A device for measuring in situ, the optical density of a sample in a medium, in which the immersed sample has a main part with a slot at one end. A mobile part can be lodged in this slot. This mobile part has an open chamber which moves with the mobile part into the slot. This open chamber will contain a sample and will be aligned with an optical device in the housing, when moved into the slot. The mobile part and open chamber are, therefore, movable between an open position in which the medium to be studied circulates in the chamber, to a closed position when the mobile part is lodged in the slot whereat the open chamber is isolated from the medium to be studied. Measurement is carried out while the device is in the closed position by measuring the optical density of the isolated medium.