This invention discloses an approach and a sensor designed to accurately determine fluid/gas interface position. By sending out a signal into a wire which immersed in the fluid and detecting the reflected signal from the fluid/gas interface (water echo method) and the reflected signal from the wire’s end face (end echo method), the fluid/gas interface position can be determined. This approach measures the fluid level using a specific signal wave with a specific technique. The water echo method has higher accuracy and lower attenuation in the same type of acoustic probe in the current market. This application is suitable for condition monitoring of power plants and petroleum storage tanks.