Abstract: The present invention related to a method and a device for use in measuring the gradient of density in a fluid. The device is comprised of a laser for providing a columnar light beam and a photodetector axially aligned with the laser. The photodetector detects the displacement of the light beam caused by the gradient of refraction of the fluid. A unit is provided for converting the displacement into an indication of the gradient of refraction and thereby the gradient of the density of the fluid. For density gradients caused by concentration gradients, an additional unit is provided for converting the displacement into a measure of the quantity of substances in the fluid.