the assembly comprises an electrically insulated linear array of a plurality of active plates working in conjunction with common return plate means to provide a line of capacitors . the electrically insulated active plate array and common return plate means extend through a column of multi - phase fluid . a discrete oscillator circuit is positioned adjacent to and is permanently connected to each active plate for charging and discharging it . means are provided for sequentially enabling the oscillator circuits to produce signals which are each indicative of the dielectric constant of the thin horizontal slice of fluid extending between the active plate and the return plate means . the signals are collected by a microprocessor which analyzes them to provide a dielectric constant profile of the fluid column . interfaces of phases can be located from the profile and the composition of the fluid phases can be determined .