0 Introduction The traditional method for measuring minute pressure differences is the use of a U-tube manometer. This manometer is simple in structure, inexpensive, and reliable. However, it cannot record transient pressure changes, resulting in slow readings and large errors. Even with manual estimation, the maximum accuracy is only 0.5 mm of liquid column height. To improve sensitivity and reduce reading errors, tilt-tube manometers were developed. If the tilt angle of the measuring tube in a tilt-tube manometer is 30°, the measurement accuracy can be doubled. With the development of pressure sensor technology, electronic micro-differential pressure sensors have emerged in recent years, which can directly convert minute pressures into electrical signals. These sensors are easy to use, have a fast response time, and can achieve high accuracy; currently, the best micro-pressure sensors have a measurement resolution of around 10 Pa. However, their stability is not good, with significant temperature and time drift, and they are expensive. Each of the above micro-pressure measurement methods has its advantages and disadvantages. If further improvements in measurement accuracy are desired, the existing measuring devices are no longer sufficient. To address this situation, we designed a novel micro-manometer—an ultrasonic micro-manometer. This design, while retaining the advantages of U-tube manometers such as simple structure, high reliability, and intuitive operation, utilizes the principle of ultrasonic waves for precise and rapid measurement of the liquid column height in a U-tube. It features high measurement accuracy, accurate readings (digital display), and fast measurement speed. It can be connected to a computer via an RS-232 interface for data and chart processing, and has broad application prospects. 1. Ultrasonic Waves and Ultrasonic Transducers Ultrasonic waves are a type of mechanical wave, representing the propagation of mechanical vibrations in a continuous medium (gas, liquid, solid). Therefore, mechanical vibration is the root cause of ultrasonic wave generation. Ultrasonic waves refer to elastic waves with a frequency f > 20 kHz. Ultrasonic transducers are key components for transmitting and receiving ultrasonic signals; they can convert electrical energy into high-frequency acoustic energy or electrical energy into acoustic energy. There are various types of ultrasonic transducers; we have chosen piezoelectric transducers, which are electroacoustic transducers based on the piezoelectric effect of certain crystals to achieve electroacoustic energy conversion. Piezoelectric transducers exhibit both direct and inverse piezoelectric effects. The direct piezoelectric effect refers to the phenomenon where, when stress is applied to a dielectric material, the resulting deformation causes a relative displacement of the positive and negative charge centers within it, leading to polarization. This results in bound charges of opposite signs appearing on the two end faces of the dielectric, with charge density proportional to the stress. The direct piezoelectric effect can be used to convert mechanical energy (sound energy) into electrical energy; a transducer used to receive this energy is called a receiving transducer. The inverse piezoelectric effect refers to the phenomenon where, when a piezoelectric material is placed in an electric field, the positive and negative charge centers within the material are displaced due to the electric field. This displacement macroscopically manifests as deformation (or strain), which is proportional to the electric field strength. This effect can be used to generate ultrasonic waves. Applying a suitable alternating electrical signal to a material causes vibration, with the vibration frequency matching the frequency of the alternating voltage, thus forming ultrasonic waves. The emission of sound waves utilizes the inverse piezoelectric effect. A transducer used to emit sound waves is called a transmitter. Piezoelectric transducers exhibit both direct and inverse piezoelectric effects. Based on this principle, an ultrasonic transducer can be used as both a transmitter and a receiver. 2. Principle of Ultrasonic Pressure Measurement The principle of ultrasonic pressure measurement is shown in Figure 1(a). Two glass tubes form a communicating vessel. An ultrasonic transducer is installed at the bottom of tube A. The pressure to be measured is introduced through points A and B. This connection method can measure the pressure difference between two pressures. If gauge pressure needs to be measured, simply connect either point A or B to the atmosphere. Before measurement, zero-point calibration is required. Connect both points A and B to the atmosphere. At this time, since the pressures A and B are equal, the liquid columns in the two tubes of the communicating vessel are at the same height. Measure the height h0 between the ultrasonic transducer's emitting surface and the liquid surface. This height is the reference height (also called the zero-point height). During measurement, point B is connected to the atmosphere (measuring gauge pressure), and the pressure to be measured is introduced from point A. Since there is a pressure difference between tubes A and B, the two liquid columns form a height difference of 2Δh. Assuming that the liquid in the two glass tubes is pure water, then the pressure is expressed in mmH2O (1 mmH2O = 9.80665 Pa). The pressure difference to be measured is: As long as h0 and h1 are accurately measured, the pressure can be accurately measured. From equation (1), it can be seen that the pressure is independent of the installation position of the ultrasonic transducer. When using ultrasonic waves to measure the height of the water column, the ultrasonic waves are emitted by the transducer, reflected after encountering the liquid surface, and the reflected wave returns to the transducer, generating an electrical pulse. As long as the time Δt between the ultrasonic wave being emitted by the transducer and the reflected wave returning to the transducer is accurately measured, the pressure can be determined. If a 24 MHz high-frequency pulse is used as the time measurement unit, and the total number of pulses obtained within a time interval Δt is n, then: Let Δt0 be the time required for the ultrasonic wave to travel a distance of 2h0, and Δt1 be the time required to travel a distance of 2h1, recording n1 pulses. Let the speed of ultrasonic wave transmission in water be ν, then the measured pressure