Tunable Diode Laser Absorption Spectroscopy Sensor for Calibration Free Humidity Measurements in Pure Methane and Low CO 2 Natural Gas

Abstract

We report a new direct tunable diode laser absorption spectroscopy (dTDLAS) sensor for absolute measurements of H₂O in methane, ethane, propane, and low CO₂ natural gas. The sensor is operated with a 2.7 µm DFB laser, equipped with a high pressure single pass gas cell, and used to measure H₂O amount of substance fractions in the range of 0.31–25 000 µmol/mol. Operating total gas pressures are up to 5000 hPa. The sensor has been characterized, addressing the traceability of the spectrometric results to the SI and the evaluation of the combined uncertainty, following the guide to the expression of uncertainty in measurement (GUM). The relative reproducibility of H₂O amount of substance fraction measurements at 87 µmol/mol is 0.26% (0.23 µmol/mol). The maximum precision of the sensor was determined using a H₂O in methane mixture, and found to be 40 nmol/mol for a time resolution of 100 s. This corresponds to a normalized detection limit of 330 nmol mol⁻¹·m Hz^−1/2. The relative combined uncertainty of H₂O amount fraction measurements delivered by the sensor is 1.2%.