Why do transmitters choose 4-20mA to transmit signals

Usually all kinds of liquid level gauges, pressure instruments, and flow instruments use 4-20mA to transmit signals, so why does the transmitter choose 4-20mA to transmit signals? The selection of 4-20mA for the transmitter as the transmission signal is determined from three aspects: the safety of the field application, the current source for transmitting the signal is superior to the voltage source and the selection of the current starting and ending points. 1. The first is the safety of the field application. The focus of safety is to consider the explosion-proof safety spark-type instrument, and to control the energy of the instrument as the premise, to reduce the static and dynamic power consumption to maintain the normal operation of the instrument to a minimum. For transmitters that output 4-20mA standard signal, the power supply voltage is usually 24VDC. The main reason for using DC voltage is that large-capacity capacitors are not needed, because in explosion-proof occasions, the filter capacitance of the instrument exceeds 50μF, which is very dangerous. The instrument only uses small-capacity capacitors and inductors, and only needs to consider the distributed capacitance and inductance of the connecting wire between the transmitter and the control room instrument. For example, the distributed capacitance of a 2mm² wire is about 0.05μF/km; for a single wire, the inductance is 0.4 About mH/km; much lower than the value of detonating hydrogen, which is obviously very beneficial to explosion-proof. 2. The current source for signal transmission is better than the voltage source because the distance between the site and the control room is far, and the resistance of the connecting wire is large, if the voltage source signal is used for remote transmission, due to the voltage division between the wire resistance and the input resistance of the receiving instrument If the current source signal is used as the remote transmission, as long as there is no branch in the transmission loop, the current in the loop will not change with the length of the wire, thus ensuring the transmission accuracy. 3. Reasons for choosing 20mA for the maximum current of the signal The choice of the maximum current of 20mA is based on the considerations of safety, practicality, power consumption and cost. The safety spark meter can only use low voltage and low current. 4-20mA current and 24VDC are also safe for flammable hydrogen. For 24VDC hydrogen, the detonating current is 200mA, which is far above 20mA; in addition, the production site must be comprehensively considered. The connection distance, the load and other factors; there are also factors such as power consumption and cost, requirements for electronic components, and power supply requirements. 4. The reason for choosing 4mA for the starting point current of the signal is that the transmitters with an output of 4-20mA are mostly two-wire systems. The two-wire system means that the power supply and the load are connected in series, with limited common points. Only two wires are used for signal connection and power supply between them. Why is the starting point signal not 0mA? This is based on two points: First, the transmitter circuit will not work without quiescent operating current, and the signal starting point current 4mADC is the quiescent operating current of the transmitter. Second, the electrical zero point of the instrument is 4mADC, which does not coincide with the mechanical zero point. This 'live zero point' is conducive to identifying faults such as power failure and disconnection. The externally mounted liquid level gauge developed and produced by Shaanxi Kaidi adopts standard two-wire system 24V, 4-20mA transmission signal, which is widely used in petroleum, chemical, pharmaceutical, electric power, metallurgy, military, storage and transportation and other industries. It can accurately measure the liquid level without opening the tank. It can be applied to the liquid level measurement of various flammable, explosive and strong corrosive media in vertical tanks, horizontal tanks and spherical tanks.
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