An apparatus for monitoring the effective dissolving of a polymer when the use region is not accessible. The apparatus includes a pipe on which are inserted, consecutively: a pump, a flowmeter, a water or brine inlet mechanism for diluting the mother solution flowing in the pipe, a mixer capable of in-line homogenization of the diluted mother solution, a first tube calibrated to simulate the distance and the conditions for moving the diluted solution in the main pipe between the point where the mother solution is diluted and the use region, a mechanism capable of reducing the pressure of the diluted solution flowing in the pipe upstream of the first tube that is calibrated from 10 to 10000 kPa (from 0.1 to 100 bar), a second calibrated tube for creating a head loss, and a device for measuring differential pressure between the inlet and the outlet of the second calibrated tube.