Device that can be used in an enhanced oil recovery method by injection of a solution of water soluble polymer and brine having a viscosity below 1000 cps that includes a mixer, and preferably a static mixer, capable of homogenizing the solution on-line; a device capable of measuring the viscosity of the homogenized solution, continuously, downstream of the solution injection pump, at a pressure below or equal to 250 bars, and preferably between 50 and 250 bars, and at a temperature below or equal to 120° C., and preferably between 40 and 120° C., by measuring a pressure drop in a calibrated tube, at a constant rate of flow.