I have a temperature measuring instrument giving 0 - 10 Volts (30 mA)
corresponding to 0 - 100 celsius. This voltage signal needs to be
transfered 500 feet away on an ammeter. Is this possible by just using
a series resistor? What are the drawbacks if neglecting environmental
temperatue and noise pickup disturbances.
Quite feasible, as your instrument can source 30mA. The usual problem
people hit with current loops is, it is easy to make a current *sink*
(ie variable resistor) but you sometimes find there's no power at either
end of the loop to drive it.
Next problem people hit is voltage drops: you will have some volts
dropped across the series resistor, but people tend to overlook the
cable. 500 feet each way = 1000 feet = several ohms. When we used to
drive 4-20mA over a kilometre or so, we needed to make sure the voltage
drops were OK *at max current*, ie 20mA. (Actually many instruments use
24mA to mean "fault", so you needed to be sure the supply could give
enough volts not to go non-linear at 24mA with a kilometre of cable,
doubled 'cos it's round a loop, plus the drop across the variable
resistor.) Include the tolerances of the supply and resistors and so on
and you can hit problems. It's just Ohm's Law, simple algebra, but it's
odd how many people just think of cable as a zero ohm resistor.
You can test this by looking up the specs of the cable you intend using.
Calculate the DC resistance of 1000 feet, put that on the output of your
instrument, add a current limiting resistor such that the total output
at 10V is say 10mA, and see if the output is indeed 10mA. If the
instrument only has a 10V supply within it it could have trouble
sourcing over 10mA.
Next problem - ground loops. If it's a simple unpowered ammeter at the
far end, no problem. If you end up connecting 2 earths together, you
could have an offset (that's one reason why 4-20mA loops start at 4mA)
and need to get an (expensive) galvanic isolator to sort the issue, or
discuss it further here.
We generally used unshielded twisted pair for current loops.