Method and filling device for filling a transportation container with a fluid

In the case of a method for filling a transportation container (12) with a fluid, the fluid in a purification circuit (1) is conveyed multiple times through the purification installation (7), and by way of a contamination-measurement installation (15) a key contamination indicator of a fluid-specimen quantity in the purification circuit (1) is determined, wherein a filling procedure of the transportation container (12) with the fluid is terminated only once the key contamination indicator undershoots a first threshold value. The fluid-specimen quantity that is provided for determining the key contamination indicator may be diverted from the purification circuit (1), be infed to the contamination-measurement installation (15), and upon determining the key contamination indicator be returned to the purification circuit (1). In a container-purification step, a container-fluid quantity from the purification circuit (1) that has previously been introduced into the transportation container (12) may be again retriev