The origin of 4-20mA gas pressure sensor
The pressure sensor output value commonly used in industry is 4-20mA, which is the conventional sensor output value, but why is it 4-20mA instead of other numerical output?
It is well-known that the reason for using a current signal is that it is not susceptible to interference, and that the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can be transmitted several hundred meters in an ordinary twisted pair.
The upper limit of 20mA is due to the explosion-proof requirement: the spark energy caused by the on-off of the 20mA current is not enough to ignite the gas. The reason why the lower limit does not take 0 mA is to detect the disconnection: Normal operation will not be lower than 4 mA. When the transmission line is disconnected due to a fault, the loop current will drop to zero. Always take 2mA as the disconnection alarm value. A current-type transmitter converts the physical quantity into a 4-20mA current output, which inevitably requires an external power supply to supply it. The most typical is the transmitter requires two power lines, plus two current output lines, a total of four lines, called four-wire transmitter. Of course, the current output can be shared with the power supply for a common line of VCC or GND, which saves one line and is called a three-wire transmitter. In fact, you may notice that the 4-20mA current itself can power the transmitter. Transmitter in the circuit is equivalent to a special load, the special point is that the power consumption of the transmitter varies between 4~20mA according to the sensor output. The display meter only needs to be in the circuit. This transmitter requires only two external wires and is therefore called a two-wire transmitter. The lower limit of the industrial current loop is 4mA, so the transmitter has at least 4mA of power within the range.
Therefore, the 4-20mA signal output is generally not susceptible to interference and safe and reliable, so the industry is generally used two-wire 4-20mA power output signal. But in order to better deal with the sensor signal, there are still more other forms of output signal: 3.33MV/V; 2MV/V; 0-5V; 0-10V and so on.