Do you check your cells at 6m?

I had a lot of problems on my Optima with current limiting.
Found O2 flush at start/ end of dive a waste of gas.
Current limiting / wrong readings progress throughout the dive not at the start( apart from having overdue old shitty sensors in your unit)
If I suspect messy behavior of the sensors I just wind back my set point to 1.0 ( as they calibrated fine at )
Sometimes I just keep adding dil at depth to confirm correct reading but that's it. No splashing around with gas and over pumping the lungs - waste of time in my eyes.
My problems were actually the wiring as it turned out not the sensors.
Changed both of my units ( Optima and Hammerhead to Nerd and Petrel setup in combination with Vandergraph cells which works at treat and no further dramas .
Obviously there are divided opinions on this topic but it seems to work just fine for me and I am doing 95% of my dives solo
 
My checks are as follows:-

Switch on unit without calibrating - make sure that all cells read about the same since last calibration - all inspiration cell failures in past have been preceded by one or more of the cells reading differently prior to calibration

Do calibration and watch response

Run pure O2 until 6-10 meters and make sure I can spike unit upto around 1.7 then exhale and dil flush with adv.

Switch to high set point at 15m and keep descending.

Check Po 2 though dive and make sure numbers are still moving on each cell.

Occasionionally dil flush with adv to make sure readings make rough sense.
 
Sharkjaw said:
I had a lot of problems on my Optima with current limiting.
Found O2 flush at start/ end of dive a waste of gas.
Current limiting / wrong readings progress throughout the dive not at the start( apart from having overdue old shitty sensors in your unit)
If I suspect messy behavior of the sensors I just wind back my set point to 1.0 ( as they calibrated fine at )
Sometimes I just keep adding dil at depth to confirm correct reading but that's it. No splashing around with gas and over pumping the lungs - waste of time in my eyes.
My problems were actually the wiring as it turned out not the sensors.
Changed both of my units ( Optima and Hammerhead to Nerd and Petrel setup in combination with Vandergraph cells which works at treat and no further dramas .
Obviously there are divided opinions on this topic but it seems to work just fine for me and I am doing 95% of my dives solo
Click to expand...


Your totaly confusing current limiting with electrical faults on the unit.

Current limiting is a 02 cells inabuility to output a hugh enough current to display the actual PP02 in terms of MV and its is a cell error totaly limited to the cell.

Wireing faults be they broken wires with intermitent contact or bad contacts due to corosion, are a totaly seporate issue and IMHO a far more dangerouns one

Many people just calibrate blindly over wireing issues which is incredably dangerous but a whole seporate thread of discussion.

ATB

Mark
 
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I dont know why we are even having this discussion

Spiking cells at the begining of a dive and again mid dive to check for current limiting takes a few seconds, costs bugger all and is novice simple to do.

Why anyone would consider it not worth doing is totaly beyound me

But then i read about people dieing because their 02 was switched off or their hand sets were switched off and I am totaly stunned that they could miss so many oportunities to check such simple things before it happened

They are obviously far less concerned with staying alive than i am or they have some goundless beleife that its the units responsabuility to keep them alive

ATB

Mark
 
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Bobjr said:
I am suggesting knowing your cells, watching during calibration, spiking above set point on dive occasionally and most importantly not cutting corners on replacing cells 12mo after manufacture but not luring myself into a false sense of security and wasting time checking at 6m at start of dive.
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I would agree with that.

All this checking and spiking would be vital if current limiting was a completely unpredictable trap just waiting to spring on you.

My understanding is: it's not. Cells become current limited (1) for one reason: they've consumed too much O2. For a given design that depends on what ppO2 it's been exposed to, for how long, and at what temperature, and that's it.

So if (2) the cells are manufactured consistently enough and are reliable enough over their recommended lifetime, then all that's needed is to stick to that lifetime (to minimize the chance of any becoming current limited)(and possibly lower it if diving intensively in the tropics) and make sure they have had different O2 consumptions (so that even if, only the oldest is). That's simple enough: stagger the replacements.

