[johnwalkr]

Those voltage standards are kind of meant for very local things. If you really get into industrial things, one should look to industrial standards that work over longer distances. That is things like RS422, RS485, and increasingly industrial versions of ethernet that use differential signals. One should also learn what a PLC is and understand that in an industrial context, implementing controls in an Arduino or rpi is probably reinventing the wheel to achieve less reliability than industry standards.

4 to 20mA sensors are great. Invented in the 50s (!) to replace pneumatic controls and to this day work great. iirc they are usually 24V these days. You missed an important detail; the first 4mA (96mW) powers the sensor/local microcontroller (no local power supply required), and the remaining 4-20mA gives a calibrated current output for voltage/pressure/whatever you are measuring. If the output is less than 4mA or more than 20mA you know something is wrong (and many devices will output 20.1, 20.2 etc currents as a kind of fault code).