The liq. is driven along a tube (6) having a section (7) of e.g. plasticised PVC elastically deformable by rollers (5) at the extremities of the rotor (4) of a peristaltic pump (3). The inlet portion (8) of the tube is equipped with an e electromechanical transducer (10). The movingg part (22) of the transducer is spring-loaded (23) and its position is measured (24) for an indication of pressure in the incoming liq. The output of the transducer (10) is amplified (14) and compared with a reference value from a potentiometer (16). The signal from the comparator (15) is amplified (18) to operate an alarm lamp (13).
