Abstract: This paper introduces the development overview of differential pressure transmitters, elaborates on their basic and extended applications, and looks forward to their development trends. Keywords: differential pressure transmitter; general application; extended application; development trend 1. Development Overview of Differential Pressure Transmitters Like other electric regulating instruments, differential pressure transmitters are in a new era of transformation. The force-balanced differential pressure transmitter, which has been used for over two decades, is well-known. It utilizes the principle of deep negative feedback to reduce the influence of factors such as temperature changes in the elastic element's modulus, elastic hysteresis, and nonlinear deformation, thereby ensuring measurement accuracy. Therefore, the force-balanced transmitter has a complex and bulky structure and suffers from persistent static pressure errors. The development of new materials has led to the emergence of elastic materials with very small elastic moduli and temperature coefficients. In particular, the development of electronic detection technology has made the detection of minute displacements possible, thus allowing for small deformations in elastic materials, further reducing the variation caused by nonlinearity and elastic hysteresis. These factors have created conditions for the emergence of new open-loop transmitters. Currently, the capacitive differential pressure transmitter, widely used in the field, is one such example. This type of transmitter not only represents a fundamental shift in principle from closed-loop to open-loop, but also features simple structure, reliable operation, no static pressure error, and convenient maintenance. With the continuous development of new technologies, various other new types of transmitters, such as vibrating wire and diffused silicon transmitters, have also been successfully developed and applied in production. In recent years, intelligent transmitters have also been introduced. Some of these intelligent transmitters feature newly developed measuring mechanisms coupled with microprocessors; others are based on existing transmitters but equipped with microprocessors. Therefore, they are products of the integration of microcomputer and communication technologies into transmitter instruments. Intelligent transmitters offer high signal conversion accuracy, are minimally affected by environmental temperature changes, static pressure, and vibration, and have a particularly large range ratio. Therefore, a single instrument specification can meet the needs of various measurement ranges, significantly reducing the number of spare instruments and parts required. Another prominent feature of intelligent transmitters is their communication function. Through simple keyboard operation of the intelligent field communicator, remote setting, modification, and calibration can be achieved. Furthermore, they possess self-diagnostic functions, all of which greatly facilitate use and maintenance. 2. General Applications of Differential Pressure Transmitters 2.1 Flow Measurement Differential pressure transmitters are used in flow measurement and metering due to their simplicity, reliability, and ease of use. For example, flow measurement in the cooling water detection system of a metallurgical plant's crystallizer uses a differential pressure transmitter. A throttling device is installed on the pipeline through which the fluid flows. As the fluid flows, a differential pressure is generated before and after it. The magnitude of the differential pressure is related to the flow rate of the fluid. Therefore, a throttling device (orifice plate, nozzle, or Venturi tube) can be used with a differential pressure transmitter to measure flow rate. The relationship between flow rate and differential pressure is as follows: When measuring flow rate with a differential pressure transmitter, a square root extractor is usually connected in series at the output. After adding the square root extractor, the flow rate is directly proportional to the output signal of the square root extractor. 2.2 Another typical application of differential pressure transmitters for liquid level measurement is liquid level detection. When Δp = λH and λ = constant, Δp and H are directly proportional. Therefore, in situations where the density is almost constant, a differential pressure transmitter can be used to measure the liquid level. Differential pressure transmitters can only be used to measure liquid level when the density is relatively stable; changes in density affect the measurement accuracy. 3 Extended applications of differential pressure transmitters 3.1 Used as a micro-positive or micro-negative pressure measuring instrument: A differential pressure transmitter has two pressure tapping chambers. When the negative pressure chamber is open to the atmosphere and the positive pressure chamber is connected to the pressure tapping point, it can be used for pressure measurement. Conversely, when the positive pressure chamber is open to the atmosphere and the negative pressure chamber is connected to the pressure tapping point, it can be used for negative pressure measurement. 3.1.1 Differential pressure transmitters are used to measure the slight negative pressure in a boiler furnace. Due to the need for proper fuel combustion, boilers require furnace pressure to be controlled within -50 Pa. This slight negative pressure can be measured using a differential pressure transmitter. 