On top of that, it makes sense to add some spiking - which I do, because that's what I was told, and because I prefer to discover problems at a time of my choosing - but the most important thing is diligent cell replacement. The best way to diagnose an issue is to make it not happen at all.

Obviously, if cells fail more or less randomly and sooner rather than later, or if mixing cell types, ignore all that :)

My 2c.

Cheers,

Matthieu

(1) Cells are always current limited in the broad sense, I mean "at high setpoint".
(2) If!
 
Ben Field said:
I do very similar to Mark but I am on an MCCR.

Sitting on the bench ramp loop to as close to 1bar (98% ish usually as have SW electronics)

Get up and walk to jump point, if in doubt adding more O2, check for close to 1bar as I jump.

Check once more at shot buoy or on surface if direct descent.

Stop at 6-7mtr, and if Dil has been injected dump and O2 flush or just thumb in more O2 to spike sensors while bubble/buddy check.

(So basically everything from turn on to 6mtrs is pure O2, any issue at this point bin the dive- I only ever decide whether the dive is on at 6mtrs)

At the bottom dil check then reestablish nominal set point and go diving.


Obviously this process is interspersed with other activities, scooters, deco stations, faffing buddies etc but my rule is O2 only till its safe not to be. Messing about near the surface when checking PPO2 can be hard is alot safer with pure O2 loop than unknown.
Click to expand...

I follow same SOP.
 
Mark Chase said:
I dont know why we are even having this discussion

Spiking cells at the begining of a dive and again mid dive to check for current limiting takes a few seconds, costs bugger all and is novice simple to do.

Why anyone would consider it not worth doing is totaly beyound me

But then i read about people dieing because their 02 was switched off or their hand sets were switched off and I am totaly stunned that they could miss so many oportunities to check such simple things before it happened

They are obviously far less concerned with staying alive than i am or they have some gounndless beleife that its the units responsabuility to keep them alive

ATB

Mark
Click to expand...


Why do a test that basically gives you no useful information. I don't care if cells can hold a spike for a short time at start of dive - I care whether they can make set point for the duration of the dive.

Has anybody actually canned a dive because a cell couldn't make 1.6 at start of dive at 6m for 0.5 minute on the other hand at least one fatality resulted from a cell voting logic failure due to current limited cells on a dive where a 6m check had been done.
 
emmbee said:
I would agree with that.

All this checking and spiking would be vital if current limiting was a completely unpredictable trap just waiting to spring on you.

My understanding is: it's not. Cells become current limited (1) for one reason: they've consumed too much O2. For a given design that depends on what ppO2 it's been exposed to, for how long, and at what temperature, and that's it.

So if (2) the cells are manufactured consistently enough and are reliable enough over their recommended lifetime, then all that's needed is to stick to that lifetime (to minimize the chance of any becoming current limited)(and possibly lower it if diving intensively in the tropics) and make sure they have had different O2 consumptions (so that even if, only the oldest is). That's simple enough: stagger the replacements.

On top of that, it makes sense to add some spiking - which I do, because that's what I was told, and because I prefer to discover problems at a time of my choosing - but the most important thing is diligent cell replacement. The best way to diagnose an issue is to make it not happen at all.

Obviously, if cells fail more or less randomly and sooner rather than later, or if mixing cell types, ignore all that :)

My 2c.

Cheers,

Matthieu

(1) Cells are always current limited in the broad sense, I mean "at high setpoint".
(2) If!
Click to expand...


Cells do fail randomly.

I never trust a new cell and only fit one at a time so I have two proven cells I can trust

the whole 12 month thing is just a guide line with a reasnoble safety margin. I have dived 3 year old cells that work fine and I have had new cells fail.

Checking for current limiting on cells also confirms the calibration at the high end. Doing the Schamerhorn Slide on low set point flushing with diluient again checks the cell at the lower end of the scale and gives a direct comparison between a known PP02 and a displayed PP02

When i hit max depth I am on virtualy pure dill and if i am on 14/60 at 70m I know my PP02 should be 1.12. If its reading 0.8 0r 1.3 i know I have a calibration error.

I can if i chose then dive arround the problem or i can abort.

