Rudimentry sensor mV monitoring

Gilles

Unlikely a fill whip issue as the cell mV behave as expected (i.e. always 0.99/0.21 x mV(air)).

Perhaps the colder conditions you're in results in a lower vapor saturation in your loop? What is your loop temp? and what type of cells do you use?

__________________
Gilles
http://www.dirrebreather.com

Gibbon

I saw this happen to one of my cells, all was well then one cell crept up and up, I did a dil flush to confirm.

The cell read higher and higher, I cant remember exactly where it got to but it was translated to over 2.0pp when I was on 1.1.

After 10 mins the cell was 100% dead.

Yes it was a teledyne.

Peter Piemonte

Gilles: In response to you questions. All cells have some evidence of moisture (water beads on the cell membrane). Number two however is the cell most likley to require recalibration on multi dive trips where I do not remove the KISS canister and dry out the area where the O2 sensors are located.
I cannot really tell whether any cell is "wetter" than the others but the number two position seems to require the most attention/recalibration.
Regarding my attempt to dry out the area as much as possible prior to calibration. I do not really know if that is a better procedure or not. It results in my sensors reading very close to each other and I am reluctant to recalibrate one sensor by as much as 4% when prior to diving, it had been calibrated to 100% O2 along with the others.
When the cells and head have dried out at my home, I normaly must recalibrate that number two cell, as it will read slightly high. The cells in question are not current limited, are replaces annually, and this phenomenon has been observed over a wide range of conditions with a variety of cells. Peter

silent running

Hi sabagia, I was refering to what I assumed were mv readings of the cells post cal with what you thought was 100% O2. If the post cal readings in air are .24 or .25, then you didn't have 100% O2 during the high point cal. But I'm not that familiar with the meg cal procedure, so I might be missing something.

As for my loop temp, I don't dive here in NY. I only dive in warm water-Indo, PNG, P.I., Caribbean, etc-and there's no temp sensor in the Prism. The Prism has a radial scrubber which flows opposite the Meg-in to out. So most of the condensate winds up on the scrubber bucket wall and then collects on the bottom of the bucket and is absorbed by a pad. The Prism sensors are higher output, around 22mv new, like the ones in MKs and specially made for the Prism. I can't remember the name of the company. But I've only had a sensor voted out 2 times from what I assumed was condensation on the face. I say assumed because when the sensor got voted out due to lagging, I shook around and it came back into line and I guess the big drop of moisture fell off. But this happened with the same sensor both times and since I replaced it I've had no cells voted out, so it could have just been faulty. Galvanic O2 sensors are not the most dependable things. -Andy

lbihler

Hello,

I observe the same symptoms than Gilles.
I observed it on 2hours + dives on multiple days of diving
I do the first calibration on the boat, with air out of a scuba tank and O2 out of my onboard O2 using a "flow reducer type" calibration kit like most MEG divers I ve seen.
My sensors face definitely got wet after the first dive and I couldn t get a decent pO2 reading (doubting my flushes at the beginning but no, nothing did, I got only a 1.4-1.5 max).
I do not doubt my calibration at all either as I got proper readings with dil flushes at the beginning of 1st dive.
I recalibrate with wet sensors and then no pb.
I noticed that my Mv readings dropped quite a bit over the trip (down to 40-45Mv instead of 45-50), probably because of this oisture on the sensors
I still recalibrated every day of the trip as sensors remained wet and only CCR diver so I was not sure about my cells getting "used" a bit too fast or if it was coming from this moisture or both together.

Laurent

Gilles

I think I binned a couple sensors too quickly as a result of the apparent non-linearity when using a dry air mV reading to calculate a wet O2 mVoltage. I now have sensors with about 50 hrs diving time, all of which have a dry to wet mV attentuation, and all have remained perfectly linear when calculating from a wet air mV reading.. Gotta love those smileys.

__________________
Gilles
http://www.dirrebreather.com

NickK

Do not think it is unit specific problem. Very much the same situation on my Evo. Noticed with sells being in use only 30 h, in waters with temperature 5-15 C, Pacific Ocean or high altitude lake. So, for now PPO2 set point 1.0 -1.15 range. Do not want to take my chances.

Cheers,
Nick

wedivebc

Just came upon this thread because I was arriving at the same conclusion. My post dive calibrations seem to hold better than my dry predive calibrations. Also the dry to wet offset does not seem constant between cells so cells that are wet calibrated seem to agree better during the dive.
Good article Gilles.

__________________
Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

AD_ward9

The failure mode you describe was almost certainly a leaking cell. If you let the electrolyte leak from an O2 cell, the output increases, then when enough has leaked, it dies.

I understand Teradyne have introduced some changes at the back end of last year to reduce the risk of electrolyte leakage.

When replacing the cell, use gloves and goggles as the electrolyte is extremely caustic.

All cells have this "feature" when the electrolyte leaks. If the electronics has a sensor load check capability, then this failure can be detected by the electronics because the load characteristics change: details of a circuit that does this is in the Compliance data and FMECA Vol 3, on our web site. We have not patented it, deliberately, so anyone can use this safety feature in their rebreather controller or PPO2 controller. It is possible to have all cells leak at the same time if you decompress them nastily, put differential pressures on them, or put your Rebreather at the back end of a RIB.

The PSR-11-39-MD is a plug replacement for the Teledyne R22D (and R22-2BUD). Others in the range offer high output etc, so you should have an exact match.

Alex

Alex

gtzavelas

I found this tread very interesting (Gilles got green) as I seem to have the same problem:

In long dives my cells develop a lot of condensation and at the end of the dive “under-read” substantially.
In 3+ hour dives I have seen 0.90 instead of 0.99 with O2 (using the calibration kit after the dive) and 0.19 on air (thats around 10% off!!)
I seem to have the same problem in long dives both in cold (11 deg c) or warm (24 deg c) water

Of course as it has been suggested a solution is to do a wet calibration- but that’s a work around not a real solution.

I wonder if anyone has to suggest any ideas ?
Also:
Why do some people have this problem and some not (on the same units)
Could it be a “sensitive” batch of sensors?
Could there be any diver habit that aggravates the condensation?

(on a side note: this is one more reason I am happy I was taught to run my unit on a low po2.
Running with 1.3 and having this kind of problem could mean really having 1.43ish, that for long dives would not be nice, especially if you would bring the po2 up for deco)