Comparison of hygrometers for monitoring of water vapour in natural gas

Hygrometers for monitoring of water vapour (moisture) in natural gases were compared with respect to accuracy, long term stability and response time. The hygrometers were based on capacitor sensor, quartz crystal microbalance (QCM), electrolytic cell, fibre-optic sensor and conversion of water to ethyne with calcium carbide (CaC2-GC), respectively. In the latter technique, ethyne was quantified by a gas chromatograph (GC). Monitoring of water vapour concentrations between 20 μmol/mol and 120 μmol/mol was carried out in the laboratory, using nitrogen as the matrix gas. The capacitor hygrometers and the electrolytic cell hygrometer tested in this work showed a tendency to drift, which reduced their accuracy. The QCM, fibre-optic and CaC2-GC hygrometer showed good accuracy and long term stability. The QCM hygrometer had the overall shortest response times. The results demonstrated the need for careful quality control of the hygrometers, and monitoring systems which are thoroughly fitted to the requirements of individual natural gas applications.