traceability to International Standards is compromised. Operation at near atmospheric pressure do provide traceability and offer other significant benefits, such as enabling the direct validation against known moisture content. Spectroscopy Absorption spectroscopy is a relatively simple method of passing light through a gas sample and measuring the amount of light absorbed at a 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 the 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). A TDLAS analyzer has a higher upfront cost compared to most of the analyzers above. However, tunable diode laser absorption spectroscopy is superior when it comes to 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, assuming the gas composition does not change. See also Karl Fischer titration References Desiccation Measurement Psychrometrics Natural gas