It outputs a "4-20 mA" output, which is an old fashioned standard for engineering sensors.
You need to measure this output current and then scale the result according to the description
of the device.
This description may helphttp://www.bapihvac.com/CatalogPDFs/I_App_Notes/Understanding_Current_Loops.pdf
What you need to do, is feed the current from the device through a suitable resistor, and then
measure the voltage across the resistor using the analog input pin of the Arduino. A 100 ohm
resistor is probably going to work.