Abstract: Addressing the challenges of controlling liquid nitrogen cryotherapy, this paper introduces a liquid nitrogen flow measurement and control system comprised of an ultrasonic sensor and an AT89C52 microcontroller, utilizing the frequency difference method to detect liquid nitrogen flow. The paper details the ultrasonic flow detection principle, system hardware structure, and working principle. The system is simple in structure, easy to operate, safe to use, and exhibits stable and reliable performance, effectively solving the flow measurement and control problems during liquid nitrogen cryotherapy. Keywords: liquid nitrogen; cryogenics; ultrasonic wave sensor; single-chip microcomputer Abstract: To address the monitoring difficulties of liquid nitrogen cryogenic therapy, this paper introduces a system for measuring and controlling liquid nitrogen flow using an ultrasonic wave sensor and an AT89C52 single-chip microcomputer, employing a frequency-difference method for liquid nitrogen flow detection. The principle of liquid nitrogen flow detection, hardware design, and the operating principle of the entire circuit are also described in this paper. The system structure is simple, easy to operate, safe, and stable. It can resolve the problems of flow measurement and control in liquid nitrogen cryogenic therapy. Key words: liquid nitrogen; cryogenics; ultrasonic wave sensor; single-chip microcomputer 1 Introduction In recent years, with the rapid development of cryotherapy, liquid nitrogen cryotherapy has been widely used in various medical fields due to its rapid efficacy and low cost. Especially in the treatment of tumors, it can not only eliminate the tumor but also preserve the tissue shape and organ function to the greatest extent possible, leaving no scars, thus gaining popularity among patients and medical personnel. Existing liquid nitrogen cryotherapy devices in clinical practice are generally handheld accumulator-type devices. The liquid nitrogen injection volume is adjusted by intuitively controlling the pressure and angle of the injection valve lever, or by intuitively adjusting the opening of the external plastic clamp at the end of the injection tube to regulate the gas output. However, because liquid nitrogen cryotherapy requires a high level of skill, it is difficult for medical personnel to accurately control the entire treatment process intuitively and achieve precise treatment. If the injection volume is too low, the therapeutic effect will be less than ideal; if the injection volume is too high, local edema and pain may occur, and even damage normal tissue, causing unnecessary suffering and harm to the patient. To address this situation, this paper introduces an ultrasonic liquid nitrogen flow measurement and control system designed using the AT89C52 microcontroller. This system uses the microcontroller as the control core to improve the existing manual control of liquid nitrogen injection flow in liquid nitrogen cryotherapy devices. Utilizing the characteristics of ultrasound, an ultrasonic sensor is introduced to detect the liquid nitrogen flow rate. Based on the detection results, the AT89C52 controls the injection valve via a stepper motor, changing the intuitive manual control of the liquid nitrogen cryotherapy flow rate. This not only automates the treatment process but also achieves more precise treatment. The ultrasonic flow detection principle, system hardware structure, and working principle are discussed in detail below. 2. Ultrasonic Flow Detection Principle Ultrasonic flow detection is based on the fundamental principle that the speed of ultrasound propagation in a fluid is equal to the vector sum of the fluid velocity and the ultrasound velocity. Specific implementation methods include the propagation velocity difference method (including the difference frequency method, time difference method, and phase difference method), beam offset method, Doppler method, correlation method, spatial filtering method, and noise method. Currently, the most commonly used methods for flow detection in small-diameter hydraulic systems are the difference frequency method and the time difference method to detect the flow velocity and then convert it into fluid flow rate. Ultrasonic flow detection is a novel non-contact measurement technology characterized by fast transient response, strong dynamic measurement capability, convenient installation and use, and ease of automation and network management. For details, please click: Liquid Nitrogen Flow Measurement and Control System Based on Ultrasonic Sensors