A device for determining the partial pressure or the concentration of a steam in a volume, includes a sensor body that can be oscillated. The temperature of the sensor body can be controlled to a temperature below the condensation temperature of the steam, and the oscillation frequency of the sensor body is influenced by a mass accumulation of the condensed steam on a surface of the sensor body. Means are provided for generating a gas flow from the sensor surface in the direction of the volume through a steam transport channel that adjoins a window to the volume. In order to increase the maximum service life of the sensor body, the means for generating a gas flow has a slit nozzle designed as an annular channel.