Do you know why the general industrial transmitter signal is 4-20mA?

Do you know why the general industrial transmitter signal is 4-20mA?
It is generally required in the industry to measure various types of electrical and non-electrical physical quantities such as current (AD), voltage (VD), power (WD), frequency (FD), temperature (TT), weight (LD), position (PT), pressure. , speed (RT), angle, etc., need to be converted into a receivable DC analog electrical signal to be transmitted to a control room or display device hundreds of meters away. This type of device that converts the measured physical quantity into a transmittable DC signal is called a transmitter.

The traditional output DC signal of the intelligent pressure transmitter has 0-5V, 0-10V, 1-5V, 0-20mA, 4-20mA, etc. The most widely used current is to transmit analog quantity with 4-20mA current. The most widely used in the industry is to transmit analog quantities with a 4-20 mA current. So do you know the reason for this?
The reason for using the current signal is that it is not susceptible to interference. And the internal resistance of the current source is infinite, the wire resistance is connected in series in the loop without affecting the accuracy, and can be transmitted hundreds of meters on the ordinary twisted pair. The upper limit is 20 mA because of the requirements of explosion protection: the spark energy caused by the current interruption of 20 mA is not enough to ignite the gas. The reason why the lower limit is not taken as 0mA is to detect the disconnection: it will not be lower than 4mA during normal operation, and the loop current will drop to 0 when the transmission line is disconnected due to a fault. Usually take 2mA as the disconnection alarm value.
Current-mode transmitters convert physical quantities into 4-20 mA current outputs, which must be powered by an external power source. The most typical is that the transmitter needs two power lines, plus two current output lines, a total of four lines, called a four-wire transmitter. Of course, the current output can also be shared with the power supply, which saves one line, which is called a three-wire transmitter.
You may notice that the 4-20 mA current itself can power the transmitter. The transmitter is equivalent to a special load in the circuit. The special feature is that the power consumption of the transmitter varies between 4 and 20 mA depending on the sensor output. The display meter only needs to be stringed in the circuit. This type of transmitter requires only two external wires and is therefore called a two-wire transmitter. The lower limit of the industrial current loop standard is 4 mA, so the transmitter only needs to be powered by at least 4 mA over the range. This makes the design of the two-wire sensor possible.
In industrial applications, the measuring points are generally on site, and the display device or control device is typically located in the control room or control cabinet. The distance between the two may be tens to hundreds of meters. According to the distance of one hundred meters, the elimination of two wires means that the cost of nearly one hundred yuan is reduced, so the two-wire sensor is still the primary choice in the application.
Read more:Correct calibration method for pressure transmitter