Here`s an effective for a temperature sensor to receive power from a 4-to-20 mA loop without actually affecting the loop current (see the figure). This particular temperature sensor IC (AD590F) conducts 1 / when powered by a supply in the range of 4 V to 40 Vdc. The scheme uses a 5-V Zener diode (Dl) to regulate the power source for AD590F. Most of the current flows through the Zener diode and a small current flows through AD590F.
The device can read the temperature information across Rl, which is a 1 mV/K in the range of-55C to 150C. The waste of power is negligible in this arrangement.