Significance of Current Loop 4 to 20 mA Standard
Subject: delabs comments - Tell Me. message: why 4-20mA standard. why not 2-22mA or any thing else
Mail from MU
they knew you would come up with a question like this.
so see this page en.wikipedia.org--4-20_mA
crouse-hinds.com--intrinsically_safe--4-20mA
tell your friends about it too too, we need to share the knowledge.
...transmitting 0mV or 0mA will be lost in the noise too. 4mA is enough power to go over long wires with low error due to leakages. and current ensures RFI-EMI immunity as the picked noise does not have any juice just some voltage.
Hence it could be 4-20mA not mV. 0-20mA is not good enough, but is used when you just have to interface two cards or instruments close by. 4-20 mA can go from your street to the next or even much more depending on cables and voltage.
delabs