An instrument adapted for the determination of fluid levels within a vessel by the comparison of the density of the fluid along the length of the instrument, said instrument comprising: (i) at least one generally linear source array comprising a plurality of sources of penetrating radiation, shielding and collimation means and a source supporting structure for supporting the sources and shielding and collimation means in a linear array (ii) at least one generally linear detector array comprising a plurality of radiation detectors, supported on a detector support structure, capable of detecting radiation emitted by the sources, and (iii) means to position the source and a respective detector array such that (a) radiation emitted from each of the sources is capable of following a linear path from the source, through the vessel and material to at least a respective one of the detectors forming a part of the respective detector array, and (b) the linear axis of the source array and the linear axis of its respective detector array are substantially parallel to each other (iv) means for analysing the detector output signals to determine the density of the medium traversed by the beams of radiation in passing from a source to a detector characterised in that the detector support structure and/or its associated electrical circuits and connectors is formed from a plurality of modular portions, each modular portion being adapted to be joined to an adjacent portion to provide a detector array of the required length.