Calibration is a PITA if you have to open it up. Ask Brent/Divetheworld for his thoughts. Homebuild where you DO have to open it up is very simple, as mentioned above. However, if you know someone who can sort you with a PIC programmer, there are LOADS of examples on here where you could get a pic-based PO2 meter array set up - including the code.
Or take the lazy but sensible way out and buy one of Paul's rEvoDreams!
