Comparison of continuous moisture monitors in the range 1 to 15 ppm

Electrolytic, chilled mirror, and aluminum oxide dielectric hygrometers were evaluated by exposing them to known moisture levels in gaseous argon. The experiments were performed over a period of several months to determine the long-term hysteresis effects which may occur under continuous operation in the range of 1 to 15 ppm (parts per million). Air exposure effects were also examined. The results indicated hysteresis and drift problems in the case of the aluminum oxide dielectric hygrometers. The electrolytic hygrometer required a ‘‘recovery’’ period following exposure to high moisture levels. The chilled mirror hygrometer (with certain specified modifications) proved to be the most accurate and responsive in the 1- to 15-ppm test range.