A chest drainage system for creating and maintaining a sub-atmospheric pressure within the pleural cavity and/or the mediastinum of an associated patient (P), the system comprising: - a flexible chest tube adapted for insertion into the chest of said associated patient, - a chest drainage unit (CDU, 10) being connected to the said flexible chest tube, the chest drainage unit comprising ∘ an internal cavity (C) having a first chamber part (C1, 11) with an air inlet into the CDU, and a second chamber part (C2, 12) with an air outlet from the CDU, and ∘ a liquid seal chamber (LSC, 13) separating the said first chamber part and said second chamber part so as to prevent air from passing from the second chamber part back into the first chamber part, at least when having sufficient liquid in the liquid seal chamber, - an optional pump being connectable to said outlet of the second chamber part, the pump being arranged for creating a suction pressure within the CDU, at least in an operational pumping condition of the chest drainage system, wherein the second chamber part (C2, 12) of the chest drainage unit is connected to a carbon dioxide sensor (CO2S, 15) for detecting carbon dioxide in any air passing through the liquid seal chamber (LSC, 13), characterized in that the carbon dioxide sensor is capable of detecting carbon dioxide by a visible color change from a chemical reaction occurring in the carbon dioxide sensor between carbon dioxide and a detector reactant (DC) positioned in the carbon dioxide sensor, said detector reactant (DR) in the carbon dioxide sensor is positioned, or dissolved, in the liquid seal chamber (LSC) at least when having sufficient liquid in the liquid seal chamber and wherein the carbon dioxide sensor - after a carbon dioxide positive detection- returns to a mode of indicating negative carbon dioxide presence in the air passing the liquid seal lock after at least 2 hours. - a flexible chest tube adapted for insertion into the chest of said associated patient, - a chest drainage unit (CDU, 10) being connected to the said flexible chest tube, the chest drainage unit comprising ∘ an internal cavity (C) having a first chamber part (C1, 11) with an air inlet into the CDU, and a second chamber part (C2, 12) with an air outlet from the CDU, and ∘ a liquid seal chamber (LSC, 13) separating the said first chamber part and said second chamber part so as to prevent air from passing from the second chamber part back into the first chamber part, at least when having sufficient liquid in the liquid seal chamber, ∘ an internal cavity (C) having a first chamber part (C1, 11) with an air inlet into the CDU, and a second chamber part (C2, 12) with an air outlet from the CDU, and ∘ a liquid seal chamber (LSC, 13) separating the said first chamber part and said second chamber part so as to prevent air from passing from the second chamber part back into the first chamber part, at least when having sufficient liquid in the liquid seal chamber, - an optional pump being connectable to said outlet of the second chamber part, the pump being arranged for creating a suction pressure within the CDU, at least in an operational pumping condition of the chest drainage system, <EOS>