Abstract: A settlement tank wherein suspended solids or denser fluids are separated from a lighter parent liquid by gravitational settlement is provided with a monitoring arrangement to permit determination of the prevailing distribution of suspended matter, including that depth of the lighter liquid at which an interface may exist with settled material of greater density, to permit selective removal of one of the fluids from the tank, for utilization or disposal. The monitoring arrangement comprises a probe displaceably mounted for movement downward through the liquid having detection means for sensing the instantaneous hydrostatic pressure acting on the head of the probe. One embodiment utilizes a display wherein the variation in density is evidenced as a change in the gradient of a pressure depth and/or pressure time characteristic.