Instrument for measuring fluid pressure. Usually the measured pressure is compared with a reference pressure (such as atmospheric pressure or other given pressure), so the relative pressure or pressure difference is measured. According to different working principles, it can be divided into three types: liquid column type, elastic type and sensor type. Liquid column type, such as U-tube pressure gauge, row pressure gauge, etc., is based on the principle of hydrostatics to convert the pressure signal to the liquid column height signal, often using water, alcohol or mercury as the pressure measuring medium. The elastic type, such as the Boden tube pressure gauge, converts the pressure signal into the amount of mechanical deformation of the elastic element, and outputs the signal in a way that the pointer is deflected. Such pressure gauges are often used in industrial systems. The principle of the pressure sensor is to convert the pressure signal into some kind of electrical signal, such as strain type, which changes the resistance through the deformation of the elastic element; piezoelectric type, using the piezoelectric effect, etc. The liquid column pressure gauge has a simple structure, high sensitivity and accuracy, and is often used to calibrate other types of pressure gauges. The disadvantages are large volume, slow response, and difficulty in automatic measurement. The elastic pressure gauge is easy to use and has a large pressure measurement range, but its accuracy is low, and it can not automatically measure. All kinds of pressure sensors can be miniaturized, more accurate and fast measurement, especially can measure dynamic pressure, realize multi-point roving detection, signal conversion, long-distance transmission, connection with computer, timely processing, etc., so it has developed rapidly and widely .