Why do we have 4-20mA AS THE INTERNATIONAL STANDARD for sensor output? Why specifically 4mA AND 20mA? And why do most DCS I/O cards have 1-5 V as the accepted i/p?? Why not some other value??
Need further reasoning here.

 

This has been discussed at length on this forum before. Rather than rehash it, why don't you just use the search function and look it up yourself.

--
Bob

 

4-20ma signal came to existence from pressure standard of 3-15psi, which were the more common prior to 4-20ma. 3psi correspond to 4ma and 15 to 20ma. If they would have gone with 0ma as the lower range, then it would be difficult to differentiate it with fault signal (open ckt). That is another reason why the lower value is not 0ma.
I am not sure about the second question.

 

Why do you need further reasoning? 4 to 20 ma was the agreed upon industry standard (surely you don't want every manufacturer using different ranges!).

Actually, the original electric signal that replaced 3-15 psi instrument air was 10 to 50 ma. That usually needed a 30 to 40 volt power supply, which was a fairly common supply voltage for discrete device electronics. Once integrated circuits became common, a lower power supply voltage was wanted, so the 4 to 20 ma standard replaced the 10 to 50 ma standard to permit the use of 24 volt power supplies.