In this paper is described a new type of evaporimeter, the principle of operation of which is based on physical laws governing the surface tension phenomenon. The measurement of evaporation in, the new evaporimeter, described as Evaporimeter IA-58, is made by simple volumetric procedure, thus avoiding the use of precise and expensive mechanisms. A mathematical analysis of the principle of operation is made, and proves that the new evaporimeter can offer satisfactory xeaults for practical use. The operation of the new evaporimeter is very simple, since only volumetric measurements are made, with a graduated cylinder of 1000 ml, both of the added water to restore the water level to the same initial level and the outflown or excess water. It is believed that enough details are given to allow the understanding of the construction, installation and operation of the new evaporimeter, bo that interested persons or organizations could have the model reproduced and verify the advantage of its use.