Ultrasonic liquid level meter

An ultrasonic echo-ranging system to measure the level of a liquid in an open channel or other liquid container located in a gaseous environment subject to changes which affect the velocity of acoustic propagation;

• said system comprising:
• (A) an ultrasonic transducer disposed at a fixed position directly above the liquid;
• the gaseous environment therebetween determining the velocity of ultrasonic energy propagated therethrough;
• (B) means to excite the transducer to emit periodic pulses of ultrasonic energy which are directed toward the surface of the liquid and reflected thereby to produce liquid echo pulses which return to the transducer and are detected thereby, said transducer having a radiation field pattern that includes a center path normal to the liquid surface and a side path which diverges from normal;
• (C) reference means at a fixed position relative to the transducer to intercept energy from said side path and to return it to the transducer to produce reference echo pulses, said reference means being constituted by a reflector placed in said side path;
• (D) receiver means coupled to the transducer to derive therefrom at separate outputs the liquid echo pulses representing center path reflection and the reference echo pulses representing side path reflection, said receiver means including means to exclude liquid echo pulses except pulses from the center path;
• (E) measuring means coupled to said separate outputs to measure the time elapsed between the emitted transducer pulse and a subsequent reference echo pulse along the side path to determine the reference transit time, and to measure the time elapsed between the emitted transducer pulse and a subsequent liquid echo pulse along the center path to determine the center path transit time;
• and
• (F) computer means coupled to the measuring means to calculate the ratio of the reference and center path transit times and to yield an output representing the level of the liquid independent of changes in the gaseous environment.