Ultrasonic level measurement : Indirect method : Indirect method
Ultrasonic level instruments operate on the basic principle of using sound waves to determine liquid/solid/slurries level. In addition to standard level or volume measurement, they can monitor open channel flow, determine the actual volumetric throughput in lift stations, measure differential level and control the pumps.
The theory of sonic elctronic level measurement is based on a sound wave emission source (transmitter) and the reflection of a sound wave pulse (echo) to receiver. Measurement of the transit time of this pulse provides a means for level detection and measurement.
The following figure is an illustration of commercial units available:
Ultrasonic Level Transmitters consist of two elements; 1) a high efficiency transducer and, 2) an associated electronic transceiver. Together, they operate to determine the time for a transmitted ultrasonic pulse and its reflected echo to make a complete return trip between the non-contacting transducer and the sensed material level. As shown in Figure, a top-of-tank mounted transducer directs waves downward in bursts onto the surface of the material whose level is to be measured.
A piezoelectric crystal inside the transducer converts electrical pulses into sound energy that travels in the form of a wave at the established frequency and at a constant speed in a given medium. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of level in the tank. The time lapse between firing the sound burst and receiving the return echo is directly proportional to the distance between the transducer and the material in the vessel. The medium is normally air over the material’s surface but it could be a blanket of some other gases or vapours. The instrument measures the time for the bursts to travel down to the reflecting surface and return. This time will be proportional to the distance from the transducer to the surface and can be used to determine the level of fluid in the tank.
This basic principle lies at the heart of the ultrasonic measurement technology and is illustrated in the equation: Distance = (Velocity of Sound x Time)/2. These noncontact devices are available in models that can convert readings into 4–20 mA outputs to DCSs, PLCs, or other remote controls.
Minimum measuring distance (Xm ): (also known as the “Dead Band”) is a feature common to all m ultrasonic level meters. This is a short range in front of the sensor within which the ultrasonic device can not measure.
Maximum measuring distance (X M): The longest range under ideal condition within which the device can M measure. No measurement is possible beyond this distance.
The frequency range for ultrasonic methods is in the range of 15…200 kHz. The lower frequency instruments are used for more difficult applications; such as longer distances and solid level measurements and those with higher frequency are used for shorter liquid level measurements.