MV reading on cell

Dsix36

Short example of MV readings at O2 flush during pre-assembly:

#1 55
#2 51
#3 51

#1 54
#2 51
#3 51

#1 53
#2 50
#3 48

#1 53
#2 49
#3 45

Each set of readings was 7 days apart, in prep for a dive. As you can see the #1 sensor is dropping very slow and steady. The same with #2. Sensor #3 has started to drop much quicker on the last two weeks of readings.

As I was taught - It is not so much what the actual mv reading is as the amount of change from reading to reading that may be an early indication of cell failure. In this instance, cell #3 was voted out on the last dive of the day. The warning was there, but I ignored it.


And YES, I still record all readings and pressures!

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THE MORE THAT I LEARN, THE MORE THAT I STILL NEED TO LEARN!!!!!!

Gilles

Close but not quite. Your conclusion is based on the following 2 premises;

1) The nominal air mV's have not changed during this time
2) You have been using the same O2 purity

Did you check both of these? Nominal mV changes happen and I don't understand the reason why, just that it does. If it happens to drop in a healthy cell, so will its corresponding O2 mV. This is why there needs to be periodic cell cal's.

Actually, you are likely to observe nominal mV drop (and a corresponding reduced PO2) after diving when your cells are still damp. The same occurs when flushed with O2. (see above attached essay). Hence there is a third premise;

3) Did all these measurements occur on dry, or damp cells, or some combination of both?

As long as the linear response is maintained (or better), the cell remains healthy.

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Gilles
http://www.dirrebreather.com

dave t

So if you cal in o2, flush it back with air and get 0.21 whats the point of doing a 2 point cal (and its potential for more error)? and if you dont get 0.21 it could be the o2 cal that is off not the cell being non-linear. And anyway how much non-linear do we actually care about!!!
I take your point about maybe a bit of non linear at the cal point MAY show an impending current limit and it is my belief that all rebreathrs should be able to calibrate at or above your chosen setpoint. Thats what they do on the Divex stealth!



Actually I agree with almost everything you say but I still dont understand why you cant do all of that with a po2 reading as opposed to a MV reading ? especially as you agree the are the same output!


I guess I am just interested with the opinion of some that unless you have a millivolt output reading you cant catch a bad cell and although I have been diving rebreathers for over 15 years I am not stupid enough to think there is nothing more to learn!! but I seem to have managed so far with just a po2 reading and one cal point (no sarcasm intended)

Now about this erectile dysfunction

best

Dave

Gilles

No point of 2 point cal in this case. We should be checking and knowing our O2 purity before using it

If/when you're extrapolating from PO2=1 (like almost everyone does), I utilize a 2 mV under-reach/ED cut-off.

In water PO2 cal? That's an idea but would require pretty good skill.

Actually you can do all with PO2 only, using 1 point cal and checking your air reading after cal. Now that we have had this conversation, I now see that it is only those of us having to do 2 point cals, that need the mV readings. The 2 point allows air to O2 calibration at a product different than the above-mentioned 4.8x, and we need to check this. In your case, the same check is done when you let the cell relax in air. If/when it falls back to .21, then Bob's your uncle.

Thanks for the insightful discussion.

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Gilles
http://www.dirrebreather.com

dave t

Ah ok now I understand

Actually I didnt mean cal underwater I meant pressurise the cannister to get the cal. As I understand it you can pressurise the stealth rebreather to 1.3 using just the scrubber/head assembly. If you then run your setpoint at 1.2 you should alway be able to spot a current limited cell (should!!)


good stuff, thanks

Dave

Dsix36

1) The nominal air mV's have changed slightly. I have noticed that the mv readings will drop about 1-2 every month. Thus recalibration needs to be done every once in a while (checked every time though).
2) I am using the same O2 purity and analyzed to exactly 100.00% each time.
3) All readings were done with sensors and ambient temperatures the same. All sensors were dry (1 week since diving).

Like I said ".....as the amount of change from reading to reading that may be an early indication of cell failure."

I was told that there is no guaranteed way to check cells each and every time by mere mv readings or calibrations.

IMHO The cells are the least reliable part of any Rebreather and the failure rate is alarming for anything that is life supporting. Makes me wonder why any of us trust them! OH YEAH - 3 cells.

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THE MORE THAT I LEARN, THE MORE THAT I STILL NEED TO LEARN!!!!!!

Gilles

For one, I have to agree with you,

I would say this is only for major changes in nominal mV (+- 20% or so). I observe that it quite normal for nominal voltages to change, thus when multiplied by 4.8 (or when flushed with O2), the changes are amplified. As long as this change remains the same, the cell is healthy. The mV readings by themselves are relatively meaningless

Following the discussion with dave t, I am a convert. I green those that I learn from, such as in this case.

A more practical and better way to calibrate is using a single point high PO2 cal utilizing PO2 readings only, and do away with these mV readings. They become redundant in this case.

__________________
Gilles
http://www.dirrebreather.com

Scubydoo

[quote=Dsix36;164665]Short example of MV readings at O2 flush during pre-assembly:

#1 55
#2 51
#3 51

#1 54
#2 51
#3 51

#1 53
#2 50
#3 48

#1 53
#2 49
#3 45

Each set of readings was 7 days apart, in prep for a dive. As you can see the #1 sensor is dropping very slow and steady. The same with #2. Sensor #3 has started to drop much quicker on the last two weeks of readings.

quote]

You don't seem to have measured the atmospheric pressure. The output of the cell will be higher on a high pressure day, compared to a low pressure day. The fact that your cells are dropping is most likely an indicator that the atmospheric pressure is dropping. Cells measure oxygen pressure. PO2 = O2% x pressure ( even at the surface)

Dsix36

I agree about the atmospheric pressure and the mv readings. The readings do not always drop, sometimes they raise.

It is about the amount of change in readings from one time to the next AND compared to the amount of change in the readings from the others cells also.

cell 1 changed by 2mv
cell 2 changed by 1mv
cell 3 changed by 6mv
Now is the time to begin suspecting cell 3 MAY be on the verge of failing.

__________________
THE MORE THAT I LEARN, THE MORE THAT I STILL NEED TO LEARN!!!!!!

Gilles

The easier and more reliable way to tell is to observe where your PO2 reading falls to in air after you have done your oxygen cal (I'm assuming your system uses a 1-point calibration protocol). If it falls back to somewhere substantially above .21, then you a have a more positive indication of cell failure. I'll only elaborate further if requested.

The above mentioned 6mv change can be due to a ~1.2mV (6/4.8) change in the ambient mV of your cell, easily caused by dampness, or other.

mV readings are only of any use in 2-point cal systems. They are redundant in 1-point cal systems, and 1-point cal systems are a more simple and superior method of controller calibration, IMHO.

__________________
Gilles
http://www.dirrebreather.com