the device comprises a laser source which emits a beam of a predetermined wavelength towards an optically transparent closed container and detectors arranged to detect the laser beam which is attenuated by the gas absorption . the detectors provide the first data of absorption representative of a first absorption spectrum of the gas including distorted absorption lines and noise . the invention implements a method of calculation aimed at receiving and elaborating said first data . the output of the operation are parameters that represent a second absorption spectrum free of noise and distortion in the absorption lines . from these parameters we are able to determine the gas pressure and concentration in the container .