Inconsistent ppo2 readings

Muddweller

I've been diving a CCR'd draeger for a while now with home made ppo2 monitors, I've mounted the sensors (2 of) in P port connectors either side of the inhale bag roughly facing each other, so when I'm horizontal one is facing the shell and the other my back.

The problem is this, at the start of the dive save for reaction time of the cells the readings are the same, however after 10-20mins the readings start to drift from each other sometimes by as much as 0.4 Obviously this is way to much and I end up flushing to asses which is reading true and turn the dive. Back on the surface after an hour or so the readings are back to the same again and never drift. I haven't noted down which of the cells or in which position is playing up yet, I'll do that next dive in the shallows.

Does this sound like moisture on the face of one of the cells ? They both look dry on the membrane though. Would I be better off getting a seperate housing like the ones Martin makes at TECME ?

Any suggestions.

Regards
Karl

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O.C.Diver

While the cl might be partially blocking one sensor's face and not the other, I think moisture or a bad sensor is a more likely the culprit. Do a test. Switch the sensors and see if it always happens in the same position or to the same sensor.

Ted

wizbang

Hi Muddweller, I have the same setup. Apart from the two sensors lagging each other (one more in the flow than the other) mine read the same as long as I do my part with the calibration.

I would be looking at the sensors. Test them in 100% and air for linearity and also test them at 6m with an O2 flushed unit to make sure they can read well above 1 for a reasonable period.

Some cells look the goods to start with but with a little time have a nervous breakdown and give sporadic readings.

It might just be some moisture or the CL occluding the face somewhat but I would be looking at the sensors first and hey it is an excuse for another dive.

Hope that helps. Wiz
PS which brand of sensor.?
How old are they.?
How do you calibrate.?

Allan_o

Hi.
I dont know what kind of measuring equipment you use, but normally the O2 cells from teledyne (r-17 r-22) needs a 10kiloohm resistor connected due to the rahter high entranceimpedance from your measuring device.
I noticed that you were on a draeger ccr and therefore i assumed that the ppo2meter might be a home made one, leading to my answer of the drifting cells!

Hope you can use my ans...

Regards Allan

MHD

Not sure what sort of connections your sensors have but if they are molex connectors, check that you have gold coated pins/connectors, if not you can get large amounts of surface corrosion effecting conductivity. as they get moist during the dive they can cause variations in impedance. I nthat environment they can build up surface corrosion very quickly

Assuming by the way to way done this with various sensor and have eliminated them first

Matt

wizbang

What he said, very important. Never had trouble during a dive but a week after the dive when the system had dried out and I wanted to calibrate again and it would keep moving around.

Changed to the GOLD pins. All is right with the world again.
Thanks ABowie.

Joseph Grech

These sympthoms are very familiar to me. In fact I dive a similar setup with my Azi CCR.
After changing sensors and also molex plugs I was freaking out thinking that something wrong with my diy meters. Alas, all it was I had a broken wire from the sensors to my meters and this was givving erratic readings.
Also something I notice is that sometimes, with moisture in counterlungs the readings are a bit adrift, Sympthoms like this might reduce when running loop at a higher volume. I mean that when the counterlung collapses and hits sensors it has more chance of getting them wet.

Joseph

Muddweller

Hi Wiz, Analytical Instruments, 8 months and I cal in 100% via 2 variable resistors. I think it's gotta be moisture since it dry dives perfectly, I know its not a thurough test since I can't get a ppo2 >1.0 but short periods of bag squeezin brings it up a bit .

Hi Allan, I designed it with a 10k accross the output of each sensor, thanks anyway.

Hi Matt, I was unaware of a corosion issue with the pins, they are not gold plated but the molex connectors are fully enclosed and sealed inside a P-port cell holder, worth a look but if this was the problem then wouldn't it drift top side ?

Hi Joseph, again with a broken wire I would expect it to vary top side also, I've checked and have full continuity. You say your readings are a bit adrift sometimes can you tell me by how much on average.

Thanks for all your replies.

Karl

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Joseph Grech

They might drift by a 0.1

Scuba_Vixen

I'm not aware of any cell holder that actually seals the connection point from the rest of the cell.

The connection is subject to whatever the loop is.

Warm, moist, oxygenated environments, promote rather rapid oxidation.

As the guys have mentioned, gold plated pins are best, and particularly the phosphor bronze, gold plated pins. That material has better spring characteristics so that they maintain a better more consistant contact pressure.

Also the temperature differentials between loop gas temperature and surrounding water temperature, which seeks to cool the cell holder, may be causing condensation. ...either at the connection area, (gold pins would be much less affected than tin plated) or (as you speculated) on a cell face menbrane that faces "up" during the dive. Neither would be occuring on a dry dive. Detrmining which cell, the "up" one or the "down" one, is the dodgy one is quest number 1.

Last thought, not knowing how you have your display(s) connected to the cell holders, is it possible you're getting condensation creating a poor connection at the display? This probably would not occur on a dry dive either.


Good luck tracking it down,


Darlene