Collar Locate Tool (ECCL) The Collar Locate Tool (ECCL) works on the principle of a changing magnetic flux within the tool sensor coil and generates a voltage signal across the output of the sensor coil. The magnetic field of the ECCL is affected by any close magnetically sensitive material, such as the casing of the borehole. A collar or joint in the casing will change the magnetic flux field, including the flux passing, through the sensor coil ends adjacent to the magnets, causing an electric voltage signal. The voltage signal is sent to the surface system after been processed by the downhole tool.