The invention relates to a spirometer, more particularly an ultrasonic spirometer, in the case of which a pair of cells, of which one is a transmitter cell and the other is a receiver cell, are arranged in a test section with an oblique or perpendicular alignment to the axis of the duct containing the test section. For this purpose in accordance with the invention a readily replaced duct, which if necessary is sterile, is inserted into the test duct. In order to design such an ultrasonic spirometer so that it may be employed for clinical purposes, it is necessary for it to be kept sterile in a simple fashion. For this purpose in accordance with the invention a sterile duct in inserted into the test duct with an accurate fit and at the transition between it and the test section it has test windows such that inserts are inserted into corresponding openings which are transparent for acoustic waves but substantially impermeable for microorganisms and dirt.