When using LAN and GSM sensors by HW group, it is important to understand the difference between a 0..20mA current loop and a 4..20mA current loop. This Application Note explains the difference.
4-20mA / 0-20mA analog current loop is the output interface of many industrial sensors (water level measurement, current transformers, gas concentration sensors, and so on). The output current is proportional to the measured value.
http://hw-group.us/support/application-note-42-0-20ma-or-4-20ma-current-...