So I'm trying to explain to my co-workers (automation company of which I am the most versed in how the industrial electronics work) why a 4-20mA current loop is used instead of a 0-10V signal in industrial automation. I'm using an example that a 0-10V signal and a 4-20mA signal originate from the same panel, travel along identical shielded twisted pair wire, travel the same 200 feet (or just a generally long distance), and experience identical interference from the environment. The 0-10V signal will be more noisier than the 4-20mA signal, even though the only difference between them is the type of signal. This is because the 4-20mA transmitter is actively working against any voltage induced in the wire by the interference. Most controls engineers just stop at "4-20mA loops are less noisy" and never really ask why if voltage signals are transmitted in the same type of wire in the same environment, etc, etc
OK.
So in explaining the above, I had to introduce the concept that there will be no voltage drop in a wire if no current is flowing, as many of them just made the statement that the 0-10V is losing voltage because of "voltage drop". (Industrial Control analog input cards have sufficiently enough high impedance to consider the current negligible for the purposes of the conversion). So I made a claim that if you put 10.0000V on a pair of wires and run this pair 70 miles and measure the voltage on the other side, you would get 10.0000V on the other side. It would be noisy, but it would still be around 10V. I of course immediately backtracked because a multimeter will induce some current even with a 10MOhm impendance, but again for the purposes of the conversation the point still stood. No current, no voltage drop. Period.
So someone who has worked a lot with PoE mentioned that a general rule of thumb used in PoE architecture is that you get 2V drop per 100feet of wire. I asked him what was the specified load that that this frule of thumb is applicable for. He said, no load, you just get 2V drop per 100feet. I asked him if you have 48.000V on one side of the PoE wire, and run it 100feet and then test the voltage with a high Independence multimeter you will get ~46V? He said yes. I said since there is no current flowing (open circuit) there would be no voltage drop. (UPDATE: Has has since come around after playing with an online voltage calculator)
However, I don't want to mislead my co-workers. So my question is this: Is everything I stated above correct?
And more specifically if you have 48.000V measured on a 22awg (16.14mR/foot) PoE line and measure it let's say 1000 feet later, with a 10MOhm multimeter, in the REAL world you would read 47.99984506V right? 0.01614R * 2000ft = 32.38R total wire resistance. 48V input / 10000000R multimeter impendance = 0.0000048A. 48V - (0.0000048A*32.38R) = 47.99984506V (in the REAL world)? Yes it would be noisy, but if you could say average the value over 10 minutes or use a low pass filter it would be that voltage right? He's making me question my sanity!