0 Introduction The cotton airflow meter studied in this project is based on the relevant provisions of GB6468-1992, the test method for "micronaire value" of cotton fibers. Combined with current microcomputer monitoring system technology, it adopts a single-chip microcomputer C8051F, which features fast testing speed, easy maintenance, low cost, and high efficiency. Micronaire value is a measure of the air permeability of a wad of cotton under specific conditions. Practical experience shows that micronaire value is closely related to spinning; excessively high or low micronaire values ​​result in poor spinnability of the cotton fibers. Only cotton fibers with a moderate micronaire value can achieve comprehensive spinning economic benefits. 1. Basic Principle of the Airflow Meter This instrument is an airflow-type micronaire value measuring instrument. The basic principle of the micronaire airflow meter is that airflow at a certain pressure passes through a fixed mass of fiber plugs within a fixed volume. Due to the different surface areas of the fibers, the resistance to airflow varies, resulting in different pressure differences or flow rates across the fiber plugs. Fibers with smaller surface areas experience less resistance to airflow and higher flow rates (or smaller pressure differences); conversely, fibers with larger surface areas experience higher flow rates (or larger pressure differences). Therefore, the air permeability of the fiber can be indicated by changes in flow rate or pressure difference. The mass and volume of the sample are constants for the instrument, and the change in air permeability can be calibrated using the micronaire value. In the formula: A is the cross-sectional area inside the sample cylinder; L is the height of the sample cylinder; ΔP is the pressure difference between the two ends of the sample cylinder; S is the specific surface area of ​​the fiber; μ is the air viscosity coefficient; ε is the porosity of the fiber inside the sample cylinder (the ratio of the spatial volume within the fiber assembly to the total volume of the assembly); K is a constant. From the formula, it can be seen that the smaller the surface area of ​​the fiber (i.e., the coarser the fiber), the greater the flow rate, indicating better air permeability, a higher measured micronaire value, and better fiber maturity. Therefore, generally, the better the fiber maturity, the higher its micronaire value; that is, the two are positively correlated. 2 Hardware Design Based on the principle of the airflow meter, we use a C8051F023 microcontroller as the core control. The core of this microcontroller is a high-speed 8051 microcontroller. The instrument has two sensors: a pressure sensor and a weighing sensor. Based on the accuracy requirements of this instrument, a microgravity sensor and a micropressure sensor are selected. The hardware block diagram is shown in Figure 1. Instrument operation: After weighing 8 g of cotton sample on an electronic scale, it is evenly placed into the sample cylinder to form a fiber plug of fixed density. An air pump fills the air storage cylinder to generate constant pressure. The constant pressure airflow flows into the instrument through the air resistance (the fiber plug in the sample cylinder), creating a pressure difference between the two ends of the sample cylinder. Because cotton fibers with different micronaire values ​​have different resistance to airflow, the resulting pressure difference is also different. The pressure sensor converts this pressure difference signal into an electrical signal. The microcontroller processes the data and displays the micronaire value and its level, while also calculating the average value. 3. Software Design The instrument control program is designed according to a structured programming method, subdividing the entire program into several subroutines for easy debugging and checking. After the instrument is powered on, the microcontroller first checks whether the instrument needs weight calibration. If so, it performs weight calibration; otherwise, it determines whether to perform micronaire value calibration. If micronaire value calibration is required, it performs it; otherwise, it determines whether to weigh. After weighing 8 g of cotton, the program checks if port P1.7 is low. If it is, it performs a Mach value measurement; otherwise, it proceeds to the weighing subroutine. Since our instrument can only measure Mach values ​​within the range of 2.5 to 7.0, the program first checks if the Mach value is full-scale. When the measured value is below 1 V (i.e., below 2.5), it is considered full-scale and displays E000. If the Mach value is not full-scale, the true Mach value is calculated using an empirical formula. Finally, the Mach value grade is determined. According to national standards, Mach values ​​between 3.7 and 4.2 are grade A, 3.5 to 3.6 and 4.3 to 4.9 are grade B, and below 3.4 or above 5.0 are grade C. After the determination, the microcontroller calls the display subroutine to show the Mach value and corresponding grade of the cotton sample. Figure 2 is the main program flowchart. The entire software subroutine includes a weight calibration subroutine, a Mach value calibration subroutine, a weighing subroutine, a Mach value measurement subroutine, a display subroutine, and some algorithm subroutines. Through the design of the sensor and its amplification circuit, we know that when the air pressure is zero, the voltage is 0.500 V, and when the air pressure is 400 Pa, the voltage is 2.46 V, as shown in Figure 3. Considering the accuracy requirements, we choose A/D conversion to 10 bits, with a reference voltage of 2.5 V. When the input analog quantity is 2.5 V, it is converted to a digital quantity of 210 = 1024 (800H). According to the hardware circuit, when the sample tube is not filled with cotton, the air pressure difference measured after the airflow is 0, and the voltage sent to the microcontroller is 0.500 V, which is converted to the corresponding digital quantity of 0CDH by the equation: According to the equation (4) and the data in Table 1, a correspondence table between the Mach value and the digital quantity can be calculated. This table is input into the program as the basis for Mach value calculation. Based on the micronaire values ​​of standard cotton samples specified by national standards, we experimentally determined their corresponding voltage values. We found that their relationship is not linear and cannot be represented by a simple mathematical expression. Therefore, a segmented processing method was used to represent their logical relationship. Mathematical modeling was then performed based on this pattern. The specific flowchart is shown in Figure 4. The above true micronaire values ​​do not consider calibration deviations; therefore, micronaire compensation is necessary. First, determine whether the micronaire value is low, medium, or high. Then, retrieve the deviation obtained during micronaire calibration from the EEPROM memory and calculate the micronaire value that matches the actual value. 4. Experimental Testing The designed instrument was calibrated according to the relevant provisions of the national standard GB6468-1992, "Micronaire Value" test method for cotton fibers. Using different cotton samples with micronaire values ​​of A, B, and C grades, at a room temperature of approximately 21℃, the measured data error did not exceed ±0.1. The instrument accuracy fully meets the requirements. 5. Conclusion This instrument is designed with close consideration to China's national conditions, featuring a user-friendly interface, convenience, speed, small size, and high cost-effectiveness. The instrument employs single-chip control, uses electronic weighing and a pressure sensor to measure micronaire value, and features automatic calibration with standard cotton samples or standard plugs, as well as automatic micronaire value grading. It can intelligently reject abnormal data, making it a new generation product for micronaire value determination.