The Omron 61F-GP-NL 4KM AC120 is a High/Low Level liquid level controller from the 61F series, built for remote tank monitoring where the sensor sits a long way from the control panel. It operates on a 120VAC supply and uses a SPDT (2 Form C) relay output to drive pumps, valves, or alarms based on conductive probe readings. The headline spec is the 4 km maximum distance between the controller and the sensing electrode — that is not a typo. It means this unit handles tank farms, elevated reservoirs, or remote sumps where a standard level relay would drop out from line resistance. The sensitivity range of 0 to 0.5 kOhm lets it work with clean water or moderately conductive process liquids.
The relay contacts are rated 3 A at 250 VAC, which is enough to switch a small pump contactor coil or a pilot light directly. For larger motor loads, use this relay as the pilot device to energize a contactor — the 3 A rating is the switching limit, not the load it drives continuously. Termination is via an 11-pin octal plug (socketable), which means the base stays wired in the panel and the controller module swaps out in seconds — useful for MRO spares or quick replacement on a down line.
Wire the base once for the 120VAC supply, the probe connections, and the relay output; the controller module plugs in after wiring is complete. This keeps the electronics out of the panel during construction and simplifies troubleshooting — pull the module, check continuity on the base, and swap if needed.