I try to investigate oxygen consumption in my cell culture. However, the oxygen level stays at 100 % O2. Is there something wrong with the SDR measurements, and what can I do?

If the oxygen level stays at 100 % O₂ oxygen ingress into the sample is greater or equal to the oxygen consumption of your cells. If increasing the oxygen consumption of your cells (e. g. using more cells) is not possible, try using a layer of mineral oil (e. g. 100 μL/well). The oil reduces oxygen ingress. It is not recommended to use adhesive foil, as the remaining headspace between sample and foil also contains oxygen. Even using a maximum filling volume to reduce this air phase can result in large well-to-well deviations, as the remaining, mostly inevitable air bubbles will be different in each well.