3.1.2 Measurement of slight positive pressure in a heating furnace. Besides considering proper fuel combustion, heating furnaces also need to maintain an oxygen-deficient state during combustion. Oxygen-rich conditions easily lead to increased oxide scale on the surface of heated components, resulting in losses. Therefore, a slight positive pressure is generally maintained. Differential pressure transmitters can be used to measure the slight positive pressure in heating furnaces. 3.2 Application of differential pressure transmitters in solid mass flow meters. In the raw coal loading measurement system of a ball mill in a power plant, a differential pressure transmitter can be used. Raw coal enters the ball mill for grinding via a coal hopper, feeder, and hot air damper. Each ball mill has a certain coal loading capacity. Grinding efficiency is highest at this rated loading capacity. Generally, the pressure difference between the inlet and outlet of the ball mill is used as an indirect measurement of the loading capacity. When the loading amount increases, the resistance of hot air through the ball mill increases, and therefore ΔP also increases. Conversely, when the loading amount decreases, ΔP also decreases. Therefore, by measuring ΔP, the amount of loading can be indirectly known. 3.3 Using differential pressure transmitters to measure specific gravity From the principle of differential pressure transmitters for measuring liquid level, we know that when specific gravity is constant, the output of the differential pressure transmitter is proportional to the liquid level; similarly, when the liquid level is constant, the output of the differential pressure transmitter is proportional to specific gravity. Therefore, differential pressure transmitters can be used to measure specific gravity. 4 Issues to be aware of when using differential pressure transmitters 4.1 Preventing oscillation The measuring sensitive element of a differential pressure transmitter is a diaphragm (or membrane), with positive and negative pressure measuring chambers before and after it. During the measurement process, these two pressure measuring chambers move back and forth like a swing, causing the output signal to constantly jitter, forming an inherent oscillation characteristic. Generally, flow transmission pipelines are transmitted through pumps or compressors, which inherently have regular pulsation phenomena. Combined with the contraction and expansion in the fluid pipeline, this forms oscillations during the transmission process. Oscillations can be eliminated by adjusting the resistance of the differential pressure transmitter's pressure tap. Additionally, placing a 0.5mm to 1.5mm orifice gasket on each of the two measuring lines can increase damping and reduce the pulsation effect on the flow lines, thus eliminating oscillations. 4.2 Do not operate below 1/3 of the range. For flow measurements below 10, the differential pressure transmitter will hardly reflect changes in ΔP. The lower the range, the lower the accuracy; generally, differential pressure flow meters do not specify accuracy below 1/3 of their range. 5. New Developments in Differential Pressure Transmitters With the development of modern industry, automation systems are becoming increasingly large-scale and complex. The focus has shifted from the stability of production processes to large-scale centralized and optimized control. Coupled with the expansion of application areas, increasingly higher demands are placed on differential pressure transmitters. Specifically, these demands include high accuracy, strong adaptability to the measured object and operating environment, miniaturization, ease of installation and maintenance, and especially high reliability, as the stability and reliability of the transmitter are crucial to the reliability and safety of the entire automation system. New differential pressure transmitters have been developed to meet these requirements. 5.1 Intelligent Transmitters Intelligent differential pressure transmitters are no longer simply instruments that convert measured process parameters into standard signals. They also possess remote control operation, A/D conversion, and self-diagnostic functions, moving towards mechatronics integration. The adoption of new composite sensor technology integrates differential pressure, static pressure, and temperature sensors onto the same semiconductor chip, significantly reducing temperature and static pressure errors and greatly improving accuracy. In systems requiring high measurement accuracy, such as transmitters used in flow metering loops, the accuracy of the transmitter has a considerable impact on the accuracy of the measurement results, making the use of intelligent transmitters essential. 5.2 Development Towards Miniaturization Traditional differential pressure transmitters are large and bulky, requiring additional supports for on-site installation. Currently, differential pressure transmitters are developing towards miniaturization. Using stabilized integrated circuit sensors in the transmitter results in stable performance and a small size, allowing direct installation on the measuring pipeline, which is both convenient and saves installation costs. 5.3 Digital Signal Transmitters Currently, the standard for analog signal transmission of DC current from 4mA to 20mA has been largely established in process industry control instrumentation. With the development of microcomputer control systems, digital signal interfaces have been established, and new and higher requirements will be placed on transmitters with digital signal output.