Lazy cells: how much is too much?

Gilles

Who said anything about shutting off the O2? and what for?

So you piss away some 40-60L of gas. There's only another 2150L left in the offboard?

Establish 1.4 and let it settle back down to normal SP.

Spiking to 1.6 is better, but 1.4 seems good enough especially if the gas your checking with is 1.4, and all else is ok. Spiking to 1.6 using a much more limited supply of O2 burns away a much more limited supply, and my personal pref is to do this at end of dive to check both the cells their cal.

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Drmike

I assumed from your post that you are flushing in offboard 1.4 in order to verify cell calibration (as well as current limitation) I assumed that because it makes absolutely no sense what so ever to be flushing in 1.4 offboard just to spike cells!

If you just want to test for current limitation - you can do this by spiking the cells at anytime:

- At depth squirt in o2 you will spike it and then you will need to dil flush to get it back down = waste bit of dil
- On descent you squirt in o2 to spike it and let adv bring it down

using off board gas of higher ppo2 than setpoint at depth to spike the cells its kinda silly imo. as you gain nothing and waste more gas and screw up your loop mix. - just press the o2 button and flush back down with dil.

In other words using O2 is far better than offboard 1.4 to spike cells if your just testing for current limitation


But if you are using the 1.4 off board to verify calibration and cell condition (and like I said I assume you are cos it makes absolutely no sense to be using of board gas to spike ) then you will need to be 100% sure that you have no O2 being injected (so solenoid/o2 is off) and you will need to do many very good flushes (not easy or fast to do, wastes a lot of gas) Not doing a thorough flush will render results increasingly meaningless the deeper you go.

Quote: (Originally Posted by Gilles)

Who said anything about shutting off the O2? and what for?

See above: how on earth will you flush and get the expected bail gas ppo2 (1.4) with your o2 on? or are you going to assume your solenoid wont fire at any time during venting/flushing? of course you would hear it fire if it did yes? No. not always not on a deep dive not wearing a hood not with high CO2 from just descending. if your cells are screwed and you pump in 1.4 they see 1.2 the solenoid will fire - if your venting as it fires you can get higher than 1.3 in your loop. Now you see 1.4 and stop cos your happy - but it can be BS. I can get that result anytime with any mix by flushing venting with o2 on.
on a deep dive you will be blowing away more than that to do a correct flush and not doing a full and correct flush will render your method useless. Id be looking at minimum 3 more likely 4 good flushes, lets say even at modest 100m depth that's 120L+ of gas from an onboard 600L (1/5th of gas supply) and same from bail gas.

I'm sorry but the only way that will work is if you have same dil as offboard, otherwise you have a different mix in your loop than your dil/deco calcs are based. Your example you talked about bail gas off 1.4, so you haven't got same mix. Your deco will be screwed. On a deep dive it can take a long time to breathe down 1.4.


So to summarise, your method is more time consuming, more dangerous, wastes unnecessary gas, leaves you with the wrong mix in your loop and too high a ppo2 right when you least should have one (immediately after descent with max co2, max depth, max exersion) ...uhuh. [Again this is assuming you are trying to verify cells reach correct ppo2 (1.4) as a way to verify cal as wella s current limitation]


nonsense. One squirt of O2 during descent will do the trick - one squirt. You squirt it in during descent (lets say at 40m, your loop ppo2 is already 1.3 so a single squirt on descent pushes it up easily. Your solenoid doesn't fire again as ppo2 is high and as you continue descent your adv firing in dil brings the ppo2 down to 1.3 before the solenoid takes over again. Only wasted gas = 1 squirt of O2. No stopping and multi tasking at depth all done smoothly effortlessly during descent

sure its good to do cell check at the end of the dive...assuming you haven't toxed and died during the dive Its too late at the end of the dive - the horse has bolted. Its better to check cells before the dive (descent) End of dive o2 flush is to monitor condition of cells for next dive and a little to ensure if cal was off you can extend 6m deco a little (in part but in practice may be too late)

What would you do when you hit depth when you flush and find cells are stuffed? Turn the dive? Now consider this is a 150m+ dive....turning the dive is still going to give you a lot of inwater time - hours on the loop. Better to check cells on descent in the shallows before you get into any deco obligation IMO.

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Gilles

If you know your gas is 1.4, and your solenoid is firing away with this gas going across the sensors, then that's a fairly positive indication your cal or cell is off. Time to SCR (or OC) your way out, no? There's no assumption about the solenoid not going off. It can be heard with a hood, and if there was any doubt, it'd be best to change the SP down further.

Granted your point about testing current limitation, but calibration is nearly/just as important, no? Even on a very deep and extensive dive?

Granted your point about same on and off board diluent. But hypothetically one can use the offboard to inflate wing/suit, test cells and once happy, you flush out your loop with a different onboard gas (using only 1/5 of your onboard supply as you said). Still plenty of room using thirds (not including OB supply).

What I find useful with the plumbing system you helped me derive is that when both supplies are on, the onboard is the preferred feed. The offboard feeds only if the onboard tank is shut-off. So there is only 1 valve to control both supplies and this is an easier valve (onboard) to open/close as needed.

Getting in the water and sinking to bottom is a fairly busy time for me. I used to do the 1.6 spiking routine but it is a (similiar) task more easily done on bottom when bouyancy is set and less occupation.

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kieranu

What he said - the Meg does seem to have the habit of occasionally voting out a sensor because it is reading low and/or slow from getting moisture on its surface. However, in my experience the offending sensor has only read out by 0.1 at most on a SP of 1.2. If the cell was consistently reading out by 0.2 I'd be pretty concerned. You might want to switch around the position of the sensors in the pod though to make sure its the position and not the sensor.

