A safety insert for an extra-corporeal circuit includes a first half-shell and a second half-shell forming a shell defining an inner duct, at least part of the second half-shell being made with a translucent material a first connector to be connected to the extra-corporeal circuit, and a second connector to be connected to a transducer and a first hydrophobic semi-permeable membrane having a first area A1 and a second hydrophobic semi-permeable membrane having a second area A2, with the two membranes being arranged in series along the inner duct. In a front view, the extent of an overlapping area A12 of the membranes is between 0% and 50% of the smaller of the two areas A1, A2. An extra-corporeal circuit includes the safety insert.
