壓力變送器是工業實踐中最為常用的一種壓力儀表，其廣泛應用于各種工業自控環境，涉及水利水電、鐵路交通、智能建筑、生產自控、航空航天、軍工、石化、油井、電力、船舶、機床、管道等眾多行業。一般大量程壓力變送器(測量范圍0-r50MPa)、螺紋連接安裝型壓力變送器，能精確測量流體的壓力，并把它轉換成DC4- 20mA的輸出信號。在檢測部內，輸入壓力被轉化為靜電電容，在傳輸部對與壓力成正比的檢測信號進行放大運算，發送輸出DC4-2OmA的電流信號。

2、整體結構和受力分析         

大量程壓力變送器的機械部總組件結構如圖2所示。

壓力變送器與現場壓力管道通過連接管采用NPT1/2圓錐管螺紋密封連接，正常工作時被測流體充滿連接管，管內部壓力為流體工作壓力，流體壓力通過密封膜片一和變送器內部填充介質進行傳遞，大氣壓力通過密封膜片二和變送器內部填充介質進行傳遞，因此壓力傳感器高壓側和連接管內部承受的壓力為流體工作壓力，壓力傳感器低壓側承受的壓力為大氣壓力。     

所以大量程壓力變送器正常工作時，其機械部總組件的連接管壁和焊縫是受壓部位中最為薄弱的環節。如果管壁設計較薄、焊縫熔深太淺，將使管壁、焊縫破裂導致被測流體或填充介質泄露。由于低壓側大氣壓力1.01X105Pa，約為高壓側工作壓力50M Pa的11500，受壓非常小，不再計算低壓側部位的強度。一般情況下，大量程壓力變送器內部填充介質和被測液體無強腐蝕性，所處溫度變化范圍較小(范圍在一150C-85"C)。

Pressure transmitter is the most commonly used pressure instrument in industrial practice, which is widely used in various industrial automatic control environment, involving water conservancy and hydropower, railway transportation, intelligent building, production automation, aerospace, military, petrochemical, oil wells, power, ships, machine tools, pipelines and many other industries. Generally large-range (measuring range 0-r50MPa), threaded connection mounted pressure transmitter, can accurately measure fluid pressure, and convert it into DC4-20mA output signal. In the detection unit, the input pressure is converted into an electrostatic capacitor. In the transmission unit, the detection signal proportional to the pressure is amplified and the current signal of the output DC4-2OmA is transmitted.

2, the overall structure and stress analysis

The overall structure of the mechanical part of the large range pressure transmitter is shown in Figure 2.

The pressure transmitter and the field pressure pipeline are connected by the NPT1/2 conical pipe thread sealing. The fluid under test is filled with the connecting pipe during normal operation. The inner pressure of the pipe is the working pressure of the fluid. The fluid pressure is transmitted through the sealing diaphragm-1 and the filling medium inside the transmitter. The atmospheric pressure is transmitted through the sealing diaphragm-2 and the transmission. The pressure on the high-pressure side of the pressure sensor and the inner part of the connecting pipe is the fluid working pressure, and the pressure on the low-pressure side of the pressure sensor is the atmospheric pressure.

Therefore, when a large range pressure transmitter works normally, the connecting pipe wall and weld of its mechanical assembly are the weakest links in the pressure part. If the pipe wall is thinner and the weld penetration is too shallow, the pipe wall and weld will break down and lead to the leakage of the fluid or filling medium under test. Since the atmospheric pressure on the low side is 1.01X105Pa, which is about 1,1500 MPa of the working pressure on the high side, the pressure is very small, and the strength of the low side is no longer calculated. Generally speaking, the packed medium and the tested liquid in the large range pressure transmitter are not strongly corrosive, and the temperature range is small (ranging from 1 150 C to 85 "C").