data-acquisition.us

[_JBurns_]

Why it is better to use a 4-20mA signal than a 0-10V signal. How/why were "4" and "20" values originally decided?

[daqmaster]

There are two MAIN 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-independent 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 did a transmitter fail?

The 4 to 20 mA standard was adopted after a long series of debates and conflicts among manufacturers and users exactly as we are facing with establishing the fieldbus standard. You may be 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/etc., A/D, and even more complicated functions. See Maxim http://www.maxim-ic.com or Linear Technology http://www.linear.com web sites to see what you can find if you're curious.