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Power input is an important engineering and scale-up/down criterion in stirred bioreactors. However, reliably measuring power input in laboratory-scale systems is still challenging. Even though torque measurements have proven to be suitable in pilot scale systems, sensor accuracy, resolution, and errors from relatively high levels of friction inside bearings can become limiting factors at smaller scales. An experimental setup for power input measurements was developed in this study by focusing on stainless steel and single-use bioreactors in the single-digit volume range. The friction losses inside the air bearings were effectively reduced to less than 0.5% of themeasurement range of the torque meter. A comparison of dimensionless power numbers determined for a reference Rushton turbine stirrer (N-P = 4.17 +/- 0.14 for fully turbulent conditions) revealed good agreement with literature data. Hence, the power numbers of several reusable and single-use bioreactors could be determined over a wide range of Reynolds numbers between 100 and > 10(4). Power numbers of between 0.3 and 4.5 (for Re = 10(4)) were determined for the different systems. The rigid plastic vessels showed similar power characteristics to their reusable counterparts. Thus, it was demonstrated that the torque-based technique can be used to reliably measure power input in stirred reusable and single-use bioreactors at the laboratory scale.