An apparatus for determining a value that represents an amount of liquid in a vessel, wherein the vessel is configured to be a tube or a cuvette. The vessel for the liquid is basically transparent, with at least one radiation unit, that is arranged alongside the longitudinal axis of the vessel and that radiates light into the vessel, as well as at least one light detector with at least one light receiver that is assigned to the radiation unit and that receives the light that is coming through the vessel and forwards it to the light detector. The apparatus comprises a data processing unit that determines the value that represents the amount of the liquid in the vessel from the light that was detected by the light detector. The apparatus is characterized in that the light detector detects the sum total of the light that was radiated through the vessel and was received by all light receivers. Furthermore, the invention relates to the use of the apparatus in an analyzer.