A measuring vessel in which a gas to be analyzed by spectrometry is intended to flow, the vessel being in the form of a hollow tube provided with a reflective material forming an optical-reflection layer, including a hollow tube is produced from a non-metallic material, and a removable supple optical article is applied against the internal surface of the hollow tube, the article including a supple flexible support, one face of the support being covered with a reflective metal material, the article being inserted in the tube so that the reflective metal material forms the optical-reflection layer.