The output of pressure transmitters is usually a 4-20mA current signal. The reason for using current signals is that they are not easily disturbed. And the internal resistance of the current source is infinite, and the wire resistance connected in series in the circuit does not affect accuracy. It can transmit hundreds of meters on ordinary twisted pairs.
The upper limit of 20mA is due to explosion-proof requirements: the spark energy caused by the current of 20mA is not enough to ignite gas. The reason why the lower limit is not set to 0mA is to detect wire breakage: during normal operation, it will not be lower than 4mA. When the transmission line breaks due to a fault, the loop current drops to 0. 2mA is often taken as the alarm value for wire breakage.
So when selecting a pressure transmitter, it is generally a 4-20mA signal output, which is less susceptible to interference and safe and reliable. The two wire 4-20mA output can save sensor costs, which makes the transmitter widely used in industry as a two wire 4-20mA output. Of course, many transmitters now have other output styles such as 0-5V, RS485/RS232, etc., all of which are designed to handle transmitter signals well.
