The TDR-1000 operates on the principle of Time Domain Reflectometry (TDR), transmitting pulses of low-power microwaves along a counterweighted stainless steel cable at the velocity of light. At the material surface, the pulses are partially reflected and travel back to the device via the cable. The TDR-1000 measures the elapsed time between the transmission and reception of pulses, which is directly proportional to the distance between the device and the stored material. In this way, the TDR-1000 indicates the instantaneous level of material in the vessel.