I still own my 20 yrs old manual Bacharach kit. You know the one with the smoke gun pump and Red gauge fluid filled CO tester. Still works great...yes I'm a old timer but still learning...doing Controls now.
O2 sensors are $140 for just the sensor I calibrate myself with N2. There is no fluid for the CO there is just a CO sensor that looks like the O2 sensor that can be replaced. But the lifespan on the CO sensor is considerably longer than the O2 sensor. The O2 sensor has a shelf life and will expire. The CO sensor will not and should last for many years.
I used to clean mine out every 2 years or so and fill with new fluid. We tested mine along side the new electronic ones when the first came out. The old one was about +/-2 percentage points. The old ones just don't have the "wow" factor. lol
i use a ck75 made by uei, it''s ok, the only thing is you have to send in for calibration every 2 years to certify it's working,and change sensors, it runs about 2 1/2 digits if you get my drift, especially if you use the infered printer for a document for readings. if it's not re calibrated after 2 years and something happens it could create issues!