Method for measuring a concentration of a gas in a container having a wall with at least one deformable portion, the gas absorbing electromagnetic radiation at least in a specific spectral range, wherein the method comprises the steps of: - biasing said deformable portion and a further portion of said wall opposite said deformable portion between first and second opposite positioning surfaces, thereby forming a biased volume of said container between said first and second opposite positioning surfaces and defining a length of a radiation path inside said biased volume, - during a measuring time , transmitting electromagnetic radiation into said biased volume between said first and second opposite positioning surfaces and receiving transmitted or reflected radiation of said transmitted radiation from said biased volume along said length of said radiation path, - relatively moving , during said measuring time, at least one of said deformable portion and of said further portion and at least one of said radiation paths, and - determining said concentration of said gas from the radiation received, characterized in that said transmitting , said receiving and said relatively moving are performed simultaneously, said relatively moving is induced by moving said first positioning surface of said opposite positioning surfaces with respect to said second positioning surface of said opposite positioning surfaces, thereby holding said length of said radiation path substantially constant, and in that said moving said first positioning surface holding said length of said radiation path substantially constant is one of: - a translation perpendicular to said radiation path, - a rotation around an axis along said radiation path, and - a tilting around a point defining said radiation path, such as a point on a reflector. - biasing said deformable portion and a further portion of said wall opposite said deformable portion between first and second opposite positioning surfaces, thereby forming a biased volume of said container between said first and second opposite positioning surfaces and defining a length of a radiation path inside said biased volume, - during a measuring time , transmitting electromagnetic radiation into said biased volume between said first and second opposite positioning surfaces and receiving transmitted or reflected radiation of said transmitted radiation from said biased volume along said length of said radiation path, - relatively moving , during said measuring time, at least one of said deformable portion and of said further portion and at least one of said radiation paths, and - determining said concentration of said gas from the radiation received, said transmitting , said receiving and said relatively moving are performed simultaneously, said relatively moving is induced by moving said first positioning surface of said opposite positioning surfaces with respect to said second positioning surface of said opposite positioning surfaces, thereby holding said length of said radiation path substantially constant, and in that said moving said first positioning surface holding said length of said radiation path substantially constant is one of: - a translation perpendicular to said radiation path, - a rotation around an axis along said radiation path, and - a tilting around a point defining said radiation path, such as a point on a reflector. - a translation perpendicular to said radiation path, - a rotation around an axis along said radiation path, and - a tilting around a point defining said radiation path, such as a point on a reflector. <EOS>