[en] This work presents the preliminary results of ²²⁶Ra activity measurements of fifteen samples of bottled mineral water acquired at markets of Curitiba-PR, Brazil. The measurements were performed at the Laboratory of Applied Nuclear Physics of the Federal University of Technology - Parana (UTFPR) in collaboration with the Center of Nuclear Technology Development of Brazilian Nuclear Energy Committee (CNEN). The experimental setup was based on the electronic radon detector RAD7 (Durridge Company, Inc.). The measurements were carried out with a special kit of accessory vessels (vials) RAD7 H₂O, which allows one to identify the ²²²Rn activity concentration in small water samples of 40 mL and 250 mL in the range going from less than 30 pCi/L to greater than 10⁵ pCi/L. During each measurement a vial from RAD H₂O was poured with a sample of water. The air pump, included in the close loop aeration circuit and connected to the vial and RAD7 detector, operated for five minutes to snatch the sample of air maintained above the level of water sample and transporting it from the vial through the system. Evaluation of the concentration of soluble radium (²²⁶Ra) salts in water and their activity was performed after 30 days when ²²²Rn in the water samples reached secular equilibrium. The background measurements were performed using the samples of the distilled water. Considering the importance of background measurements, it was found that the value suggested by user Manual protocol (RAD7) for the case of low activity radon measurements, has to be slightly modified. (author)