Measuring Humidity in Methane and Natural Gas with a Microwave Technique

The results of microwave measurements with a quasi-spherical resonator in humid methane samples realized under laboratory conditions at the Istituto Nazionale di Ricerca Metrologica (INRiM) and under industrial conditions in a natural gas sample made available at the facilities of the Technical Manager of the Spanish Gas System and main supplier of natural gas in Spain (ENAGAS) are reported. Measurements at INRiM included vapor phase and condensation tests on methane samples prepared with amount fractions of water between 600ppm and 5000ppm at temperatures between 273K and 295K and pressures between 150kPa and 1MPa. ENAGAS measurements were performed at ambient temperature, 750kPa on natural gas sampled from the pipeline and successively humidified at amount fractions of water between 140ppm and 250ppm for completeness of the comparison with several humidity sensors and instrumentation based on different technologies. To enhance the sensitivity of the microwave method at low humidity, an experimental procedure based on the relative comparison of the dielectric permittivity of the humid gas sample before and after being subject to a chemical drying process was conceived and implemented. The uncertainty budget and the final sensitivity of this procedure are discussed.