Is my Cootwo Crazy: My o2 bottles have CO

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JahJahwarrior

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I preach to always check o2 and CO, and I mostly inspect for them. The times I don't, are because the Cootwo discharges itself too quickly between dive trips, and sometimes it's just finicky and refuses to work right. For example, it wouldn't connect to the app for a year, then magically one day it started connecting again.

I've never found CO with this thing. The CO sensor was replaced a year ago and calibrated, it just always reads 0-1, unless I breathe in it or stick it in exhaust. But my tanks have always been 0-1, no big deal.

Today analyzing o2 on my o2 bottle, I didn't even notice the CO, but when I switched to check DIL, I noticed it was reading 16ppm!

I grabbed three other oxygen bottles, filled at atleast 3 different shops, along with two dil bottles and one bailout, and ran some tests that have my questioning if the Cootwo is going crazy.

That is:
-Calibrate in air. 20.9 o2, 0ppm co.
-Turn on o2 bottle, o2 starts climbing. When it hits mid 70's level, CO starts climbing, reaching 6-9ppm when O2 is done.
-Switch to a dil or bailout and as the o2 drops, CO typically jumps up to 13-16ppm, but then quickly drops. -When the O2 is at the expected value, the CO is 0 or max 1.

It's roughly repeatable between all of those bottles, and that pattern has my questioning if there is something funky going on. Because I inspect just before diving, I don't have labels on these to show that they were 0ppm co before, but I am relatively certain that all were inspected before and had 0, but I can't prove that.

But, I tried filling a gallon Ziploc baggie with some air from an o2 bottle. After letting it sit for a while, as without the tank pushing air through it, it takes longer to equalize...I had 3ppm co at 47% o2, 5ppm co at 67% o2, and that seems to support the fact that my o2 bottles do have some CO for whatever reason.

I took the unit apart and sprayed Deoxit on the contacts before this test.

Anyone else dove with 5-9ppm in rebreather o2? I will try to stop at a shop and use their CO analyzer to confirm if the cootwo is nuts.
 
you don't want to use it on ccr. it will concentrate over time in the loop. 5ppm is low enough you can use it for open circuit.
 
Use a second meter to confirm or dump the gas and refill.
 
It does seem very unlikely that three shops have that kind of CO level, doesn't it? Not impossible, but unlikely. If it were me, I would try to get access to some "bump gas", which has CO at a known concentration. Perhaps one of the places you get fills has some and would let you test with it. CO sensors are not the most stable, and fundamentally all the CooToo is giving you is an interpretation of the voltage the sensor is producing. Depending on the sensor, the presence of other gases could cause a reading, for instance. All kinds of things could be going wrong, and it's certainly worrisome, but I would guess there's an issue with your unit, not the gas.
 
I would suspect the analyzer isn't set up to do pure O2. I've done hundreds of lab grade analyses on oxygen and never saw a speck of CO. Methane, yes, it's elevated in pure O2 as compared to air but not CO.
 
I have had the same experience with my Cootwo as you are describing. My sensor was more than one year old so I bought cal gas and calibrated it. It would seem like it was working right, on breath hold it would show 2-3 ppm and all my air cylinders read 0. I tested some Nitrox cylinders I mixed and was seeing 5-10 ppm. Checked my T of aviator O2 and just like you, it showed 0ppm for quite a while, then as it got close to finishing the O2% the CO starts climbing. It got to 10ppm when I quit. I checked my rebreather O2 tanks and got readings all over the place. From 3-10ppm, but most were 4 or 5ppm.
I bought a new sensor, installed it, calibrated it. I still see 2-3ppm show sometimes on my O2 on if I leave it connected well after it O2 reading stabilizes. If it does, it’s a real slow creep that doesn’t start until the analyzer is nearing its completion. The same Nitrox bottles now show 0.
I have 0 confidence in this analyzers ability to accurately detect CO in anything other than a cylinder with air, and now skeptical of that.
 
Well, the good news is, I survived my dive. I swung by a shop I use frequently and borrowed their Oxycheq CO analyzer.

That analyzer showed NEGATIVE 6ppm on my o2 bottles, but positive few PPM when breathing into it, 0ppm when waving around inside the shop.

I have no clue how you get negative CO, and I have no clue how the Cootwo handles that type of situation, but I was reasonably satisfied that their analyzer was measured when there is CO with a positive value, and since my tanks had a negative value, I'm assuming they have no CO inside of them.

My friend suggested I look over the Cootwo for signs of cracked solder joints, but that will require that I do some work to extricate the board that is held in not with screws, but with melted plastic standoffs. His theory is that there is voltage leakage from the o2 circuit that is bleeding into the CO side, and it might be the result of a damaged trace or cracked solder joint.

I will get some bump gas and investigate for cracked joints, but I will also save up for a analox, palm or oxycheq CO analyzer, because I don't expect to solve it and I want a reliable sensor.

I'd be interested to know, why can an o2 analyzer be built with a simple circuit to do some basic math on a mv reading, but a CO analyze rcosts so much? Are the other brands using a better/expensive sensor compared to the Cootwo? Or is there reading not just simple math on mv?
 
I'd be interested to know, why can an o2 analyzer be built with a simple circuit to do some basic math on a mv reading, but a CO analyze rcosts so much? Are the other brands using a better/expensive sensor compared to the Cootwo? Or is there reading not just simple math on mv?

You get what you pay for. My lab analyzers for CO, CO2, etc cost $50K for accurate, reliable <ppm results. You are expecting ~$400 units to do the same.

You also trying to transpose the technology for measuring one analyte (O2) to the technology for measuring another (CO). Don't do that.
 
you get a negative reading with a non-zero calibration intercept performed initially from an environment containing several ppm of CO. sensors can drift over time also which is why periodic recalibration with traceable bump gas is necessary. would be interesting to see what sensor the different manufacturers are using as I suspect it is far less standardized than you see with the o2 sensors that are all variations of the r22d
 
http://cavediveflorida.com/Rum_House.htm

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