Options for Keeping CELLS DRY

[bubblemonkey2]

A momentary spike is no big deal. Sustained high PPO2 is a different story.
What I am talking about is simple. At 30' and time to move to 20'. A quick blast of O2, a couple of breaths to get it past the cells, verify they still spike past the 1.6 mark. Inside of 30 seconds of initiating this you are ascending and the PPO2 is back down again. No need for 100% flush of O2 in the loop to check for limiting. If you are so close on your O2 units you can't take a 30 second blast of O2, there are other issues with your planning.

I'm not talking about spiking the O2 at 30' then hanging around for 15 minutes at that depth and super high PPO2. Check, get out of there.

If you want to see a number above 1.6, then I imagine that works. But that is unnecessary and misguided, because:

Trying to get 100% o2 in all parts of the loop/lungs/canister/head at once is not a depth dependent process. Just because getting the loop up to 95% o2 can give a reading above 1.6 at a deeper depth, doesn't mean there is actually 100% o2 in there. It just means your ppO2 is too high.

Like other posts say, physically and mathematically it should be impossible to have 100.0% o2 in the system at all times unless you aren't metabolizing. It's going to be asymptotic at any flush depth. Getting 1.60 or above steadily just means you are below 6.0 metres.

You would get the same (or actually faster) nitrogen/inert off-gassing at a ppO2 of 1.3 at ~3 metres, because this would be the same "100%" o2 but at a lower ambient pressure.

 

[broncobowsher]

If you want to see a number above 1.6, then I imagine that works. But that is unnecessary and misguided, because:

Trying to get 100% o2 in all parts of the loop/lungs/canister/head at once is not a depth dependent process. Just because getting the loop up to 95% o2 can give a reading above 1.6 at a deeper depth, doesn't mean there is actually 100% o2 in there. It just means your ppO2 is too high.

Like other posts say, physically and mathematically it should be impossible to have 100.0% o2 in the system at all times unless you aren't metabolizing. It's going to be asymptotic at any flush depth. Getting 1.60 or above steadily just means you are below 6.0 metres.

You would get the same (or actually faster) nitrogen/inert off-gassing at a ppO2 of 1.3 at ~3 metres, because this would be the same "100%" o2 but at a lower ambient pressure.

I'm not trying to reach 100% O2. Only thing I am doing is checking that the cells are still valid and not current limited. They can still read above 1.6 after being stressed by a dive, I am happy with them. Strictly a hardware function check, not trying to get through deco any faster.

 

[bubblemonkey2]

I'm not trying to reach 100% O2. Only thing I am doing is checking that the cells are still valid and not current limited. They can still read above 1.6 after being stressed by a dive, I am happy with them. Strictly a hardware function check, not trying to get through deco any faster.

Ok so the idea is to make sure the cells are capable of reading a potentially dangerous threshold (≥1.6), by simulating that dangerous situation directly: excess o2 in the loop below 6 metres. The logic works out. I wouldn't be teaching that in courses though, it sounds as risky as the "turn off your o2 while underwater" drill, which while survivable, has also resulted in drownings.

 

[rddvet]

Ok so the idea is to make sure the cells are capable of reading a potentially dangerous threshold (≥1.6), by simulating that dangerous situation directly: excess o2 in the loop below 6 metres. The logic works out. I wouldn't be teaching that in courses though, it sounds as risky as the "turn off your o2 while underwater" drill, which while survivable, has also resulted in drownings.

then how are you doing checks for current limiting other than a pressure pot? I try to stay between 20-30 ft and try if I can to keep it shallower than 25 ft when I do them. I don’t see it as unsafe. Even at 30 ft

 

[broncobowsher]

Ok so the idea is to make sure the cells are capable of reading a potentially dangerous threshold (≥1.6), by simulating that dangerous situation directly: excess o2 in the loop below 6 metres. The logic works out. I wouldn't be teaching that in courses though, it sounds as risky as the "turn off your o2 while underwater" drill, which while survivable, has also resulted in drownings.

You are just doing the 20' PPO2 adjustment as you are leaving the 30' mark instead of waiting to arrive at the 20' mark. Remember that people have made planned dives with a PPO2 of 2.0 in the past, for a long time 1.6 was how Nitrox classes were taught. If you want to check for 1.6 at 20' (6m) you are doing a lot of loop flush to get a full loop of O2. Now you have a full loop of O2 and should flush that down a touch. Instead just do a quick spike, not a full loop of O2. Drop you PPO2 by ascending instead of flushing. It really is easier and I will even say safer. There is no way that a ~20 second spike of 1.7ish is going to create a problem. I don't even wait for the numbers to stabilize. Do the blast, watch all the cells turn red, I'm heading up before they are stable. Stop at 20' and purge some loop gas, numbers are good again.

I won't do a shut down drill unless I am with another rebreather diver I trust and it has been planned ahead of time.

