The CO sensor does not need to be replaced when calibrating.
The MonOx is new and seems to only be in the store section:
monOx - DiveNav Store
I'm not sure what you mean by "the other two self-calibrate"? The Cootwo/MoxOx can be CO calibrated by the user if you buy bump gas - you need a "zero" CO reference gas and a CO reference gas (they reco 10, 15 or 20 ppm, I believe - you enter what you used into the unit during calibration). I would imagine that the zero reference/bump gas requirements and calibration processes are similar with the other 2 units too?
ediving.us • View topic - cootwo & monOx: Carbon Monoxide sensor calibration
Yes, but with a single bump gas to calibrate each one. By self-calibrate, I mean they can be done in the field, by the operator, like the monOx and cootwo. Being on the store page but not the products page explains how I missed the monOx. The store pages have a lot more info.
They don't sell the bump gas as far as I can tell. Except for OxyCheq, the places I found that do sell it sell a small tank of it with a regulator and hose, in the $225-$345 range depending on how much gas you get. So let's look at 5-year COO for the monOx.
$229 for the monOx, another $225 for bump gas, unknown cost for 0 PPM test gas, and $70 for the sensor every two years. $225 + $140 is $365 vs. $429 for the OxyCheq, but that leaves out the 0 PPM test gas. Or, you can send it in for four calibrations at $60 each, including shipping back to you, for $240, adding $15 for a total of $380.
If you trust a zero reading from the monOx for a tank you can mark and dedicate to using once a year, maybe you can use that for the zero test, but then you have to factor in the cost of the tank ($100 for a 6 cu ft pony from Leisurepro today), or the cost of a tank of actual 0 PPM calibration gas, which I did not research.
Back to the cootwo, for which I missed field calibration and 2-year CO sensor life, but also missed the 1-year O2 sensor life, while most O2 analyzers use a sensor that costs about the same but usually lasts 5 years. So to the above, add $75 plus an O2 sensor every year at another $280.
With any luck, I've added things up right this time...