An arrangement for evaluating an effective flow resistance of a breathing circuit (4) connected to a mechanical breathing assist device (2), the arrangement comprises: a flow controller (8,28) operable to temporarily introduce an occlusion to gas flow within the breathing circuit (4) at a time after the end of an inspiration phase of a breathing cycle provided by the breathing assist device (2)； a sensor unit 34 having a flow sensor for measuring gas flow within the circuit (4) and a pressure sensor for measuring gas pressures within the circuit (4)； and an evaluating unit (6) adapted to receive measurements from the sensor unit (34), to determine for a measured gas flow a value of a pressure drop within the breathing circuit (4) consequent on the introduction of the occlusion and to establish a relationship between the calculated pressure drop and the measured gas flow, preferably based on the known Blasius formula. <；IMAGE>；
