Hello,
One of the methods to measure the level of solid / liquid matter in a container is ultrasonic sensing.
I have several questions on this method:
1) What are the advantages and disadvantages of this method?
2) Since the speed of ultrasonic sensor is affected by the temeperature, how this effect is compensated in practice?
3) For which products (solid / liquid) is this method appropriate / inappropriate?
Thanks in advance for sharing your knowledge?
Best regards,
Muharrem Keskin
Muharrem,
What is the nature of the project on which you are working? I have used ultrasonic sensors for level sensing in cryogenic applications - but cannot advise unless you can clarify the nature of your research.
Hello Keskin,
The ultrasonic method typically consists on placing transducers (One transmitter and one receiver or a single transducer performing both functions) at the top of the recipient. By measuring the time-of-flight and knowing the speed of sound one can estimate the distance between the transducers and the surface of the liquid. The speed of sound depends on the air's temperature. Therefore, if you want to compensate for this change, you need to measure the temperature and use it to estimate the speed of sound before the estimation of the distance.
Some advantages: non-contact measurement, relatively cheap, relatively simple
Best regards.
Dear James and Ricardo,
Thanks for your answers.
We tested an inexpensive ultrasonic sensor (HC-SR04) with Arduino Uno board in a class lab for instrumentation course. We had a small container of about 30 cm height. We put water in different amounts and managed to predict the depth of the water. Then, we wanted to figure out if the system works with soilds as well. So we used granular solid material (sawdust + wood chips) but the measurements was not good as compared with water.
We are wondering if this method is applicable for granular agricultural materials such as wheat, rice, etc. with uneven surface. Does uneven material surface affect the measurement accuracy negatively?
Thank you.
Best regards,
Muharrem Keskin
Keskin,
The acoustic impedance of the material, as compared to that of air, will affect the amplitude of the received signal, therefore the Signal to Noise Ratio (SNR). Also note that the structure/position of the material affects how the wave is reflected.
Best regards.
Ultrasonic level sensing is applicable to granular materials as well: some 30 years ago, a dear colleague used US sensing in coal storage. I also know about snow height sensing using ultrasound.
The issue one might experience is about "acoustic impedance": the HC-SR04 e.g. is a rather simple device targeting acoustically "hard" (high impedance) surfaces. OTOH, snow height measurement is certainly one of the most demanding applications and may have limitations regarding "powder snow" due to its "near-air" properties.
Regarding temperature compensation, 2 methods apply: a) measuring the temperature or b) having a second, constant and well-known distance which is measured repeatedly to assess the current speed of sound. The latter also applies very well when ambient gas compensation changes (e.g. if a storage container may or may not be flooded with inert gas to prevent explosion).
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