Industrial Ears: How Ultrasound ‘Hears’ Changes in Liquid Level

Industrial Ears: How Ultrasound "Hears" Changes in Liquid Level

Let's talk about what ultrasonic waves are. The frequency range of sounds we can hear is approximately between 20 hertz and 20,000 hertz. However, the frequency of ultrasonic waves is much higher, typically ranging from 20 kilohertz to 100 megahertz. Therefore, our ears cannot detect ultrasonic waves. In fact, ultrasonic waves are a type of mechanical wave. They can propagate in elastic media and, due to their high frequency and short wavelength, they have strong directionality, significant energy, and strong penetrating power during propagation.

Welcome to the Solidat Measurement and Control Laboratory. I'm your instrument and equipment measurement and control manager. Today, let's talk about the application of ultrasonic waves in level measurement.

When it comes to the history of ultrasound, it can be traced back to 1793. At that time, an Italian scientist, Spallanzani, discovered through experiments that bats use ultrasonic waves to sense their surroundings, thus unveiling the mystery of ultrasound. Later, with the development of technology, ultrasound was widely applied in fields such as detection, measurement, and medicine. In industrial production, level measurement is particularly important. Level measurement refers to measuring the height of materials in containers or spaces, such as liquids and granular solids. Through level measurement, we can know how much material is in the container, thereby ensuring the material balance in the production process. If the level can be precisely controlled, it can also ensure the output and quality of the products, as well as ensure safe production. So, how is ultrasound used in level measurement?

In simple terms, ultrasonic waves have very little attenuation in liquids and solids, and have extremely strong penetrating ability. Especially in opaque solids to light, they can penetrate a distance of several tens of meters. Moreover, ultrasonic waves have strong directionality and can be emitted directionally. During measurement, the sensor emits ultrasonic waves. When the waves encounter the surface of the material, they will reflect back. After the sensor receives the reflected wave, it can determine the distance by calculating the time difference, and thereby obtain the liquid level height. The entire measurement process does not require direct contact with the measured medium, so it is very suitable for corrosive and erosive environments and is widely used in industries such as chemical engineering, petroleum, food, pharmaceuticals, and environmental protection.

Next, let's take a look at the working principle of the ultrasonic level gauge. Generally speaking, an ultrasonic level gauge consists of a transducer, a signal processing unit, and a display or output module. The specific measurement steps are as follows:

1. **Ultrasonic emission**: The ultrasonic level meter emits ultrasonic pulses at a fixed speed towards the target material surface through the probe, for example, five times every two seconds.
2. **Ultrasonic propagation**: Ultrasonic waves propagate at a certain speed in the air. When they encounter the material surface, some of them will be reflected back to form an echo. The intensity and return time of the echo are related to the characteristics of the target surface.
3. **Reflection wave reception**: The probe receives the ultrasonic wave signals reflected from the material surface and converts them into electrical signals. At the same time, it measures the time it takes for the ultrasonic pulse to travel.
4. **Calculating level**: By measuring the propagation time of the ultrasonic pulse, calculate the time difference from emission to reception, and then use the formula to calculate the distance from the sensor to the material surface. The formula is: D = V × Δt ÷ 2, where V is the speed of sound in the medium, Δt is the time difference from the emission of the ultrasonic wave to the reception of the echo, and D is the distance from the sensor to the material surface. Additionally, since the geometric shape and height parameters of the container are known, the level height can be calculated using the formula L = E - D, where L is the measured level height, E is the distance from the sensor installation base to the bottom of the container (which is the empty tank height or total tank height), and D is the distance from the sensor to the material surface.

However, there are some points to be noted in practical applications. Firstly, the speed of sound is affected by the medium and environmental conditions, such as temperature, pressure, humidity, etc. For example, in air, for every 1℃ increase in temperature, the speed of sound will increase by approximately 0.6 meters per second. Therefore, in actual measurements, temperature sensors are usually installed for temperature compensation to ensure measurement accuracy. Secondly, ultrasonic waves may not be able to propagate in a vacuum or under extreme pressure conditions, so the applicable environment also needs to be carefully considered.

In addition, the installation position and orientation of the ultrasonic sensor are also very important. The sensor should be aligned with the surface of the measured material, and obstacles should be avoided as much as possible to prevent interference with the echoes. If there is a stirrer or other structures inside the container, false echoes may be generated. At this time, signal processing technology needs to be used to identify the correct echoes. Moreover, dust, steam or foam in the air may also affect the propagation and reflection of ultrasonic waves. In such cases, other measures may need to be taken to deal with the interference.

Finally, there is one minor detail that needs attention: The ultrasonic level meter has a certain distance near the probe that cannot be measured. This is because the emitted ultrasonic pulse has a certain time width, and the sensor will still have residual vibrations after emitting the ultrasonic wave. During this period, the reflected echo cannot be detected. This distance is called the blind zone. Therefore, the highest part of the measured material should generally not enter the blind zone of the sensor.