Question Strange behavior of O2 sensor

[LFMarm]

Hello everyone,

I am diving a ChOptima (4 O2 sensors 1,2,3C going to controller and 1,2,3H going to monitor).

Recently I noticed that 3H was reading in line with other 3 sensors (eg 1.28) until it jumped to about half the reading (eg 0.75). After a few minutes it jumped back to being in line with other 3 sensors (eg back to 1.28).

I suspected the sensor going back so I replaced it but got the same behavior.

I have not started troubleshooting (eg replacing the sensor wires, replacing the M8 cable from electronic to shearwater cable).

Has anybody else come across this behavior? Any ideas on what to try next?

Many thanks!

 

[tbone1004]

if the issue doesn't follow the sensor then it's a wiring issue somewhere, intermittent point of high resistance of some variety. It could also be on the board that does the ADC function but it's most likely wiring

 

[beldridg]

if the issue doesn't follow the sensor then it's a wiring issue somewhere, intermittent point of high resistance of some variety. It could also be on the board that does the ADC function but it's most likely wiring

My thinking exactly. Since I'm assuming 3H is separate and not split and going to monitor only, it is likely a wiring issue.

- brett

 

[makar0n]

Hello everyone,

I am diving a ChOptima (4 O2 sensors 1,2,3C going to controller and 1,2,3H going to monitor).

Recently I noticed that 3H was reading in line with other 3 sensors (eg 1.28) until it jumped to about half the reading (eg 0.75). After a few minutes it jumped back to being in line with other 3 sensors (eg back to 1.28).

I suspected the sensor going back so I replaced it but got the same behavior.

I have not started troubleshooting (eg replacing the sensor wires, replacing the M8 cable from electronic to shearwater cable).

Has anybody else come across this behavior? Any ideas on what to try next?

Many thanks!

Had exactly the same a while ago, one cell would suddenly drop to ~0.7 . One of the molex pins on the cell was almost broken (dunno why, those were the cells installed from new), and basically sometimes was contacting, sometimes not. Issue would usually show at the beginning on the dive, maybe later the moisture helped the connection or something. Either way, changed the cell (Revo, Vandagraph) and all was well. Do not remember which cell number it was.

EDIT:
Just found the old cell - it was not even 1 pin, but all. Careful when getting it out, getting pins out from the cable plug can be a pain.

 

[LFMarm]

Will check the pins but given that 2 separate sensors showed the same behavior, I think the problem will be somewhere else

 

[Rollin Bonz]

I had the same issue pop intermittently. Twice since service 9 months ago. Assumed a bad cell. Initial discussion with DR advised likely moisture on the face of the sensor, if it continues, let us know. And it went away. Issue came back a few months ago so switched that sensor out with a new (but expired) cell. Still readings all over the place. Switched sensors around that were already in the unit (2>3 and 3>2), problem stayed with what the nerd is calling "cell 2". Tells me it's in the #2 connection or otherwise related to that particular connection. Hit the socket with Deoxit and all SEEMS to be good since. Keeping an eye on it...

 

[kensuf]

Try replacing the wiring harness for the specific cell. You should have gotten one in the spares kit that came with the unit, but you can also order them from dive rite.

 

[Brad_Horn]

I had the same issue pop intermittently. Twice since service 9 months ago. Assumed a bad cell. Initial discussion with DR advised likely moisture on the face of the sensor, if it continues, let us know. And it went away. Issue came back a few months ago so switched that sensor out with a new (but expired) cell. Still readings all over the place. Switched sensors around that were already in the unit (2>3 and 3>2), problem stayed with what the nerd is calling "cell 2". Tells me it's in the #2 connection or otherwise related to that particular connection. Hit the socket with Deoxit and all SEEMS to be good since. Keeping an eye on it...

Have you considered the physical position of 'cell 2' {3H} throughout the dive...?

For a little more on waterblocked cells and how this occurs see the highlighted yellow at https://www.opensafetyglobal.com/Safety_files/DV_O2_cell_study_E4_160415.pdf

 

[LFMarm]

Try replacing the wiring harness for the specific cell. You should have gotten one in the spares kit that came with the unit, but you can also order them from dive rite.

Did it and tested today. Unfortunately not solved yet.

 

[LFMarm]

Have you considered the physical position of 'cell 2' {3H} throughout the dive...?

For a little more on waterblocked cells and how this occurs see the highlighted yellow at https://www.opensafetyglobal.com/Safety_files/DV_O2_cell_study_E4_160415.pdf

Great report, thanks for sharing. Based on the description of the water block problem ("If water covers part of the O2 cell membrane, a dramatic increase in the response time can occur. If the water blocks the whole membrane, then the sensor output will “freeze” at whatever it was before the block occurred. The output of a blocked sensor will reduce only very slowly over time"), I would rule it out. I am not noticing slow response time, I am noticing immediate drop of readings to values that are about half of what is in the loop (e.g., instantaneously from 1.4 to 0.7 for about 3-5 mins and then back to normal).

I have ruled out so far (1) bad sensor by noticing exact same behavior on a different sensor from a different batch, (2) bad sensor connector by having noticed the same behavior after replacing the brace with a brand new one, (3) water blocking because I see a different time dynamic.

I will move on to try to clean with deoxit the female connectors to the braces.

Where does the analog to digital signal conversion happen? Is it possible that this is a problem that is generated at that stage or on the digital side of the circuit?