The Model 5700 operates on the principal 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 Model 5700 measures the elapsed time between the transmission and reception of pulses, which is directly proportional to the distance between the device and the liquid. In this way, the Model 5700 indicates the instantaneous level in the vessel.

The intensity of the reflected pulses depends only on the dielectric constant of the liquid. The Model 5700 is effective in measuring level in materials with a dielectric constant as low as 2.3. Provided this condition is met, the Model 5700 accurately measures level independent of changes in the dielectric constant of the liquid.