1. A breath analysing and training assembly adapted to detect the carbon dioxide (CO2) concentration in the breathing gas of a user, and a mobile unit comprising an image capturing means, a display and a processing means, the assembly comprises a selective colorimetric CO2 detector provided with a detector surface adapted to change color rapidly and reversibly with the concentration of CO2, when exposed to CO2, and an adapter comprising a docking part for receiving and attaching said mobile unit, wherein the docking part is configured to position said image capturing means in a fixed relation to said colorimetric detector, such that said image capturing means is adapted to capture images of said detector surface, wherein at least a part of said display is arranged to illuminate said detector surface during image capturing, a detector holding part adapted to receive said colorimetric detector and attach said detector, a breathing conduit to lead breathing gas to and from said user such that at least a part of the breathing gas passes the detector surface, and a detector surface protecting means adapted to protect the surface from ambient air in an airtight fashion when no measurement is performed, the detector surface protecting means being movable from a protection position to a non-protection position when said mobile unit is received by said docking part, said detector surface being available for measurements, when said detector surface protecting means is in said non-protection position, and wherein said processing means is adapted to execute an application program adapted to perform a measurement session including a measurement step that includes measuring CO2 concentration changes in breathing gas by identifying changes in the optical characteristics of said detector surface captured by said image capturing means.