After the first couple of times I noticed this happening to me (only with the the lesser discrepancy) I ended up swapping out a cell only to get the same behaviour with the new cell on a subsequent dive. I've noticed this happens less frequently now but this could be because something I'm doing is preventing so much moisture getting on the sensor, I'm typically using a lower setpoint for the dives that I've been doing, or shear luck.

Drmike

so;

reach max depth,
obtain neutral bouyancy
close adv
vent loop and flush with offboard gas
vent and reflush (repeat several times)
whilst flushing and venting with those noisy exhaust bubbles going past my ears listen intently for solenoid (which Im sorry is not always possible to hear in my experinece)
listen some more, hope you didnt miss it firing
flush some more
still maintaining bouyancy
ok satisfied (and sure you didnt miss it fire)
now thoroughly flush loop back with dil

..and continue dive

yeh thats a lot easier than spiking ppo2 on descent



did I say it wasnt? Actuall fact it doesnt make any difference on a very deep dive if your cal is out a bit as bottom ppo2 has little effect on deco on deep dive. My point is I do not believe your method is a good one. You have alreday verified cal at 0.21 and at 1.0 and you have verified not current limited on descent to say 1.6....theres no doubt the cal was good and no doubt the cells are linear to setpoint....so theres no need to test cal again (or try to) at depth

I can also dive with pure He in my dil but it doesnt make it the best method.

really? Your still using a check valve (that will make it breathe like sh1t at depth whith high rmv, if so what happens if your IP is higher on your off board? what happens to your onboard IP when you inflate your wing or manually add dil? if it drops more than 0.06bar your letting in offboard gas!

I dont believe you. 90% of the time I dont even have to do anything I can spike my ppo2 simply by descending whilst pausing breathing (as I start off with 1 in my loop on surface)

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Gilles

It's just not that complex. Whilst descending, loop is being fed with offboard and the PO2 increases asymptotically to PO2=1.4 + a little more. The solenoid should not go off and it can be heard if it does.

No need to close ADV, in fact the ADV provides the quickest most efficient flush. Takes a whole 5 seconds and is efficient.

Well to my own preferences it is

If you're simply spiking on the way down, you're simply checking that the cells reach 1.6 or so without really knowing what your loop is. Is that right? If so, is this not making an assumption?

In addition, in current and swimming down working pretty hard isn't a good time to be spiking PO2 (IMO).

Breathing like shit is another assumption, although I can't say I have tested it like I would like yet. It worked fine using a 20/20 Tmx at 45m.

If I were to use 2 different dils, the dPinerts would be kept within 0.5 ATA, so it doesn't really matter IMO that a little bit of offboard may leak in. Anyways it is simple to test by simply pressuring up and closing the valves and comparing the HP depletion in both dils. And if one was really concerned, disconnect the OB.

Good for you. I'm sure it works fine for you and any others who have this preference. I never once professed the superiority of my tried (and maybe not to be trusted) method. But... I haven't received arguments that have changed this preference significantly.

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Drmike

I can tell you were trained by Cedric you have his tallent for totally changing what you say to suit the arguments/evidence...


1) thats not what you said before you talked about doing flush at bottom only , you even referenced how much gas that would waste when flushing at depth - but now your saying your descending with offboard - your totally changing what you said you were doing - He taught you well.

2) frankly - why bother??? you know your dil ppo2 at depth - why not just use that??
3) descending on one gas then switching at depth to another (a totally different method to what you originally proposed) just to spike ppo2....doesnt make any sense (like rest of protocal imo)
4) descending with dil of 1.4 (as your now saying your doing ) you will lucky to not spike above 1.6 on descent unless doing very slow descent (see own argument below re assumed risk of spiking on descent)

You have done a 2 point cal, 0.21 or whatever your dil is and 1.0, if you know the cells are not current limited to near setpoint because you have spiked on descent then yes you do know with high degree of certainty that the displayed ppo2 matches the loop contents - and id say given the massive flaws in your method (did you miss the solenoid fire, did you really flush enough) that you know it to a higher certainty than you can say for sure that you have that ppo2 in your loop at depth

why? what do you think will happen spiking ppo2 to 1.6 for 10 or so seconds -give me a break!

try it with high rmv at depth - anything apears to breathe well with low rmv.

so your talking about 2 dills now whereas before you were talking about on board dill and off board bail gas with higher FO2.....Well like I said - Why bother??

Each to his own, just in my opinion this isnt a sensible approach or set up.

But its really none of my business so ill shut up
you just did


look if you really want to check cal at depth then why not just momenatrily lower setpoint on handset and dil flush? Combine that with a spike on descent gives you all the info you need. In fact for most dives you can just leave setpoint on lower setting during descent and do dil flush at depth to verify cal just before you increase to bottom setpoint..simple - a lot less buggering around

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Wreck diving is a sport
Diving in general is a sport

'Rebreather diving' is not a sport
its the delusional obsession with a highly dangerous and often inappropriate piece of equipment

Benthic

Um, the cells in my Meg face DOWN, not up when the unit is put together. Thus if lots of condensation forms on the faces it can drip off rather than pooling there. Are you sure you don't have the sensor carriage in upside down?

Brian

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Abbo

Ah, the cell carriage is, of course, impossible to put in upside down, and you are right that the cells face down when assembled. Unfortunately, my head was in my bum when I wrote that.. Frightening!

Gilles

Calling Dr. Gadd (not you Mr. Hyde ), this discussion has become .

If you want to dwell on technical aspects, well I'm open to reasoned suggestion and impartial argument. If you're fishing for an opportunity for more antagonism, you may my hairy .

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