Abstract: Experimental studies were conducted for a special air assisted atomizer patented by Sadatomi and Kawahara (2012); in which atomization is attained by supplying the pressurized air alone into a mixing chamber inside the nozzle, and water can be sucked automatically with siphon principle. Since the energy for atomization is only supplied by an air compressor, consumption of energy to produce a fine spray is lower than usual twin-fluid-type atomizers. In our previous study, experimental and analytical results for the atomizer of small size with outlet of 7 mm i.d. were reported. In the present paper, in order to test the scaling capability of the atomizer, similar experiments have been conducted for that of medium size with outlet of 14 mm i.d. In the experiments, atomizers with a series of outlets in different lengths were tested since some spraying effects (vacuum pressure, diameter of droplet, etc.) are considered to be influenced by the lengths of outlet. In addition, six types of PET (Polyethylene Terephthalate) propellers with different blades and outer diameters were tested to expand the spray dispersion. After the optimization of the middle sized atomizer, comparisons of atomization performance were made between the two sized atomizers, and the superiority of the small sized atomizer was confirmed.