Of course if you change the loop power voltage, then you must scale the sense resistor at the Arduino analog input pin. For example, if you were using a 24V loop voltage, you would use a 1200 ohm resistor between ground and the Arduino input (which would also be the sensor - side). The 1200 ohms would develop 5V when in a 24V loop with 20mA flowing (Ohm's Law again).
This is not really how current loops work (and also a little bit confusing...)
24V can be the "open circuit" voltage, but will drop down to match the resistor (and not vice versa).
I included the10k for safety reasons if the loop would not close..
This can easily happen when you screw the wires to the board and the (-) has a bad connection...