The RAD Aqua is the fastest, most sensitive method of measuring radon in water, and it can monitor both radon and thoron levels continuously. Water passes continuously through the exchanger. Air flows in a closed loop through the exchanger and through the RAD7. Radon in the air comes into equilibrium with radon in the water. From the radon concentration in the air and the temperature of the air/water interface, the radon concentration in the water is determined.