Hopefully your instructor will verse you well about knowing the proper millivolts of your oxygen sensors at both high & low PO2’s & how they should read linearly as the PO2 changes. When I was going through my CCR course, my instructor kept me on my toes for months (I got my CCR in the winter, so used the local indoor pool to get myself thoroughly comfortable with the unit until the following spring, when the waters warmed up). He would continuously quiz me on gases, PO2’s Procedures & emergency procedures. I had close to 50 hrs on the machine in the pool, before I ever took it to the water.