ULTRASONIC LEVEL MEASUREMENT BASICS
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.
Ultrasonic detectors with single sensor:
A typical single-sensor liquid level indicator is shown in figure A and Figure B
Ultrasonic detectors with two sensors:
In this system, which is generally used for dry or solids level control, one transmitting sensor creates the sonic beam, and sound waves are picked up by a receiving sensor. See figure C. This can be accomplished by a direct path or by surface and are rejected back to the receiving sensor.
For practical applications of ultrasonic measurement method, a number of factors must be considered. A few key points are:
• The speed of sound through the medium (usually air) varies with the medium’s temperature. The transducer may contain a temperature sensor to compensate for changes in operating temperature that would alter the speed of sound and hence the distance calculation that determines an accurate level measurement. Temperature compensation is provided to account for uniform temperature variances of the sound medium. The temperature sensor is placed inside the transducer and the signal is sent to the
transceiver via the transducer's wiring. Optionally, an alternate temperature sensor can be used to provide a temperature input, rather than by using the integral temperature sensor. If the temperature of the sound medium is to remain constant, instead of using either the integral temperature compensation or the remote sensor, the desired temperature may be entered during the transceiver configuration.
• The presence of heavy foam/dust on the surface of the material can act as a sound absorbent. In some cases, the absorption may be sufficient to preclude use of the ultrasonic technique. To enhance performance where foam/dust or other factors affect the wave travel to and from the liquid surface, some models can have a beam guide attached to the transducer.
• Extreme turbulence of the liquid can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem. The transceiver provides damping to control the maximum changing rate of the displayed material level and fluctuation of the mA output signal. Damping slows down the rate of response of the display especially when liquid surfaces are in agitation or material falls into the sound path during filling.
Article Source: Indumart Inc