is: At an ambient temperature of 25℃, the speed of ultrasonic wave transmission in water is approximately 1483 H1/s. According to theoretical calculations, the resolution of this micromanometer for measuring the height of the water column is: To further improve measurement accuracy, the frequency of the counting pulses can be increased, or a liquid with a lower specific gravity can be used. For example, if pure alcohol is used as the measuring liquid, and its specific gravity is 0.8, then the measurement resolution is: Therefore, it can be seen that the measurement accuracy of this micromanometer is far superior to the currently used measurement methods, and it can meet the requirements for measuring minute pressures in most situations. In addition, this method has a very fast measurement speed, measuring at a rate of approximately 300 times per second, thus capturing transient changes in pressure. U-tube manometers or tilt-tube manometers require at least 1 second to read a single data point, making it impossible to read values ​​when the measured pressure fluctuates rapidly. 3 Hardware Design 3.1 Ultrasonic Transmitting Circuit The ultrasonic transmitting circuit is relatively simple. Since the transmitter and receiver use the same transducer, and the pressure-measuring liquid column is short, continuous wave transmission is not suitable. We adopt a single-pulse wave transmission method. The microcontroller sends a start pulse, which, after passing through a differentiating circuit, causes the transistor to conduct instantaneously, resulting in a high-voltage pulse with a very steep leading edge on the ultrasonic transducer. The transducer then emits an ultrasonic pulse towards the liquid surface. 3.2 Ultrasonic Receiving and Amplifying Circuit The input stage consists of a differentiating amplifier circuit. The first reflected wave (point 4) from the ultrasonic transducer is further amplified, making the first reflected wave signal sharper (point B). This signal is then detected and filtered by a precision detector circuit to obtain the envelope of the reflected wave (point C). To eliminate interference signals such as ground wires and clutter, and to enhance the signal-to-noise ratio, the signal at point C is further amplified by an amplifier with a threshold comparison, resulting in a relatively clean first reflected wave signal with a sufficiently large amplitude (point D). Finally, a voltage comparator outputs a trigger signal (point E) to the counting system, stopping the counting. This completes the amplification and processing of the reflected wave signal. Because ultrasonic waves undergo multiple reflections within the liquid column, and we only need the first reflected wave, the counting must be stopped immediately after processing the first reflected wave. This is handled by the control circuit. 3.3 Pulse Timing Circuit The control system uses an MCS-51 series microcontroller. When the microcontroller uses a 12 MHz crystal oscillator, its highest counting frequency is 500 kHz. However, the ultrasonic timing system uses 24 MHz high-frequency pulses. Therefore, the 24 MHz high-frequency pulses are divided by 256 using a 74LS393 to become 93.75 kHz before being sent to the microcontroller for counting. When reading the clock pulse count, the 74LS393 reads the lower 8 bits, and the higher 16 bits are read by the microcontroller's internal counter, thus achieving a 24-bit binary count. The theoretically calculated maximum water column height is: 4 Software Design The software design of this system focuses on solving real-time and synchronization problems. First, the counter starts counting at the same moment the ultrasonic wave is emitted. Second, the counter stops immediately upon receiving the first reflected wave. Third, since the same ultrasonic transducer is used for both transmission and reception, the transmitting and receiving amplifier circuits are connected to the same point. Simultaneously, the receiving amplifier circuit receives the emitted signal. Without intervention, the emitted wave might be mistaken for a valid reflected wave, causing the counter to shut down instantly upon startup, resulting in malfunction. Therefore, necessary measures must be taken to shield the amplifier's input. On the other hand, the emitted electrical pulse signal has a certain width, and the ultrasonic excitation utilizes the leading edge of the emitted pulse. Combined with the circuit's time delay effect, based on the above, the amplifier circuit's input should be shielded for a period after emission, and then reopened after a delay to prevent false triggering. Therefore, a fixed time difference exists between the emitted signal and the reopening of the receiving circuit, inevitably creating a measurement dead zone. Measurement is impossible when the liquid level is below this dead zone height. 5. Conclusion This measurement system, after practical use, fully meets the design requirements and has achieved excellent application results in the wind tunnel pressure field application at the Department of Thermal Engineering, Shanghai Jiao Tong University. Since the transmission speed of ultrasound in liquids varies with ambient temperature, we designed an ambient temperature detection circuit in the circuit and performed temperature compensation through software. Simultaneously, the volume of the liquid column also changes with temperature, but the change in height is very small and negligible. After temperature compensation, the measurement system is very stable, provides accurate measurement data, is fast, easy to read, compact, and can be used in multi-point cascading to achieve simultaneous multi-point measurements, facilitating the capture of transient pressure changes. For example, it achieved rapid measurement of 128 points in the wind tunnel pressure field application at the Department of Thermal Engineering, Shanghai Jiao Tong University. If the manual estimation accuracy of the U-tube manometer is 0.5 mm, and the ultrasonic micro-pressure measuring instrument has a measurement accuracy of 0.03 mm, the measurement accuracy is improved by 16 times. A U-tube manometer takes at least 1 second to measure the pressure of a single point, while this ultrasonic micromanometer can measure 300 points per second. This means that its measurement speed is increased by 300 times, making it a complete replacement for conventional U-tube manometers or inclined tube manometers. It is an excellent product for upgrading and replacement, with broad market prospects.