Just wondering what the problem is.
Two different sorts of loops exist:
1. The transmitter actively drives the loop because it is externally powered.(4-wire)
Then the voltage over the transmitter terminal will be same as the voltage over the rest of the loop, but inverted polarity. If there is just one consumer in the loop and wiring is short, then voltages may appear to be identical on source and consumer but inverse.
For a 50 Ohm consumer you will need 0.2V at 0% and 1V at 100%, for a 250 Ohm resistor see above.
2. The loop (not the transmitter) is powered externally by f.i. 24VDC. Then the (2 wire)transmitter is one more consumer in the loop and must limit current to give correct reading at receiving consumer's terminals. Voltage is not inverted and equals just the rest that is not consumed by consumer and wiring:
example 24VDC/50 Ohm : 24V - 0.2V = 23.8 V (wiring not regarded)=> 0% or 24V-1V=23V => 100%
250 Ohm: 24V - 1V = 23 V(0%) and 24V-5V= 19V(100%)
Btw: A disadvantage of 2wire transmitters is that in case of failure of transmitter on certain models you might get a maximum signal like 22mA. So you have to have a routine to treat readings higher than 21mA as a failure in some applications.
A 4-wire transmitter usually will just give low or no voltage at all in case of failure, which is easier to treat.
[This message has been edited by Wolfgang (edited 10-03-2005).]
