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In this work, we present a sensor system for continuous non-invasive cell growth monitoring, especially designed for single use bioreactors (SUBs). The sensor system is based on a differential transformer. In theory, using this differential setup, the influence of the primary magnetic flux is fully eliminated from the sensor signal and the sensor signal is zero for an ideally symmetrical, unloaded sensor. In practice, due to small geometric asymmetries and resonance effects, the sensor signal of the unloaded sensor can have a significant offset. Thus, complex measuring electronics is required to detect the small signal variations caused by permittivity changes in the cell culture medium. In order to use simple, low-cost electronics a new method for zero balancing of the sensor signal is presented. This method is based on an additional axially arranged compensation coil. Different mixtures of isopropanol and water were used as test liquids with defined permittivity and to evaluate the sensor system. Our results show that the additional compensation coil does not affect the sensitivity of the sensor system. Thus, a low-cost sensor system with low-cost electronics can be realized.

P2.05 - Continuous non-invasive cell growth monitoring in disposable bioreactors using an active compensated differential transformer