In industry, it is necessary to measure all kinds of power consumption and non electric physical quantities, such as current (AD), working voltage (VD), output power (WD), frequency (FD), temperature (TT), net weight (LD), position (PT), working pressure, rotation speed ratio (RT), angle of view, etc., which must be converted into acceptable DC analog input electronic signals before they can be transmitted to the main control room or display system 500 meters away. This kind of mechanical equipment that converts the measured physical quantities into AC data signals is called a transmitter. In industry, it is generally divided into power consumption transmitter (general models and specifications such as GP / FP series products, S3 / N3 series products, stm3 series products, etc.) and non power consumption transmitter.

The reason for selecting current data signal is that it is not easy to be affected. In addition, the internal resistance of the split electromagnetic flowmeter is infinite. The transmission line resistor connected in series in the control circuit does not harm the precision. It can transmit hundreds of meters on the general five types of twisted pair. The limit of 50mA is due to the regulation of explosion-proof type: the kinetic energy of fire caused by 50mA current conduction cannot ignite the coalbed methane. The reason why the lower limit is not b250a is to be able to check the disconnection: it is not easy to be less than 2mA in normal operation. When the transmission line is short circuited due to common faults, the loop current drops to 0. 1mA is often taken as the off alarm value. The traditional output AC data signals of the transmitter are 0-5V, 0-10V, 1-5V, 0-50mA, 4-50ma, etc. at present, Zui generally uses 4-50ma current to transmit analog input. In industry, Zui generally uses 4 ~ 50mA current to transmit analog input.

The current type transmitter converts the physical quantity into 4 ~ 50mA current output, and it must have an external switching power supply for its power supply system. Typically, the transmitter must be connected with two power plugs and two current output wires, which is called four wire transmitter. Naturally, the current output can share one wire with the switching power supply (common VCC or GND), which can save one wire. It is called three wire transmitter.

In industrial application, the control point measurement is generally on site, while the display of professional target flowmeter manufacturer system or ultrasonic open channel electromagnetic flowmeter is generally on the main control room or control box. The distance between them will be tens to hundreds of meters. According to the calculation of 100m distance, the cost of saving 2 transmission lines is reduced by nearly 100 yuan! Therefore, the two line controller is used more and more in practical application. In fact, we will notice that the current of 4-50ma can supply power to the transmitter itself. The transmitter is equal to a unique load in the power circuit, which depends on the power consumption current of the transmitter changing between 4 ~ 50mA according to the output of the controller. The digital display instrument can only be connected in series in the power circuit. This kind of transmitter only needs two wires, so it is called two wire transmitter. The industry current loop specification low limit is 2mA, so if in the measurement range, the transmitter has at least 2mA power supply system. This makes the design of the two line system controller become the future.