In preparation for the high-luminosity LHC phase, the ATLAS detector will be upgraded with a new silicon inner tracker, the ITk, relying on a cooling system based on carbon dioxide (CO2) evaporative properties.In order to test the key aspects of the cooling, prototypes of the cooling system for the different ITk Outer Endcap layers is built in Milan and tested at the CERN CO2 BabyDEMO cooling plant. The facility is able to provide a CO2 flow of 150 g/s with a temperature as low as -45 °C. The presentation illustrates the mechanical construction of the prototype and the use of 3D-printed titanium parts. The thermal load of the detector (up to 3 kW on the Layer 4 half-shell during normal operation) is also simulated. The sizing of the capillary present in the system, required to reach the design pressure drop of 8 bar and to trigger the CO2 evaporation, is also be discussed. The pressure and temperature sensors installed in the prototype and the data acquisition is described. The measurements performed at the BabyDEMO cooling plant, both at the nominal ATLAS operating condition and in more extreme scenarios, will be described. The systems are proved to be stable under all the conditions tested, and the total pressure drops are consistent with the requirements of the system specification.