Determination of water vapor by microwave spectroscopy with application to quality control of natural gas

The design and construction of a microwave spectrometer is reported. The aim of this instrument is to detect water vapor at low levels. Sinusoidal Stark field modulation is used in order to amplitude modulate the absorption of microwave energy by the water molecule, thus enhancing the sensitivity of the instrument. A semiconfocal Fabry-Perot resonator is used to hold the sample under test. The fourth harmonic of the Stark modulation frequency was used to show the quantitative presence of water in the sample gas. Both atmospheric air, and liquified natural gas (LNG) were used as carrier gases. The peak-to-peak intensity of the fourth harmonic was used to calibrate the instrument against different amounts of water present in atmospheric air. The observed limit of this prototype is around 300 p.p.m. water. No attempt was made to lower the detection limit at this point since the aim of this project was to examine the feasibility of using the fourth harmonic to determine water content reliably