Absorption spectroscopy is a relatively simple method of passing light through a gas sample and measuring the amount of light absorbed at the specific wavelength. Traditional spectroscopic techniques have not been successful at doing this in natural gas because methane absorbs light in the same wavelength regions as water. But if one uses a very high resolution spectrometer, it is possible to find some water peaks that are not overlapped by other gas peaks.
The tunable laser provides a narrow, tunable wavelength light source that can be used to analyze these small spectral features. According to the Beer-Lambert law, the amount of light absorbed by the gas is proportional to amount of the gas present in the light’s path; therefore this technique is a direct measurement of moisture. In order to achieve a long enough path length of light, a mirror is used in the instrument. The mirror may become partially blocked by liquid and solid contaminations, but since the measurement is a ratio of absorbed light over the total light detected, the calibration is unaffected by the partially blocked mirror (if the mirror is totally blocked, it must be cleaned).
The Tunable Diode Laser Absorption Spectroscopy (TDLAS) analyzer has a higher upfront cost compared to the analyzers above. However, the TDLAS technology is the only one that can meet any one of the following: the necessity for an analyzer that will not suffer from interference or damage from corrosive gases, liquids or solids, or an analyzer that will react very quickly to drastic moisture changes, or an analyzer that will remain calibrated for very long periods of time.