Water vapor concentration in air, often expressed as relative humidity, plays a key role in many fields including mass metrology. In addition, we discuss the possibility of using water vapor detection by spectroscopy to observe the change of the shape and the position of the absorption line produced during measurements of the dew point temperature by the optical chilled mirror hygrometer. This paper reports the measured frequency positions of the observed line at λ = 1392.5337 nm for different air pressures and compares them with the values given in the HITRAN database. Here, using the molecular absorption device the use of such an instrument is examined from the point of view of its impact on the measurement of relative humidity. The latter instrument is widely used due to its performance with regard to both accuracy and repeatability of measurements. For metrological needs, we are looking for a means to control the water vapor concentration in ambient air in near real time and especially when an optical chilled mirror hygrometer is used. First, we present the optical system that uses a distributed feedback (DFB) laser diode, emitting around λ ≈ 1392.5 nm with a power ≈ 3 mW and linewidth ≤ 10 MHz. In the case of water vapor, a particular strong absorption line around λ ≈ 1392.5 nm is suitable for the detection of molecules, not only because of its high strength but also because it is well separated from the neighboring transitions, thus avoiding any overlap. The HITRAN database (High-Resolution Transmission molecular absorption database) is an extremely helpful reference for selecting lines of molecular species.
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