear but exponential- the greater the O2 the greater the risk therefore I don’t really see a good reason to keep flushing with O2 and then letting it drift down. Except to check that the cells are linear up to 1.6 - but you only need to do that once a dive beginning or end you choose.

I am reassured that at 1.2 I have 4 hours per day diving as per the NOAA tables. Which might not mean much.

The one thing I think we should remind people is O2 is not like narcosis where you can build up a tolerance. O2 is unpredictable both hypoxia and hyperoxia- one day no issues the next - not so good. I recall a study which I read in med school (years ago) where the British HB doc put recruits in his 2atm chamber and just waited to see when they toxed. Average was 20minutes. Longest was - he didn’t tox after 8!! Hours he went home, shortest was 8 minutes.
The other issue is you can’t stop OT once the cascade is in motion, you just have to ride it out - which in the water may not be survivable.

wish I was diving - supposed to be on the Big O this week.

One does not build up a tolerance to narcosis.