Differential pressure transmitter is used to prevent the media directly into the pipeline inside the transmitter, the transmitter with the sense of laminated sheet filled with fluid by capillary connected. It is used to measure liquid, gas, or steam level, flow and pressure, and then convert it into 4 ~ 20mA DC signal output. Smart Pressure Transmitter from the smart sensors and intelligent electronic board composed of two parts, smart sensors components include: capacitive sensor, measuring diaphragm detection circuit, the temperature sensor and temperature compensation circuit, etc.; intelligent electronic plate board components include: micro-controller and external circuit to complete the pressure signal to 4 ~ 20mA dc conversion. For measuring liquid, gas, or steam pressure, and the pressure signal into 4 ~ 20mA DC signal output. Now both are generally smart transmitter digital signal another hybrid intelligent analog signal transmitter, and DCS communications are transmitted mainly 4 ~ 20mA analog signal and then digital signal superimposed thereon for remote set zero, span and on the transmitter calibration, configuration and diagnostics. It just is compatible with existing DCS and design, is truly all-digital smart transmitters (such as field bus intelligent transmitter) transition products. Compared with the conventional analog transmission, intelligent transmission in the application has the following characteristics: a wide measuring range, range over a large smart transmitters generally have a wide range, range ratio from 30:1 to 100:1, with or even 400:1. Transmitter range ratio refers to the maximum measuring range (URV) and the smallest measuring range (LRV) ratio [1]. Smart Transmitter range compared with conventional analog transmitters, the biggest advantage is that you can reduce inventory levels, while according to process requirements, in the case of the table without changing any time change the range, rather than scruples range overrun. For example Zhongyuan Dahua Group Co., Ltd. With the addition of three sets of melamine unit, intermediate byproduct CO2 must return the medium pressure absorber urea plant re-absorption, while the absorption of CO2 into the people will greatly increase the amount of ammonia absorber pressure , so that the range of the instrument used to measure the ammonia from the original 10 t / h expanded to 30t / h, then it is very easy to use smart transmitters, online span adjustments can be, do not replace the meter. If it is necessary to replace the conventional analog transmitters large-scale new table. 2 Maintenance is simple and convenient to use handheld communicators communicate with smart transmitters optional on-site transmitter terminals or at the main control room control cabinet at the terminals. Such instrument maintenance personnel can be in a relatively safe environment better range of the transmitter control room to modify parameter settings, instrument maintenance work. Zhongyuan Dahua Group Co., Ltd. urea plant has an ammonia tank level measuring instrument, in the car stage always indicates an error, because the driving phase and normal phase liquid ammonia production caused by different density meter indication are not allowed. Only when driving to the transmitter set a range, and then set up a production of normal range. In addition, the site is easy to leak when driving ammonia and other harmful gases, instrument maintenance personnel had to wear gas masks for each treatment. If you use smart transmitter replaces analog gauges that can simultaneously solve the above problems, in the control room on the instrument parameters can be modified to ensure personal safety. Note that the handheld communicator in the instrument can not be directly connected to the control circuit across the power supply to the transmitter power, because the power supply is low impedance, handheld communicators across it can not read the information, generally requires a loop resistance 250 ~ 600Ω, resistance is too small can not communicate, the resistance will work too transmitter exception. 3 has a high and low side conversion function