If you have a loop isolator between a transmitter and a analog card both with a 4-20mA supply, what measurement would you receive on the analog side? And how is the transmitter prevented from seeing an additional current? Is the transmitter side isolated with diodes?

 

> what measurement would you receive on the analog side?

If it wasn't the same measurement on both sides what would be the point?

http://en.wikipedia.org/wiki/Opto-isolator

 

There is no electrical connection between one side and the other, it's often a magnetic coupling as in transformer or optical.

Isolator means it's Isolated.

Roy

 

It would seem that what's missing here is an understanding of what a loop isolator does. It "isolates" the analog card electronics from the electronics of the field device, usually to prevent multiple ground loops from adversely affecting one or both of the ends of the loop (the analog card and the field device).

The concept is that the loop isolator replicates the signal from the field device to the analog card of the control system. So, if the isolator is adjusted correctly (some have various methods of adjustment, including potentiometers) then you should be able to read the same current value on the field device side and on the analog card side of the loop isolator. That's its function--to "pass" the same current value from the field device to the analog card without electrically coupling the circuits of the analog card and the field device, which may introduce unwanted effects (ground loops; noise; etc.).

So, as suggested if the loop isolator is working properly the readings on both sides (input and output) should be identical--that's what it's supposed to do, anyway.