Physical and chemical detection of abnormal and transient fluid transfers in hydrothermal systems are often associated with active and seismic areas. A particularly important parameter is the emission of gases in such systems. Monitoring the flow of gas bubbles, may supply some simple parameters to interpret in terms of global degassing processing. In this paper a very simple apparatus is displayed. This new sensor, which allows both the direct detection and quantization of gas fluxes and the online analysis of the bubbles-constituing gases will be described and tested. Results of laboratory test that show both the reliability of measurements and the good response time to gas injections and the first results of a monitoring test performed on a spring-water in the western Pyrenees (France) will be displayed here. This experiment shows that such measurements are adapted to the geochemical surveillance of hydrothermal environments in tectonically active areas and that parameters measured in this way may provide good indicators for the detection of precursors during seismic and/or volcanic monitoring.