Info DiveGearExpress o2 sensors

[InWay2Deep]

Can these be used interchangeably?

Yes! In fact I have an AI, Vandagraph and a Solid State (Greenflash) in my SW right now. The best thing to do is to test them regularly in a pressure pot. If you can get them to read 2.0 under pressure they are still good, if they won't get to 2.0 then the fuel has been used up. Tracking linearity is okish, but pressure testing is the best.

 

[LFMarm]

Yes! In fact I have an AI, Vandagraph and a Solid State (Greenflash) in my SW right now. The best thing to do is to test them regularly in a pressure pot. If you can get them to read 2.0 under pressure they are still good, if they won't get to 2.0 then the fuel has been used up. Tracking linearity is okish, but pressure testing is the best.

Thanks, I do pressure testing and linearity checks.

 

[Tassi Devil Diver]

The best thing to do is to test them regularly in a pressure pot.

Or spike to 2.0 on the way down, and then on deco.

 

[InWay2Deep]

Or spike to 2.0 on the way down, and then on deco.

But you don't know what the actual loop % is. Feeding it a know gas, pure o2, at a known pressure and holding it there is a more reliable way, hence the need for the pressure pot. You can almost never get a loop to 100% o2.

 

[inquis]

Or spike to 2.0 on the way down, and then on deco.

I just tossed a cell that failed reading high. It read 2.0 when the actual PO2 was 1.5. That would skew the average in an unfavorable direction until it was voted out.

 

[Tassi Devil Diver]

My typical MO is to spike O2 on the way down to see that all cells can reach 2.0, then from the 12m spike the O2 to 2.0 as I leave each stop letting it decay down to 1.4, then at the 6m stop an O2 flush to get the cells as close as possible to 1.6m. I also test my cells every 6 months in a pressure when I replace a cell following the rEvo cell replacement practise.

 

[Dsix36]

I only calibrate when absolutely necessary, like when replacing a cell or when they get to far out of alignment. This gives a quick and easy way to compare how they are acting without a bunch of paperwork and records to fumble with. I run 4 cells with staggered ages and only replace when one fails or makes a sudden noticeable variance from the other 3. I am definitely NOT advocating this method for anyone but it has been working well for me for a couple of decades without any adverse outcomes.

 

[LFMarm]

I only calibrate when absolutely necessary, like when replacing a cell or when they get to far out of alignment. This gives a quick and easy way to compare how they are acting without a bunch of paperwork and records to fumble with. I run 4 cells with staggered ages and only replace when one fails or makes a sudden noticeable variance from the other 3. I am definitely NOT advocating this method for anyone but it has been working well for me for a couple of decades without any adverse outcomes.

I get your logic but I think it's dangerous. All sensors have a rate of decline of mV over time (and this rate is different for each sensor). So if you calibrate now and use the same calibration in 6 months, the constant of all sensor will have changed while your computer will be showing the one from 6 months before. Given that constants normally go down (unless the cell has problems), you will be underestimating your pO2 (which is safe for decompression calculation but not conservative with regards to O2 toxicity). Manufacturers say that sensors can be used in the range 40-62 mV which means that if you are using a sensor calibrated at it's peak of 62 mV when new while you dive it towards the end of its life at 40 mV, you will be underestimating your pO2 by 35%. I.e. if you have a set point of 1.3, your computer reads 1.3 in the loop but you are actually breathing 2.0.

It has worked for you for 2 decades so I do not want to convince you but wanted to share my thoughts on this practice for other divers.

 

[OTF]

I get your logic but I think it's dangerous. All sensors have a rate of decline of mV over time (and this rate is different for each sensor). So if you calibrate now and use the same calibration in 6 months, the constant of all sensor will have changed while your computer will be showing the one from 6 months before. Given that constants normally go down (unless the cell has problems), you will be overestimating your pO2 (which is safe for decompression calculation but not conservative with regards to O2 toxicity). Manufacturers say that sensors can be used in the range 40-62 mV which means that if you are using a sensor calibrated at it's peak of 62 mV when new while you dive it towards the end of its life at 40 mV, you will be underestimating your pO2 by 35%. I.e. if you have a set point of 1.3, your computer reads 1.3 in the loop but you are actually breathing 2.0.

It has worked for you for 2 decades so I do not want to convince you but wanted to share my thoughts on this practice for other divers.

I too try not to re-calibrate the controller on every single setup, but I ALWAYS do a two-point test looking at the PO2 and mV values in air (with the unit open) and flushed with O2. Any drift should be easily observable this way. If anything seems imperfect I will calibrate. Granted this doesn't test up to 2.0 for limited cells, I do that underwater, but I like the idea of using a pressure pot.

 

[lizardland]

A pressure chamber is fairly cheap and easy to make. A water filter housing, a cheap pressure gauge and a few pipe/hose barb fittings. Three cheap voltmeters or an Arduino if you know what you're doing. Potting the cable exit is a bit tricky but it's not that hard.

I built one after the Great Teledyne Disaster and the supply of reliable cells was questionable for a while. To be honest, I haven't used it in years as I never found it that valuable. Spiking on descent has been adequate for me and I just stopped using it unless there's been something weird I've needed to figure out.

I've never really found a need to calibrate overly frequently. I don't know if that's an artefact of Mk15 cells but I've found them a lot more stable for a lot longer than I ever had with the more common RB cells. I'm not a big fan of calibrating over cell issues. If I'm getting a lot of drift just on pure O2 at the surface and the mV is noticeably not where it should be then I might be looking at changing that cell rather than cal'ing it away. Rebreather diving isn't a sport for penny-pinching.

I aim to change cells 12-18 months, usually at the early end of the scale so probably averages about 14-15 months old. I don't really notice my unit needing frequent calibration, it'll hold cal for a long time and I suspect it's down to not running old cells.