2 wire transmitters cannot be ranged to 0-20mA because a 2 wire transmitter uses about 3.5 or 3.6mA of current to operate its own electronics. Without the 3.6mA of current 'below' 4.0mA, a 2 wire transmitter would not operate.
So a 2-18mA or 0-20mA is not doable because there would be no current available to drive the electronics in the transmitter.
3 or 4 wire devices would conceivably be 0-20mA, like an analog output on a PLC's AO card can usually be ranged 0-20mA because power to drive the circuit's components is not part of the loop circuit; the circuit power supply is external.[/QUOTE
Thanks for the reply , I've just seen it, i agree with you , the filed instruments are supplied by the approved associated apparatus ,