Oxygen Sensor Fundamentals

[tbone1004]

This is a presentation by Joe Citelli of IANTD on the fundamentals of O2 sensors. This is information that the majority of CCR instructors and vast majority of CCR divers do not understand. Presentation was birthed by @Bobby who has had passionate discussions about this with many of us and Joe decided to run with it. It's a long presentation, but I urge everyone who is actively diving CCR's or interested in diving CCR's to truly listen to this and absorb it. We all hope for a day when we have reliable and robust solid-state sensors where we no longer have to worry about these limitations of galvanic cells, but that day is not here yet and we all need to truly understand how our current sensor technology works.

 

[The Chairman]

Many, many thanks for this.

 

[kelemvor]

We all hope for a day when we have reliable and robust solid-state sensors where we no longer have to worry about these limitations of galvanic cells, but that day is not here yet and we all need to truly understand how our current sensor technology works.

I thought solid state sensors were a thing, albeit expensive. Are they covered in the presentation? Obviously I haven't watched it yet.

 

[runsongas]

Poseidon

 

[tbone1004]

I thought solid state sensors were a thing, albeit expensive. Are they covered in the presentation? Obviously I haven't watched it yet.

Poseidon has them as @runsongas pointed out, however they have not been fully vetted by the CCR community as the Se7en is not a rebreather accepted by cave divers and most technical divers mainly due to limitations of the MK6. It is still largely targeted as a recreational rebreather and that is a good thing. The sensor is available with the M28 computer, but only in the CPOD and can only read a single sensor. Not something we will invest $5k to try. If they had one that could read a pair of galvanic cells and the SSS that we could use without the CPOD and we only had to invest say $2500, then you'd see a lot more buy in right now. Since it's prohibitively expensive and is still very new, it is not a technology that we can realistically expect to take over anytime soon.

 

[kelemvor]

I agree, m28 sucks. It's what turned me away from poseidon back when I was considering rb's. I just figured folks would have been working on the better option by now. Does the thing output digital data instead of millivolts directly or some other complication? I guess at $1200 (divegearexpress) not many have bought them for experimentation.

 

[Southside]

I agree, m28 sucks. It's what turned me away from poseidon back when I was considering rb's. I just figured folks would have been working on the better option by now. Does the thing output digital data instead of millivolts directly or some other complication? I guess at $1200 (divegearexpress) not many have bought them for experimentation.

Its digital output so you need a power source and I believe a DAC or something to translate it.

 

[tbone1004]

@kelemvor and @Southside
the SSS is a digital sensor that requires an external power source and outputs a digital signal.
The DiveCAN versions of Shearwater computers, and other canbus based systems use an ADC converter on the O2 board to convert the analog mV output from the current O2 cells to something that the computers can read. This is a much more reliable system than running analog mV all the way to the computer and have the ADC onboard. Much less risk of voltage drop and the digital communications know when there is an error in the communication line.

A DAC would be counterproductive with the SSS since an all digital communication would be significantly more reliable and would only continue to perpetuate the abomination that is the fischer cable.
What would be ideal is something like what Dive Rite has where the battery and boards could be offboard to allow retrofit options and handling of multiple cells. Alas, Poseidon has chosen to focus on implementing this technology in their own rebreathers to help them recuperate the development costs of the cell and focus on gaining market share with the Se7en. Once that demand has been satisfied they have indicated they will be exploring other options. Since it is a canbus based system, it will be easy to integrate with systems like DiveCAN once it is made available, but it will be on the CCR developers themselves to deal with how to design the O2 control board and fit everything in there. I'm excited for that day.

 

[Tassi Devil Diver]

Thanks for posting will have a look at that presentation.

Personally I have no problems with the current galvanic oxygen cells, I get 2.5 years out of the 5 cell array in my rEvo, using the rEvo B Vandagraph cells, replacing one cell every 6 months, the oldest cell might fail before it is rotated out after 2.5 years (I have had 3 cells fail in 7 years, 500hrs of diving before reaching 2.5 years, never had a cell fail before reaching 2 years), but I am fine with that as it is on my backup PO2 monitor (the Dream), so I unplug that cell and rotate it out at the next 6 month interval.

 

[RainPilot]

The sensor is available with the M28 computer, but only in the CPOD and can only read a single sensor.

All the new Se7en CCR units are SSS ready in the heads, diver's choice as to whether you want one or both as SSS or mix'n'match to reduce initial costs.

Not something we will invest $5k to try.

we only had to invest say $2500,

The M28, CPOD and SSS package ready to dive is $2800 retail. Add 200 for Martin at Tecme to do an adapter for whatever hoses you need and you can be using a SSS 3rd cell in any CCR for $3k.

Alas, Poseidon has chosen to focus on implementing this technology in their own rebreathers to help them recuperate the development costs of the cell and focus on gaining market share with the Se7en. Once that demand has been satisfied they have indicated they will be exploring other options.

That is true, about the recoup of R&D costs, however there is another reason that they have not been pushing the SSS options for 3rd party CCR, and that is their military sales. A big part of the delay in availability of SSS for recreational divers was that all the production was being used for military CCR adaptations. Poseidon does some very interesting units that are not particularly known outside of military circles and the demand for SSS in that field has been way higher than anticipated. It has also had a knock-on effect with pricing, as there are constraints around military vs civilian pricing etc.

Right now I am working with Poseidon on some integration options which would allow the end cost to the user to be substantially reduced while allowing SSS control of an existing unit. Sadly, the M28 is going to remain part of that equation for the foreseeable future. Personally, I prefer Shearwater but the M28 is a decent enough DC and has a couple of nice features, one being the nicest ascent/descent rate indicator I have ever used.