Why do instruments and meters like to use 4-20mA current to transmit signals

Why do instruments and meters like to transmit signals with 4-20mA current? Please see below 4-20mA signal system is the analog signal standard for International Electrotechnical Commission (IEC) process control system. my country has adopted this international standard signal system from the DDZ-Ⅲ type electric instrument. The instrument transmission signal adopts 4-20mA, and the contact signal adopts 1-5V, that is, the signal system of current transmission and voltage reception is adopted. Why do instruments and meters use 4-20mA current to transmit signals? In the industrial field, using an instrumentation amplifier to complete signal conditioning and long-term transmission will cause the following problems: 1. Since the transmitted signal is a voltage signal, it will be disturbed during the transmission process; 2. The distributed resistance of the transmission line will generate a potential difference 3. How to provide the working voltage of the instrumentation amplifier in the field is also a problem. In order to solve the above problems and avoid the influence of related noise, we use 4-20mA current to transmit the signal, because the current is not sensitive to interference. The 4-20mA current loop uses 4mA to represent the zero signal, and 20mA to represent the full scale of the signal, while the signals below 4mA and higher than 20mA are used for various fault alarms. Why is the maximum signal current 20mA? The maximum signal current of 20mA is mainly based on the consideration of safety, practicality, power consumption and cost. The spark caused by 30V voltage and 30mA current can ignite the average lower limit of dangerous gas. For the sake of insurance, and referring to other traditional settings, many instruments are set to 24V power supply, and the limited current is less than 30mA. In order to leave room, the upper limit of the signal is limited. is 20mA. What is the reason for choosing 4mA for the starting point of the signal? Selecting a 2-wire system for the sensor can reduce the complexity of wiring. The 2-wire not only needs to transmit the signal, but also power the sensor, and a certain energy supply is still required when the signal value is zero. Under the condition of 24V power supply, the energy provided by 4mA current is the lower limit of energy supply that most instrument manufacturers can accept when the standard was formulated at that time. At the same time, the electrical zero point of the instrument is 4mA, which does not coincide with the mechanical zero point. This 'live zero point' is beneficial to Identify faults such as power outages and broken wires, so that the 4-20mA standard is established.
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