A miniaturized trace-moisture sensor based on cavity ring-down spectroscopy

Abstract We developed a miniaturized sensor based on cavity ring-down spectroscopy (the mini-CRDS sensor) for the real-time measurement of trace moisture. The dimensions and mass were 24 cm × 9 cm × 13 cm and 2.7 kg, respectively, exclusive of the laser controller and data processing unit. The distance between the two cavity mirrors was 5 cm, and the internal volume and surface area of the cavity between the mirrors were approximately 4 cm3 and 16 cm2, respectively. The Allan-Werle deviation of trace-moisture measurement was approximately 0.46 nmol/mol at an averaging time of 10 s. The minimum detectable absorption coefficient for an averaging time of 10 s was 6.8 × 10−10 cm-1. The limit of detection (3 times the standard deviation) of moisture in nitrogen was 3.1 nmol/mol (3.1 ppb) in the amount-of-substance fraction (mole fraction). The relative standard uncertainty was tentatively evaluated as 10 % to 0.67 % for the range of 12 nmol/mol (12 ppb) to 1.3 μmol/mol (1.3 ppm). The measurement accuracy of the mini-CRDS in the above range was evaluated by comparison with a primary trace-moisture standard that is metrologically traceable to the International System of Units (SI), and the results demonstrated good agreement between the mini-CRDS and the primary trace-moisture standard.