In order to limit the thermal gradient of plates during quenching and limit flatness issues, it is necessary to control the cooling. This control requires the knowledge of the cooling performances of the cooling device. This paper describes the equipments and procedure we developed for the on-line characterization of quenching. We illustrate the application of this methodology to the quench of Burns Harbor. After the austenitzing furnace, this quench is equipped with 3 cooling sections that have been characterized for several flow rates, for several speeds of plates and for the top and bottom sides of plates. Thanks to the collected data, heat fluxes have been calculated and correlations describing the cooling performances of the equipment have been built. These correlations have been implemented in a thermal model that gives the thermal state of a plate during cooling according to parameters like thickness of the plate, flow rates of the cooling sections, speed of the plate, and temperature after heating. Experimental and numerical results are discussed.