Device for detecting parameters of a breathing gas, comprising - base body which is a breathing mask or a mouthpiece and has a breathing gas guide section , - measuring device for detecting parameters of the breathing gas, - computing unit for processing the detected parameters of the breathing gas, and - an energy store for supplying energy to at least the measuring device and the computing unit ; wherein the device is a mobile spiroergometry device, the measuring device comprises at least a first laser (L1) and a second laser (L2) for illuminating the breathing gas guide section and a detector (D) for measuring the absorption, and the measuring device is provided on the base body for detecting the parameters directly in a breathing gas flow which is guided through the base body , so that an in-situ detection of the parameters of the breathing gas is made possible; wherein the breathing gas guide section is tubular and the at least first laser (L1), the second laser (L2) and/or the detector (D) are provided flexibly on the circumference of the breathing gas guide section , so that the position of the at least first laser (L1), the second laser (L2) and/or the detector (D) relative to the breathing gas guide section is variable; and the at least first laser (L1), the second laser (L2) and/or the detector (D) are configured to vary the measuring range of the measuring device by varying the path length of the first laser (L1) and/or the second laser (L2); wherein the inner region of the breathing gas guide section has at least partially a reflective surface and comprises a circular reflector and the first laser (L1) and the second laser (L2) are configured to illuminate the inner region of the breathing gas guide section . - base body which is a breathing mask or a mouthpiece and has a breathing gas guide section , - measuring device for detecting parameters of the breathing gas, - computing unit for processing the detected parameters of the breathing gas, and - an energy store for supplying energy to at least the measuring device and the computing unit ; the device is a mobile spiroergometry device, the measuring device comprises at least a first laser (L1) and a second laser (L2) for illuminating the breathing gas guide section and a detector (D) for measuring the absorption, and the measuring device is provided on the base body for detecting the parameters directly in a breathing gas flow which is guided through the base body , so that an in-situ detection of the parameters of the breathing gas is made possible; wherein the breathing gas guide section is tubular and the at least first laser (L1), the second laser (L2) and/or the detector (D) are provided flexibly on the circumference of the breathing gas guide section , so that the position of the at least first laser (L1), the second laser (L2) and/or the detector (D) relative to the breathing gas guide section is variable; and the at least first laser (L1), the second laser (L2) and/or the detector (D) are configured to vary the measuring range of the measuring device by varying the path length of the first laser (L1) and/or the second laser (L2); wherein the inner region of the breathing gas guide section has at least partially a reflective surface and comprises a circular reflector and the first laser (L1) and the second laser (L2) are configured to illuminate the inner region of the breathing gas guide section . <EOS>