Results of the analysis of velocity data obtained by the laser Doppler velocimetry (LDV) in a flat-bottomed cylindrical stirred tank (300 mm diameter, filled with the liquid to the height equal to its diameter) equipped with four radial baffles and stirred with a pitch-blade turbine impeller are presented. Water and 85% aqueous glycerine solution were used as working fluids at three values of Reynolds number of the impeller: 75000, 1200 and 750. The fluid velocity was recorded in a rectangular region of evenly spaced points close to the stirrer region. The macro-instability (MI) of the velocity field was extracted from the experimental data using the proper orthogonal decomposition (POD) technique and spectral analysis. This approach enabled not only detection of the macro-instability in measured velocity data but also to quantify its relative magnitude (a ratio of kinetic energy captured by the macro-instability to total kinetic energy of the flow). The dependence of chaotic invariants of the MI on the Reynolds number and position of the measuring point in the vessel is constructed. The time evolution of the MI is reconstructed from experimental records.