For gas-phase oxygen measurements, the oxygen concentration is typically measured in vol% O2. If oxygen evolution occurs, but the concentration does not exceed 1 vol% O2, pressure changes from the gas evolution can typically be neglected. Then, the absolute amount of O2 (e.g. in µmol(O2)) can be calculated based on the headspace volume of the photoreactor. If higher amounts of oxygen are formed, the partial pressure of O2 can also be measured instead to account for the pressure change. In this case, the headspace volume can again be used to calculate the absolute amount of O2.