There is disclosed an apparatus 10 and method for determining an amount of non-condensable gas within a fluid flow containing both non-condensable gas and condensate liquid. The method comprises receiving the fluid flow in a fluid displacement volume 18 causing the non-condensable gas to rise within the volume 10; monitoring the liquid level of a free surface in the fluid displacement volume 18 to generate a level signal; monitoring the flow of liquid out of the fluid displacement volume 18 to generate a liquid flow signal; and determining the amount of non-condensable gas based on the level signal and the liquid flow signal.