It comprises, in addition to a sampling system (3), at the outlet of a vat (1) containing the liquid to be sampled, a selector (4) that establishes switching on an array of similar vats (1), and especially a suction unit (5) that operates by releasing compressed air into a nozzle (28); the airflow direction is controlled by a valve (29) between a direction directed toward the outlet (6), causing the suction of the liquid from the vat (1), and an opposite direction when the valve (29) is closed, feeding the liquid back into the vat (1) and cleaning the pipe (2).