We consider the flow inside an unbaffled stirred tank at Re = 600 and decompose the pressure field into components that are constant, linear and quadratic with respect to the temporal coefficients of velocity POD (proper orthogonal decomposition) modes. The pressure components are used to analyse the power number of individual blades and blade combinations. We also employ an identification algorithm to derive a linear dynamic estimator that allows the reconstruction (in real time) of the instantaneous 3D velocity field from sparse pressure measurements at the impeller blades. The first pair of POD modes are reconstructed with reasonable accuracy using the pressure signal from a single sensor. The results improve with 6 or 12 sensors. Application of the estimator to Re=500 and 700 shows that it is robust to changes in operating conditions. The work opens the possibility for real time control of mixing with targeted feeding location and rate.