A chest drainage system, comprising: - a circulation assembly having: ∘ an intake for taking fluid into the system and an exhaust for exhausting fluid out of the system; ∘ an intake flow device configured to selectively control fluid flow through the intake and an exhaust flow device configured to selectively controlling fluid flow through the exhaust, the circulation assembly having a first configuration and a second configuration such that transitioning between the first and second configurations during operation of the circulation assembly displaces at least a portion of fluid within the system with fluid from outside the system via the intake and the exhaust, wherein the intake flow device and the exhaust flow device together comprise a pair of check valves (102, 104) arranged to permit flow in opposite directions, wherein a first valve of the pair of check valves opens to permit pressure in through the intake upon a first pressure relating to a patient inhaling, and the second valve of the pair of check valves opens to permit fluid to the exhaust upon a second pressure relating to an impermissibly high pressure within the system; and ∘ a sensor arranged in fluid communication with the fluid within the system and configured to detect a concentration of a reference fluid in the fluid in the system. - a circulation assembly having: ∘ an intake for taking fluid into the system and an exhaust for exhausting fluid out of the system; ∘ an intake flow device configured to selectively control fluid flow through the intake and an exhaust flow device configured to selectively controlling fluid flow through the exhaust, the circulation assembly having a first configuration and a second configuration such that transitioning between the first and second configurations during operation of the circulation assembly displaces at least a portion of fluid within the system with fluid from outside the system via the intake and the exhaust, wherein the intake flow device and the exhaust flow device together comprise a pair of check valves (102, 104) arranged to permit flow in opposite directions, wherein a first valve of the pair of check valves opens to permit pressure in through the intake upon a first pressure relating to a patient inhaling, and the second valve of the pair of check valves opens to permit fluid to the exhaust upon a second pressure relating to an impermissibly high pressure within the system; and ∘ a sensor arranged in fluid communication with the fluid within the system and configured to detect a concentration of a reference fluid in the fluid in the system. ∘ an intake for taking fluid into the system and an exhaust for exhausting fluid out of the system; ∘ an intake flow device configured to selectively control fluid flow through the intake and an exhaust flow device configured to selectively controlling fluid flow through the exhaust, the circulation assembly having a first configuration and a second configuration such that transitioning between the first and second configurations during operation of the circulation assembly displaces at least a portion of fluid within the system with fluid from outside the system via the intake and the exhaust, wherein the intake flow device and the exhaust flow device together comprise a pair of check valves (102, 104) arranged to permit flow in opposite directions, wherein a first valve of the pair of check valves opens to permit pressure in through the intake upon a first pressure relating to a patient inhaling, and the second valve of the pair of check valves opens to permit fluid to the exhaust upon a second pressure relating to an impermissibly high pressure within the system; and ∘ a sensor arranged in fluid communication with the fluid within the system and configured to detect a concentration of a reference fluid in the fluid in the system.