Detection of liquid-liquid interface is critical in solvent extraction equipment/contactors typically used in petroleum and hydrometallurgical industries. An instrumentation system, comprising of a self-calibrating ultrasonic sensor and a novel signal processing algorithm is reported. It can be used to estimate location of liquid-liquid interface along with speed of sound in both the liquids without any manual/user intervention. The algorithm involves use of discrete wavelet transform and matched filter. The system has been tested at laboratory scale under wide range of industrially relevant conditions. The system works for any pair of liquid without apriori knowledge of their acoustic characteristics, is insensitive to changes in temperature and/or compositions. It also works for dirty interfaces contaminated with particulate matter or emulsion. The above mentioned features truly impart the sensor system “deploy-and-forget” capabilities. The range, accuracy and resolution of the system, with respect to interface location, are 40–200 mm, 0.45% FS (Full-scale) and less than 1 mm, respectively.