Abstract: An apparatus for continuous measurement of the proportion of relatively non-condensable gases in a flow of mixed gases, such as geothermal steam. The apparatus is an open system which detects the proportion of relatively non-condensable gases in a sample flow continuously. The sample flow is condensed and supplied in a tube which conditions the sample flow to consist of a series of segments of condensate and segments of uncondensed gases. The sensor senses the relative volume of the condensate and the uncondensed gases. The data processing means receives the output of the sensor and calculates the proportion of non-condensable gases in the flow of mixed gases. In the preferred embodiment, the sensor optically detects the ratio of the volume of the condensate to the volume of uncondensed gases.