Forel-Ule scale

The Forel-Ule Scale is a method to approximately determine the color of bodies of water, used in limnology and oceanography. By means of different inorganic compounds (ammonia, copper sulfate, neutral potassium chromate) a color palette is produced in a series of numerically designated vials (00-21) which is compared with the color of the water body. The result is a color index for the water body which gives an indication of the transparency of the water and thus helps to classify gross biological activity. The color graduations correspond to open sea and lake water colors, as they appear to an observer ashore or on board a vessel. The method is used in conjunction with the Secchi disk.

The method was developed by François-Alphonse Forel and was three years later improved by the german limnologist Willi Ude. The Scale is quite imprecise and thus rarely used in modern research. It was substituted by the Platin-Cobalt-Scale which is less subjective.