Abstract: A device for detecting the presence of a gaseous fluid in relation to a predetermined level of liquid in an elastically deformable vessel comprises a U-shaped body capable of accommodating within it a portion of the vessel at a predetermined liquid level. A pair of piezoelectric transducers in the body, used as transmitter and receiver, is fixed face-to-face on opposite arms of the U-shaped body. The device moreover comprises a cover capable of confining the portion of the vessel inside the U-shaped body, and an elastic device disposed between the cover and the U-shaped body and capable of producing an elastic deformation of the vessel and causing the contact surfaces of transducers to contact intimately the said portion of the vessel without entrapment of air.