Status of Prior Art about Ultrasonic Measuring
In an ultrasonic echo-ranging meter, pulses of ultrasonic energy transmitted from a transducer station placed above the surface of a liquid in a tank or open channel are reflected thereby to produce echo pulses which are received at the station. By determining the round trip transit time of the pulse energy in the gaseous medium above the liquid surface, which transit time depends on the distance between the station and the surface, one is able to provide a reading of liquid level.
The accuracy of an ultrasonic liquid level meter of the echo-ranging type is adversely affected by environmental changes; notably temperature, pressure and chemical composition. These factors alter the velocity of acoustic propagation. For example, the velocity of sound in air at 0° C. is 1,087.42 fps, whereas in carbon dioxide it is 1,106 fps. When a meter is installed in an environment in which the chemical nature of the gaseous medium undergoes change, this factor will disturb the level reading unless means are provided to compensate or correct therefor. Similarly, changes in the temperature of the medium or in ambient pressure adversely affects the accuracy of the instrument.
In prior art ultrasonic meters such as those disclosed in the Tankin U.S. Pat. No. 3,090,224, the Kritz U.S. Pat. No. 2,949,772, the Kohno U.S. Pat. No. 4,183,244 and the Asada U.S. Pat. No. 3,710,021, use is made of an automatic gain control circuit in conjunction with the received signals. Automatic gain is generally effected by a control circuit adapted to automatically modify the amplification gain of a receiver in a manner whereby the desired output signal remains at a constant amplitude despite variations in input signal strength.
In an ultrasonic echo-ranging liquid level meter, variations in the amplitude of the echo pulses received from the surface of the liquid are encountered by reason of changes in this surface as well as changes in distance due to liquid level changes. Thus an echo pulse which has a long distance to travel before reaching the transducer will be weaker than an echo pulse traveling a shorter distance.
But in the context of an echo-ranging system of the type disclosed in my prior patent '299 ; in which reference echo pulses as well as liquid level echo pulses are received, at first blush it would appear that no need exists for automatic gain control with respect to the reference echo pulses. Because these pulses are derived from a reflector having a smooth surface placed a fixed distance from the transducer, all reference echo pulses should have the same strength.
However, typical ultrasonic transducers of the same model, though seemingly alike, nevertheless differ somewhat in sensitivity and exhibit a wide spread in echo response. Thus when manufacturing ultrasonic echo-ranging instruments, all of which incorporate the same model of transducer, it becomes necessary to make an individual gain setting to match a particular transducer to the instrument.