The RAD 7 has the unusual ability to distinguish between the “new” radon daughters and the “old” radon daughters left from previous tests. Even so, a very high radon sample can cause daughter activity that can affect the next measurement (RAD7 RAD H2O). In order to determinate the background level, a distilled water sample is used as a radon-free water sample. The back-ground sample was measured using the same protocol. The minimum detectable activity (MDA) was estimated to be 0.073 Bq/l