Parasitic Echo Pulse Rejector for Ultrasonic Liquid Level Meter

1. An ultrasonic echo-ranging system to measure within a predetermined time span the level of liquid in an open channel or tank, said system comprising:

A an ultrasonic transducer disposed on an ultrasonically-reflective surface at a fixed raised position above the surface of the liquid;

B means to excite the transducer to emit periodic pulses of ultrasonic energy which are propagated in a gaseous medium between the transducer and the liquid and are directed toward the surface of the liquid and reflected from this target to produce main echo pulses which return to the transducer and are detected thereby, said transducer also detecting parasitic echo pulses originating from said ultrasonically-reflective surface;

C means including a programmed microcomputer and a distance counter responsive to pulses detected by the transducer to measure the round trip transit time of the pulse energy in the gaseous medium to provide a reading of liquid level; and

D a parasitic echo rejector operatively coupled to said microcomputer acting to discriminate between said main echo pulses and said parasitic pulses received within said span to render said system non-responsive to said parasitic pulses to provide an accurate reading of liquid level.

2. A system as set forth in claim 1, further including a receiver coupled to said transducer to receive the pulses detected thereby and to apply them to said microcomputer. 

3. A system as set forth in claim 2, further including an automatic gain amplifier interposed between said transducer and said receiver and responsive only to main echo pulses when the strength thereof is greater than the parasitic echo pulses. 

4. A system as set forth in claim 1, wherein said rejector includes a parasitic echo counter. 

5. A system as set forth in claim 4, wherein said microcomputer is programmed so that each new echo level reading related to the count provided by said distance counter is compared with the present liquid level value, and this new reading is accepted by the system when the difference therebetween lies within a given acceptable margin. 

6. A system as set forth in claim 5, in which readings outside of said acceptable margin may be based on parasitic echoes or reflect a valid level change, discrimination therebetween being effected by determining whether new outside margin readings occur consecutively a predetermined large number of times, in which event they are accepted as true level changes. 

7. A system as set forth in claim 6, wherein said large number of times is 50. 

8. A system as set forth in claim 6, wherein if outside margin readings occur consecutively said large number of times and are accepted, all subsequent new outside margin readings have only to occur for a predetermined small number of consecutive times to be accepted as non-parasitic. 

9. A system as set forth in claim 8, wherein said small number is 4 times.