The Well Casing Indicator Probe works using magnetism. While the probe sits within the magnetic field of the steel casing, an auditory and visual signal is active. When the probe leaves the magnetic field the signal stops. The well casing measurements are taken directly from the tape. The condition of the steel casing may be a factor in receiving accurate readings. Recommended Use: New Construction Only (Old/Imperfect casing may cause false or inaccurate readings when using this probe).