 

[bubblemonkey2]

then how are you doing checks for current limiting other than a pressure pot? I try to stay between 20-30 ft and try if I can to keep it shallower than 25 ft when I do them. I don’t see it as unsafe. Even at 30 ft

So those who want to be 100% sure that ≥1.6 is a possible reading on their sensors are free to do an o2 flush below 6m/20ft. I'm not sure that's safe as a general recommendation to an increasing number of rec-tech CCR divers though?'

I'll take my 1.55 to 1.59 at "6 metres" if we're talking about an ascent.

A careful o2 flush around 6m prior to descent sounds more applicable for those wishing to guarantee the upper range of the sensors before undertaking the more dangerous part of dive with respect to high o2 sensitivity. Properly carried out, the dil ADV or MAV does its job to dilute the mix safely as the descent proceeds. But that too could be done improperly resulting in excessive exposure. (?)

 

[bubblemonkey2]

You are just doing the 20' PPO2 adjustment as you are leaving the 30' mark instead of waiting to arrive at the 20' mark. Remember that people have made planned dives with a PPO2 of 2.0 in the past, for a long time 1.6 was how Nitrox classes were taught. If you want to check for 1.6 at 20' (6m) you are doing a lot of loop flush to get a full loop of O2. Now you have a full loop of O2 and should flush that down a touch. Instead just do a quick spike, not a full loop of O2. Drop you PPO2 by ascending instead of flushing. It really is easier and I will even say safer. There is no way that a ~20 second spike of 1.7ish is going to create a problem. I don't even wait for the numbers to stabilize. Do the blast, watch all the cells turn red, I'm heading up before they are stable. Stop at 20' and purge some loop gas, numbers are good again.

I won't do a shut down drill unless I am with another rebreather diver I trust and it has been planned ahead of time.

Right so that's useful if you want to ensure that a value of ≥1.6 is possible on the sensors. I guess I read things too quickly. But that should be done as a pre-descent check, if the goal is to make sure the sensors are working on the high end before you trust your life to them at depths where it's relevant.

 

[rddvet]

But that should be done as a pre-descent check, if the goal is to make sure the sensors are working on the high end before you trust your life to them at depths where it's relevant.

That's very debatable and subject to personal opinion and your personal diving. I rarely do it on the way in. I almost do it on the way out. Mainly because alot of the caves I dive are a pain to stop at around 20 ft to do an oxygen flush. I'm of the opinion that if my cells are not current limited at the end of the dive then they will probably not be current limited next time I dive them. I keep track of it and linearity over time and can note negative trends. Some will argue that doing it at the beginning of the dive is better, but I think that's all personal opinion. The argument that you want to know they're good going into the dive is a good one, but a cell can also become current limited halfway through your dive. I typically choose to check at the end, but will check at the start if the dive allows. I've been taught or have discussed different views on this topic by multiple instructors. There is no consensus.

 

[rjack321]

So those who want to be 100% sure that ≥1.6 is a possible reading on their sensors are free to do an o2 flush below 6m/20ft. I'm not sure that's safe as a general recommendation to an increasing number of rec-tech CCR divers though?'

I'll take my 1.55 to 1.59 at "6 metres" if we're talking about an ascent.

A careful o2 flush around 6m prior to descent sounds more applicable for those wishing to guarantee the upper range of the sensors before undertaking the more dangerous part of dive with respect to high o2 sensitivity. Properly carried out, the dil ADV or MAV does its job to dilute the mix safely as the descent proceeds. But that too could be done improperly resulting in excessive exposure. (?)

Are you CCR certified? If so on what unit?

This all sounds very pedantic to me. Doing an O2 flush at 30ft and ascending to 20ft is not rocket science and despite being quite common I've never heard of anyone ever toxing from this practice ever.

 

[bubblemonkey2]

Are you CCR certified? If so on what unit?

This all sounds very pedantic to me. Doing an O2 flush at 30ft and ascending to 20ft is not rocket science and despite being quite common I've never heard of anyone ever toxing from this practice ever.

Yes this might make it easy to register an exact "1.6" (or higher) on the cells. If that's the goal, then I understand that logic. Apparently that was the goal according to previous posts--so that is already resolved.

However I don't see the logic of teaching to do an O2 flush *below* 6m/20ft, if the goal is to stay at or below a ppO2 of ~1.6 (aka 100% O2 at 6m/20ft). People who feel it's fine to potentially go above 1.6 briefly will be comfortable with that, I guess. If that's what people are doing, they can continue. It's their dive... No my CCR tech training did/does not advocate premature O2 flushing. The flush is at 6m. I'd rather just stay in the water a few minutes longer, than unnecessarily risk going above 1.6. 1.55 isn't going to ruin your deco.

Even a pp02 of 1.5 at 5 metres / ~16ft for example is still 100% O2, and would be both a) safer according to oxtox theory and b) faster off-gassing of inert gases due to lower ambient pressure--as long as your deco ceiling allows it. But I'm not saying that everyone should do 5m stops instead of 6m stops. I'm saying that fixating on "1.6" as a magic value is missing the point of deco, which is not about ppO2 itself, but rather about %O2 and ambient pressure.