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SPECIAL NOTE, PLEASE: This thread is only for those who do test tanks for CO or are interested. Yes, I know that some experienced divers still don't - but that discussion is for another thread. Want me to start a new one?
This boredom thing has happened to me, especially with my first tester that didn't register anything until 5 ppm - but when it did get 5, 7, 9 at times, it made all that blank testing worthwhile. Then I dropped it water - damn, as it wasn't water resistant like my newer one.
Fortunately, I was diving with an anesthesiologist who also wanted to make sure our tanks were safe and he tested our remaining tanks - and he got 17 ppm on all tanks the last day as we were headed to a 130 ft dive!
17 is not much on a bad air day in a city, but taken to 130 feet it's like 85 ppm - still survivable probably, until you ascend, then it gets iffy. At depth, the increase in PPO helps offset the toxicity of the CO, but the latter binds with hemoglobin in your blood and remains as the PPO decreases on ascent! There are good reasons by 10 ppm CO is the maximum allowed by US regulations even tho they are weakly enforced, and of the few countries with such regs - many have lowered the max to 3 as the max! We were halfway to the site, wishing we had checked at the dock, but we just turned the boat. We shouldn't have used those tanks for even the shallow dives we did, but we survived those - and will refuse such tanks in the future even if it means a missed dive. My last trip, I did not get a single reading
and it got a little boring again by the end of the week, but I guess I should just enjoy the reassurance. I carry two CO analyzers now, and will calibrate both before my next trip. I replaced the cheaper Pocket CO, which I use with slider ziplocks, and their newer model starts reading at 3 ppm now, but it's my backup for my Analox EII CO analyzer which is much easier to use and somewhat water resistant.
There are a couple of others that will test in the ultra low range we need to see now, unlike when I first got interested, but these are my favs.And yeah, I will test even in Cozumel where the main fill facility has Analox Clear monitors and others there are receiving monitors in a DAN project. Every fill station should have those as they cost pennies/tank, but that's to protect their business quality, not me. There are things that can wrong with them too, like not having auto shut off, etc. You can trust your air source and that's nice, but you don't know about your tank unless you test it personally.
How did you get a mistaken reading? 
