E-Zine October 2011
Ultrasonic level measurement sensors emit ultrasonic energy towards the material and measure the remnants of the energy that is reflected from the material surface. One sensor emits ultrasonic energy while a second sensor receives the remnants. Some designs package both sensors in the same physical device so it may appear to be a single sensor.
Ultrasonic level measurement systems determine the material level using the time-of-flight that the ultrasonic energy takes to travel to and return from the material. The distance between the sensors and the material surface can be calculated as one-half of the measured time-of-flight times the speed that ultrasonic energy travels in the path, which is the same speed of sound in the path. Mechanical dimensions can then be used to determine the level in the vessel.
Ultrasonic level measurement is affected by the speed that ultrasonic energy travels in the path in which the ultrasonic energy travels. Note that the pressure, temperature, and composition of the fluid through which the ultrasonic energy travels can vary and cause the speed to vary. Therefore, varying operating conditions can change the speed that ultrasonic energy travels and can adversely affect the performance of ultrasonic level measurement systems.
For example, ultrasonic level measurement techniques will not operate in a full vacuum because no ultrasonic energy will be emitted, so no ultrasonic energy will be received. In some applications, compensation may be needed to correct the raw ultrasonic level measurement for ultrasonic energy speed variations that may occur under different operating conditions.
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Excerpted from The Consumer Guide to Non-Contact Level Gauges