Interface Self power Sensor with Particle Photon using 4-20mA current receiver
Please provide links to equipment used.  

Excellent tutorial here.
Images below are from the above web page. 

For a sensor powered by the loop current / voltage: If the sensor accepts a voltage, uses some of the voltage to power itself and then modulates the current to 4-20 mA to return signal, then eg placing a resistor from negative return to ground will provide a voltage across the resistor proportional to current. Who needs a receiver ? :-).   



If the sensor has its own power supply then placing a resistor across the output will provide a voltage proportional to current. Who needs a receiver ? :-).   



In both cases a receiver in place of the resistor should provide a useful signal.

Many of these may be relevant, This provides  a good tutorial.