The device comprises a connecting pipe (18) having an inlet (18A) capable to be connected to the feed and an outlet (18B) capable to be connected to the storage vat (10), as well as a retaining chamber (20) which branches off from the connecting pipe (18) and which is equipped with a sampling valve (24). The connecting pipe comprises a charging valve (22), located between the retaining chamber and the outlet of the connecting pipe. After the product originating from the feed has been conveyed to the retaining chamber (20), and a sample of the product mixture present in the retaining chamber (20) has been taken to ascertain information relative to the nature of this mixture, and if the ascertained information corresponds to expected information, a charging valve (22) is opened to have the outlet of the connecting pipe (18) communicate with the vat (10).