Capacitance probe for measurement of bubbles in molten metals

Abstract

A probe has been developed for measuring bubble size, velocity and frequency in molten metals. The probe has been tested on a low melting point alloy (Belmont alloy) and on aluminum. Bubble signals are discernible in either metal. The probe measures the change in capacitance between a wire and the metal as a bubble passes. Because there is no electrical contact necessary between the wire and the metal, the wire may be sheathed with a material (e.g. alumina) that is inert to the metal. The ruggedness of this design was demonstrated by its use for three hours in aluminum. A bridge circuit which can measure the small capacitance change was devised and a deconvolution procedure has been developed whereby bubble size distributions can be obtained. Measurements have been made of the bubble size and velocity above J-shaped nozzles through which argon was injected. Similar measurements have been made of the bubble size distribution above a porous alumina plug and the bubbles were larger than those observed in water at a comparable flowrate.