The apparatus (1) comprises a sensor (3) having a sensor axis (A), for monitoring at least one property of a fluid; a body (2) for comprising said fluid; and a connecting system for detachably connecting said sensor to said body; wherein said connecting system comprises a fixing member (9) arranged at said body, rotatable around an axis (R) of rotation; and a retaining member (6) arranged at said sensor, interacting with said fixing member; wherein said axis of rotation is arranged substantially perpendicular to said sensor axis. The method is a method for detachably connecting a sensor (3) for monitoring at least one property of a fluid to a body (2) containing said fluid, said sensor having a sensor axis (A). Said method comprises the step of rotating a fixing member arranged at said body in a first sense of rotation around an axis (R) of rotation, thus causing said fixing member to interact in such a way with a retaining member arranged at said sensor, that said retaining member moves in a substantially translational way substantially parallel to said sensor axis. Said axis of rotation is arranged substantially perpendicular to said sensor axis. Preferably, said fixing member has a cross-section having substantially the shape of a section or a segment of an ellipse, in particular it has a substantially semi-circular cross-section.