An apparatus and method for determining a level of a fluid within a vessel are disclosed. The apparatus includes: a source unit for emitting a beam of radiation into the interior of the vessel, the source unit including a source of radiation and a collimator for collimating radiation emitted by the source to provide the beam, wherein the source unit is adjustable to vary an angle of the beam with respect to horizontal; at least one detector for detecting radiation emitted by the source and having passed through at least a portion of the interior of the vessel; and a processor for: recording data corresponding to an amount of radiation detected at the at least one detector as a function of angle of the beam; and determining the level of the fluid, based on a variation of the data with the angle of the beam.