1. Introduction
In the field of aerospace, real-time monitoring and accurate measurement of liquid fuel consumption in fuel tanks are very necessary [
1,
2]. Therefore, the research and development of a liquid-level sensor are particularly important.
There are two types of liquid-level measurement technologies, which are invasive and non-invasive [
3]. The invasive types include capacitive [
4], resistive [
5], float-type [
6], magnetostriction type [
7], optical fiber liquid-level meter [
8], and many more. As the fuel tank is a closed container, its internal environment is high pressure, low temperature, etc., and its internal liquid fuel is inflammable and explosive. Therefore, it is not suitable to use a contact sensor introduced into the container to measure the liquid level [
9,
10]. Ultrasonic non destructive testing (NDT) technology has gradually become the mainstream for liquid-level detection [
11,
12].
There are some liquid-level measuring devices based on ultrasonic propagation characteristics, which are mainly divided into three categories: interface reflection method [
13], penetrative method [
14], and attenuation method [
15]. The detection accuracy of the interface reflection method and the penetrative method is greatly affected by the temperature of the internal medium. For large containers with diameters over 1 m, the long transmission distance and bubbles or impurities in the liquid will seriously affect the transmission of ultrasonic waves. The penetration attenuation characteristics of liquid medium will also seriously affect the reliability of measurement [
16,
17]. Attenuation is a relatively new technique that requires only an ultrasonic transducer to be installed on one side of the container wall. When the internal medium at the measurement point is different, the attenuation range of ultrasonic echo energy on the container wall is different. According to the time from the reception of the echo to the attenuation, it can distinguish whether the internal liquid level reaches the detection point, so as to play the role of liquid-level monitoring [
18,
19]. Therefore, the ultrasonic attenuation method has relatively good measurement accuracy and reliability.
The ultrasonic transducer emits a beam of ultrasonic waves, but due to the existence of the near field, the effective reflection echo cannot be received, resulting in inaccuracy of the measurement. Therefore, when using ultrasonic waves for measurement, it is necessary to ensure that the measured surface is in the far-field area of the sound pressure to obtain an effective signal [
20]. Buffer blocks are widely used in ultrasonic applications. At present, two kinds of rods with cylindrical and cone structures are used by researchers. Zhang et al. [
21] studied the shape and boundary conditions of the buffer block and proposed a high-performance rod with shape based on a cone reference surface. Hoppe et al. [
22] found an optimized geometry of a buffer rod for an ultrasonic density sensor. They can measure the amplitude with high accuracy and low noise. Fischer et al. [
23] used a conical buffer element with a combination of two materials to obtain a reference for the pulse amplitude of the emitted signal. The buffer material connected to the transducer is polymethyl methacrylate (PMMA), and the material in contact with the measured liquid is high-grade steel. However, the acoustic impedance of the buffer block material is not close to that of the measured liquid, so the sensitivity is low. Liu et al. [
24] made a detailed comparison description of the buffer block materials and drew the curve of the sound velocity in PMMA varying with frequency and temperature. Combined with other physical properties of PMMA, it is finally proposed that PMMA is most suitable for the measurement experiment of liquid acoustic properties.
To sum up, most of the researchers studied the material, shape, boundary conditions, and internal noise of the buffer block. For the length of the near-field area of ultrasound, the researchers only say that the acoustic beam range should be more than 3 times the length of the near field when using the p-wave testing [
22]. However, if the length of the buffer block is too short, the near field region cannot be avoided, and if it is too long, it may cause ultrasonic attenuation. At present, no team has proposed an exact value of the optimal and the minimum size of the buffer block required to avoid the near-field area.
In conclusion, based on the attenuation method, this paper builds a fixed-point liquid-level monitoring system. This method is based on the ultrasonic impedance method: the ultrasonic transducer emits a group of continuous ultrasonic waves to monitor whether the height of the liquid level is higher than the transducer by measuring the energy values of the received echo of the container wall. In this paper, a buffer block is added between the probe and the container wall. We used different lengths of buffer blocks to conduct experiments and studied the relationship between the length of the near field of the ultrasonic wave and the amplitude of the received echo. Finally, the experiment was conducted to find the minimum size of the ultrasonic probe and buffer block that can get effective results when using this method for liquid-level monitoring. The research in this paper provides an effective solution to avoid the near-field area for experiments such as liquid-level measurement based on ultrasound. It also provides a powerful basis for the selection and design of ultrasonic probes in other experiments.
4. Conclusions
Based on the ultrasonic impedance method, we built a non-contact, fixed-point, liquid-level monitoring system. Then, we studied the relationship between the length of the near-field area and the ultrasonic echo energy. The evaluations show that under the situation of 20 °C in temperature, 1 MHz in ultrasonic probe frequency, and ±15 V in amplitude of the emitted ultrasonic wave, when the probe diameter is 15 mm, the optimal length of the buffer block is 22 mm. The maximum average difference of the results is 0.49 V. When ultrasonic probe is in its minimum size of 10 mm in diameter and 2 mm in thickness, the minimal length of the buffer block is 5 mm. The maximum average difference is 0.36 V and the evaluation precision is 1 mm. Our evaluations are consistent with the theory, which proves the reliability of the research. Our approach provides an effective solution to avoid the near-field area for all experiments based on ultrasonic. It provides a powerful basis for the selection and design of ultrasonic probes. The liquid-level monitoring system built in this paper has the advantages of convenient installation, high reliability, and high sensitivity. It can be applied to industrial and aerospace applications and has important practical significance. Moreover, the parameters such as the buffer block boundary shape can be further studied. Next, our team will research the influence of parameters such as tilt angle and width of buffer block on results in the oblique-incidence ultrasonic experiment, to design a complete set of ultrasonic probe structures.