How to test water-heater thermocouple?
NO, thermocouples may operate at as low as 1-13mv (10-13 mv for operating current, 3-1mv for ECO -emergency cut-out- at low temp.).

Some older appliances used what we call thermopiles or pilot generators (which were actually a bunch of thermocouples bundled together) to supply higher current flow (like 400mv) to operate equipment and/or control circuits (gas control valves, thermostat circuits, fan control relays, etc.) 

example: