Abstract: The specification discloses an apparatus for assessing the ingress of air into a steam system which includes a steam condenser having an exit, a vacuum pump arrangement for maintaining the exit under a partial vacuum, and pipework connecting the exit to the vacuum pump arrangement. A pitot probe is positioned at a first monitoring point in the pipework to sense the total dynamic and static pressure of the entire flow in that pipework. A second probe is positioned at a second monitoring point which is displaced from the first monitoring point and adjacent the exit from the condenser, to sense the static pressure and temperature at that second monitoring point. An indication of the amount of air flowing past said first monitoring point is derived from the total pressure, the static pressure and the temperature, which are measured at the first and second monitoring points.