FG100 is Rain Bird's 1-inch wired flow sensor for ESP-ME3 (and compatible ESP-LX / 2-wire panels with the right sensor decoder). It learns expected station flow, then flags high flow (broken lateral, stuck valve, missing head) and low flow (blockage or failed valve) so the break shows up on the controller — and in the Rain Bird app when LNK2 is installed — instead of on the water bill.