welcome
yep would second that, I used to but would never dive it without a sensor system. first rule - know what you are breathing! those orifices are so tiny it wouldn't take much to block one and you wouldn't know!
You can get several systems, such as the OXY2 (uses 2 sensors) that screws onto the inhale side of the breathing loop hose and transmits it's readings to the computer wirelesly (Aladdin AirZO2)
Or you can get something like a VR3 which has an external single sensor which plugs straight into the breathing bad via the spare P-connector. This is a much more capable computer and can also do CCR.
Drager also make a single sensor system but just tells the the ppo2, is not a dive computer.
I'm currently looking at the 3 sensor system from Delta-P for the VR3 and how this would fit using a custom 3 sensor housing.... so lots of options!
Enjoy
Matt