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 and a second laser for illuminating the breathing gas guide section and a detector 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, the second laser and/or the detector are provided flexibly on the circumference of the breathing gas guide section, so that the position of the at least first laser, the second laser and/or the detector relative to the breathing gas guide section is variable; and the at least first laser, the second laser and/or the detector are configured to vary the measuring range of the measuring device by varying the path length of the first laser and/or the second laser; 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 and the second laser 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 and a second laser for illuminating the breathing gas guide section and a detector 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, the second laser and/or the detector are provided flexibly on the circumference of the breathing gas guide section, so that the position of the at least first laser, the second laser and/or the detector relative to the breathing gas guide section is variable; and the at least first laser, the second laser and/or the detector are configured to vary the measuring range of the measuring device by varying the path length of the first laser and/or the second laser; 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 and the second laser are configured to illuminate the inner region of the breathing gas guide section.