O2 Cell Replacement Time....

[jale]

Just to add an opinion...
I dive mccr. Loop flushed and filled with o2 just before going down and adv close.
Going down you need to add o2 to breathe and you see the reading going up.
Reaching or passing 6M, after another look at the monitor to see the ppo2, you breathe gas out, inject dil, open adv and you go down. Yes, you may go beyond 1.6 but it is only temporary and you can control it buy adjusting descend speed and dil addition. No need to he too anal alsoabout quick burst of high ppo2.
I also check at least twice the cells wirh a flush during the dive. Yes, it consumes dil and even o2 to go back to a good ppo2 but I don't see as a problem.

 

[michael-fisch]

Have had cells that develop current limiting after a few minutes, Oxygen flush at the beginning of the dive didn't catch them, but 60 minutes of 1.3 PO2 did. Thankfully it has never happened to more than 1 cell at a time.

Michael

 

[tbone1004]

Right, but do you worry about current limitation developing between dives? A day, a week or a month apart? Is there any interval where you would feel safe relying on the check from the last dive?

not really no. You validate calibration during setup before the dive, so the loop is already full of O2. I do my negative check first, then do positive with O2 so the loop is already flushed. You do your descent on O2, check for 1.6 at 20ft, then depending on where you are and what unit you dive, either just add dil during descent and it will come down fast enough, or dil flush if you are on a leaky valve that isn't shut off and need to hot drop.

One of the arguments from a rather well-known instructor is that it uses the cells faster so you don't want to do it for cell life. I about rolled out of my chair laughing when I heard that. It is true that it consumes some of the electrolyte, but you're about to go on a 3+ hour dive at or above 1.0 with at least an hour of it at or around 1.6 so how does less than a minute at 1.6 become relevant? I'm not entirely sure said instructor actually understands how cells work as he also says that linearity isn't a thing....

@michael-fisch 's issue is why I don't want to replace them all at the same time, and also the reason that I don't universally dive a really low dil. I try to keep my dil somewhere around the ppO2 that I want to run the unit. That way when I dil-flush, I'm validating the ppO2 reading that I want to run vs. spiking it low which doesn't really do anything IMO

 

[doctormike]

OK, so the consensus here is check on descent, not on ascent. Reasoning is good, so I will change my approach. Thanks!

 

[tbone1004]

@doctormike checking on ascent doesn't hurt. I was taught that doing the flushing on your way from 30ft to 20ft will give you the momentary spike over 1.6 to check for limiting above 1.6 but you're on your way to 20ft, so it is momentary. I'm not concerned about oxtox at 2.0 while it is rapidly dropping down anyway, but it's good to be able to see that.

 

[JonG1]

Depends on unit/duration/water temps etc. but may not get as good a flush on ascent due to humidity on cell face and presence of inerts in the head from the dive, i do both but don't fret if i am not bang on on ascent flush.

 

[kelemvor]

I thought there was some new solid state o2 sensors on the market that were supposed to fix many of the problems with galvanic. I vaguely remember them not requiring calibration and supposedly lasting nearly forever. Did those not actually become available or was the hype all just marketing bs?

 

[darylm74]

A lot of good discussion here. I will stick with the Vandagraph cells for now but may work one of the AIs in the mix to see the difference. I usually do the O2 flush on the way down too but I haven't done any hot drops with mine yet as I am usually diving the great lakes or quarries, so I am normally in no hurry. I get down to 20, do my flushes and checks and then head down. I am usually waiting on the OC divers to join my buddy and I anyway.

I think the solid state are still in the works last I read.

 

[cathal]

The argument I’ve heard about Oxygen flush at 20’ is that if you forget to flush with DIL you could tox and die. My argument to counter that is that if you are too stupid to remember to flush after you have remembered to check your cells, you’re too stupid to dive a rebreather.

There’s simply no reason not to do it at the beginning of a dive unless you’re too stupid to be diving a rebreather in the first place.

I thought the dive industry was trying to develop and encourage a 'Just Culture', whereby divers can feel free to talk about the mistakes they made in the dive community without fear of being called derogatory terms such as 'stupid'. That way everyone learns from everyone else's mistake including their own. Its called Human Error, it does not mean your stupid but labeling people as being stupid for making a mistake is one sure fire way of getting them to stop divulging their mistakes and then we all lose as no-one gets to learn from other divers errors.


Cathal

 

[tbone1004]

I thought there was some new solid state o2 sensors on the market that were supposed to fix many of the problems with galvanic. I vaguely remember them not requiring calibration and supposedly lasting nearly forever. Did those not actually become available or was the hype all just marketing bs?

Poseidon released them. Used on the Se7en right now, but have not gone to the other manufacturers yet so you can only use them in one CCR. Hoping that it will change very soon. The solid state cells are optical so wouldn't require calibration after the factory calibration, and being LED based would certainly outlast the rest of the unit on an hours basis.