If my 14% is reading 0.8 then at 1.3 set point i am actualy running 1.62pp02 and risking a tox. SO i could chose to alter set point to 0.9 and carry on the dive using my stand alone computer set to 1.3 for deco and ignoring the intigrated unit or I could just folow the intigrated unit to be on the safe side.

I have had errors but never as much as 0.3

Obviously for this to work you need a known gas (I use premix) and at least two depth gauges to conform your depth (I use two Shearwaters and a Sunnto in gauge mode)


Perhaps i am bias on this as i generaly do longish dives (3 hours is the norm sometimes as much as 4) so i am very concious of actual PP02 from a CNS tox and deco point of view.

ATB

Mark
 
Well I think I jinxed myself. Yesterday I removed the cells from my rEvo & ran them all through my Mini Cell Checker. All passed with ease. Refitted them into the unit & today ran calibration & one of them is stuck on 10mv at 0.21ppo2 :(
Typical, but goes to show that even with a cell checker it is no guarantee that the next day will be good !
 
Mark Chase said:
Cells do fail randomly.

I never trust a new cell and only fit one at a time so I have two proven cells I can trust

the whole 12 month thing is just a guide line with a reasnoble safety margin. I have dived 3 year old cells that work fine and I have had new cells fail.
Click to expand...

Hi,

They certainly do, but as far as failures go current limiting is "well-behaved". It happens because exposure to O2 and temperature. It's progressive. "It's going to be current limited at 1.6 before it's current limited at 1.3" and all that - bearing in mind what Paul said. Otherwise this thread would not exist.

So it's reasonable to say that making sure cells have a different "experience" ensures they won't fail that way at the same time.

You're right of course that 12 months is just a guideline. I have zero doubt that 12 month is in my case overkill. First I'd assume manufacturers have to give numbers that will work for all divers (almost, anyway). That's going to include a lot that dive more than I do or/and live in a warmer climate. Second I just replaced 2 month ago an analyser cell that was current limited. 3 years, almost to the day. And third it didn't escape me when I opened up one of my old JJ cells a few months back that it was nowhere near spent.

Having said that, this doesn't tell me anything about how much longer I could use them. Given that anything from how much diving to climate to how the rebreather is stored, all over a year at least, will affect it, I don't know, so I don't do it. Anything else adds an unquantifiable risk. And no amount of procedure is going to change that.

Cheers,

Matthieu
 
emmbee said:
Hi,

Having said that, this doesn't tell me anything about how much longer I could use them. Given that anything from how much diving to climate to how the rebreather is stored, all over a year at least, will affect it, I don't know, so I don't do it. Anything else adds an unquantifiable risk. And no amount of procedure is going to change that.

Cheers,

Matthieu
Click to expand...


I am baffeled by this responce.

All i need to know is will my cells last three to four hours (IE the period ill be underweater)

By ensuring they can read 1.7+ at the start of the dive I am prety confident they will read 1.3 for at least a little while

During the dive I just spike a bit to ensure they go 1.4 just to make sure they havent frozen on 1.3

I find the end of dive testing to be of little use as with moisture on the cells I never seem to get an accurate 1.6 in 02 at 6m but seeing as i always end my dives on 20mins of pure 02 I do the test by default

Thats it, dive over testing over till the next kit prep and dive

Testing in this way for the last about 8 of the 10 years I have dived CCR I have identified current limited cells and calibration issues which failed to show up in predive tests. The most common of the errors were new cells that read high which I beleive was due to the cells not being fulley "woken up" prior to calibration. However i have also discovered current limited cells doing this test and I have dived them without a problem just by running the unit manualy (to avoid voting logic) and keeping an eye on them. Because of the way i manage my cells I have never had two cells go down at once.

All the pre dive testing and kit prep with reguard to cells is of limited to zero value to the actual dive. The bit where the unit is imersed in real world dive conditions is the bit of testing I am primeraly interested in.


Anyone can argue the limitations of the in water tests. However to say they have no value at all and that going in totaly blind relying on the unit alone is the better option is totaly mad in my view. S#it happens, any of us could be the next statistic and I am acutly aware it could be me, but we can reduce the risk by takeing simple precautins



ATB

Mark
 
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