There are two advantages with 4-20mA current loop over 0-10V signals ....
1. A current loop signal is less susceptible to interference and noise, particularly over longer distances. The transmitter produces a load independant output (within the range of the transmitter) so volt drop is also generally not a problem.
2. The 4mA zero offset allows you to detect failure of the transmitter. With 0-10V, does 0V mean zero measured parameter, or transmitter failed ??
The 4 to 20 mA stadard was adopted after a long series of debates and conflicts among suppliers and users exactly as we are facing with establishing the fieldbus standard. I am personally aware of instruments supplied using 0-12mA and 0-20 mA for communicating signals.
4mA was adopted to indicate a quantity zero signal so that if the signal is 0mA it would indicate a fault of some kind.
... another advantage of the 4-20mA current loop is that remote sensors can be 'loop powered' i.e. they derive their power from the current in the loop, and signal back by modulating that current. There are several designs for such sensors that have been posted over the years that have switching, temperature/pressure/you-name-it, A/D, and even more complicated functions. Try checking out the Maxim
