Transducer Output Why 4 to 20 Ma

Zero would give very poor noise immunity*. Also if you put zero current through a sensing resistor you get zero voltage drop which is difficult to amplify and may well cause problems as if you are working with a single supply of say 0-5v you would have no offset and some amplifiers don't like input or output going down to the negative supply level (0v).
But aside from that I have no idea.
Del

*Calculate 1uA noise current as a percentage of 0mA, then as a percentage of 4mA, that will demonstrate what I mean.

One reason was mentioned -supply current for the sensor and connected conditioner in 2 wires systems- there is a second. If the signal you get is outside the limits there is a fault either in the sensor or in the connections and this helps to determine where the failure is. from this point of view it is as for the voltage ranges from 0.5 to 4.5 V which is very usual.

Because 0mA means "a wire has fallen off", or "there is a fault".

just an idea. zero current in signal transmission is somehow an open circuit and therefore it confuse the receiving end (controller) wether a fault occured or not. 4ma is logically a live zero because it represents zero variation on the measurement considering the transducer/transmitter is in service.

If we are using a 0-20mA signal we have to supply power to our electronics in a similarway as to power a voltage signal using one or two extra power supply lines.With a 4-20mA signal we always have at least 4mA flowing in the loop. This means thatif we can power our electronics from the residual 4mA we can have our power supplyAND signal on the same pair of conductors. This technique simplifies installation,especially in large plants, as only a twisted pair is needed to transmit a signal from asensing device to a measuring or control instrument.An instrument which transmits a 4-20mA signal is usually referred to as a TRANSMITTER and if it derives its power from the 4mA residual current it is called a LOOP POWERED TRANSMITTER.The principle of operation is not however immediately obvious to engineers used todealing with voltage signals.The transmitter is a current sinking circuit, which means that it will attempt to draw acurrent from an external power supply. This is usually a 4-20mA signal powered from24V DC which is often an integral part of the measuring instrument to which the transmitter is connected. Unlike voltage transducers which are wired in parallel to measuring instruments, the current transmitter is wired in a series circuit.