O2 Cell Replacement Time....

[rddvet]

I can give you two first hand accounts...
I was part of a massive Maxtec 305 recall. If you’ll look, you’ll notice Maxtec 305 was replaced with a new model, I think it’s 305F now or something like that.

Also, early on, I got a bad batch of AI’s, but that was a fluke. I certainly don’t think it’s the norm.

Today, at least a few manufacturers are stating that you should replace all sensors at once. I talked to Dive Soft recently regarding the Liberty. Because that machine is so precise, (as I understand it) it wants to calibrate cells that are exactly the same. Apparently there are very minor variances in cells prior to calibration. My optima doesn’t care. It’ll take a very broad array of cells and work seamlessly, but if I put three different cells in a Liberty, it freaks out on the dive, voting out dissimilar cells constantly.

When I took rEvo class, Paul Raemakers said to spread out cell replacement. Recent classes have told me to buy all in the same batch. I think that bad batches are rare, and we test for cell linearity before and during a dive. We have protocols to ensure our wellbeing and I’m not afraid of a bad batch of cells. But I do think it’s unlikely but possible.

The old Maxtec was one of the things my instructor talked about. But basically said that problems really haven't occurred recently. All were a while ago.

When you say you test for cell linearity during the dive are you talking about an oxygen flush at 20ft? It seems I was told checking cells on a dive is pretty standard in training, but I'm finding that's not true.

 

[Superlyte27]

Actually, there is a large faction of instructors who are teaching NOT to do O2 flush. I do not agree with that approach.

 

[doctormike]

Actually, there is a large faction of instructors who are teaching NOT to do O2 flush. I do not agree with that approach.

Yeah, what's up with that? I do a flush on ascent. Is there a case to be made against it?

 

[rddvet]

Actually, there is a large faction of instructors who are teaching NOT to do O2 flush. I do not agree with that approach.

That's the thing. I'm finding there's lots of differences in what people are taught. In my course we discussed doing the math on the surface comparing cells in air vs oxygen, oxygen flushes in the deco depth ranges, and dil flushes at depth to confirm p02 matches what's expected. My instructor basically left it open-ended for us to discover what we preferred when dealing with cells, but gave us the skills to use and figure out how we want to dive as opposed to saying "it's this way and that's the only way:

Yeah, what's up with that? I do a flush on ascent. Is there a case to be made against it?

I've been told oxygen flush on descent and oxygen flush on ascent, both work. Some people feel it should be done on the way down because doing it at the end of the dive is too late. Others have said it doesn't matter, you're looking at linearity over time so it's not like it's going to jump out bite you all of a sudden. Their point is doing them at the end of a dive on ascent is to be able to make a mental note to compare to on future dives.

I'm finding rebreather diving to be like sidemount, there's a million different ways to do the same thing and there's not a complete concensus on which is right.

 

[doctormike]

I've been told oxygen flush on descent and oxygen flush on ascent, both work. Some people feel it should be done on the way down because doing it at the end of the dive is too late. Others have said it doesn't matter, you're looking at linearity over time so it's not like it's going to jump out bite you all of a sudden. Their point is doing them at the end of a dive on ascent is to be able to make a mental note to compare to on future dives.

I'm finding rebreather diving to be like sidemount, there's a million different ways to do the same thing and there's not a complete concensus on which is right.

I understand the ascent vs. descent controversy, that wasn't what I was wondering about. My thinking is that if you diver fairly regularly, checking on ascent is just as good as checking on descent in terms of watching for non-linearity, which happens over time. If you do it at the start of your dive, you have to waste a bunch of dil to flush the loop back down before your descent. But if you do it on ascent, you get a nice rich mix in the loop for surfacing - you offgas more efficiently and you are less likely to go hypoxic.

But what's the argument for not flushing at all?

 

[tbone1004]

O2 check on descent is for current limiting which can't be checked at the surface. It can be used to validate linearity, but the far more important use is checking current limiting which can kill you a lot easier than a cell being non-linear

@doctormike there is no valid reason for not doing it. The arguments I have heard are very sketchy and unfounded.

No need to dil flush before finishing your descent in most cases. If you are hot-dropping, you probably aren't stopping at 20ft. I have an eCCR which is easier than a cmf for this, but by the time you get down, the dil you're adding brings it down quick enough. If you have a leaky valve you want it closed off though

 

[doctormike]

O2 check on descent is for current limiting which can't be checked at the surface. It can be used to validate linearity, but the far more important use is checking current limiting which can kill you a lot easier than a cell being non-linear

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?

 

[doctormike]

@doctormike there is no valid reason for not doing it. The arguments I have heard are very sketchy and unfounded.

Gotcha. I was just wondering what they were...

No need to dil flush before finishing your descent in most cases. If you are hot-dropping, you probably aren't stopping at 20ft. I have an eCCR which is easier than a cmf for this, but by the time you get down, the dil you're adding brings it down quick enough. If you have a leaky valve you want it closed off though

Yeah, I never hot drop at home, have only done it a few times. I guess it also depends on loop volume - I just worry about stopping at 20 feet, getting the PO2 up to 1.6 and then immediately descending without flushing it down a bit first. Maybe not much of an issue with a reasonably slow descent.

 

[Superlyte27]

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.

The argument that it does the same thing to do it at the end of the dive is countered by that fact that not doing it prior to the start of the dive might mean that you never make it to the end of the dive, as you lost consciousness and died earlier. Sure, it’s unlikely, but it is possible. I’d also contend that while you might dive weekly, eventually, there will be a break in diving lasting a month, a quarter or a year, and I’d rather build in memory to do it every time at the beginning of a dive after a 5 month break from diving, than to do it at the end of a dive after a 5 month break from diving.

It’s a waste of Oxgen or DIL? C’mon, I can do 6 hour cave dive and still have half my bottles left at the end of the dive. Are you really cutting it that close that you can’t afford one flush? Yikes.

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.

 

[doctormike]

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.

The argument that it does the same thing to do it at the end of the dive is countered by that fact that not doing it prior to the start of the dive might mean that you never make it to the end of the dive, as you lost consciousness and died earlier. Sure, it’s unlikely, but it is possible. I’d also contend that while you might dive weekly, eventually, there will be a break in diving lasting a month, a quarter or a year, and I’d rather build in memory to do it every time at the beginning of a dive after a 5 month break from diving, than to do it at the end of a dive after a 5 month break from diving.

It’s a waste of Oxgen or DIL? C’mon, I can do 6 hour cave dive and still have half my bottles left at the end of the dive. Are you really cutting it that close that you can’t afford one flush? Yikes.

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.

OK, you convinced me! Will flush at the start of the dive from now on.

It was more about having the high PO2 for the final ascent than about wasting the dil, but I see your point. I suppose I could do it on descent and ascent, for that matter. The point is that there is a non-zero chance that something will have happened to the cells between the dives, and I guess it is always better to check a critical value just before a dive.

When you said that there was a bad argument for not doing an O2 flush, you meant for not doing one on descent, right?