Float Switch Water Level Controller: Fill vs Empty Logic
Control Logic Guide
A float switch water level controller can refill a tank when the level falls or empty it when the level rises. Define that direction first. Then decide whether the application needs one switching condition, a separate start and stop level, or an additional alarm. Manufacturers offer several such arrangements; two visible floats do not, by themselves, identify the control function. Waterline Controls configurations
Fill and empty control reverse the start and stop levels
For two-point control, define a lower level and an upper level. During the normal operating cycle, the requested actions are:
| Control purpose | At the lower level | At the upper level |
|---|---|---|
| Fill the tank, or pump up | Start filling when the level falls below the lower point | Stop filling when the level reaches the upper point |
| Empty the tank, or pump down | Stop emptying when the level falls below the lower point | Start emptying when the level reaches the upper point |
These describe the process demand, not relay terminal states. Flowline’s LC11 instructions describe both sequences. LC10/LC11 manual, page 9
Why the same middle level can mean running or stopped
Between the two points, a latching controller retains its previous state. Consider a fill cycle: after starting at the lower point, filling continues through the middle. After stopping at the upper point, it stays stopped as consumption brings the level back through that same middle position.
Therefore, a middle-level observation alone cannot tell you whether the fill command should be active. You also need to know which threshold was reached last. This is normal control memory, not proof of a stuck float. Flowline explicitly describes the unchanged relay condition when one sensor is wet and the other is dry. LC10/LC11 manual, pages 15 and 17
One input and two inputs serve different purposes
A single threshold is useful for detecting a high or low condition. A two-point controller can maintain an operating band with separate start and stop levels. Flowline illustrates the distinction with LC10, which accepts one level input, and LC11, which accepts two inputs and provides latching fill or empty control. Flowline LC1X
That does not mean every installation needs two separate float assemblies. Waterline lists both single-float and two-float control configurations. Determine where the start/stop differential is provided in the selected system rather than counting floats and assuming its behavior. Waterline Controls
An alarm point has a different job
An additional level point may report an alarm instead of participating in normal start/stop control. The standard SJE Rhombus Type 113 arrangement uses three floats for pump stop, pump start and high-level alarm. Its documentation notes that ordered options can change the functions. Type 113 manual, pages 1–2
Flowline’s LC42 similarly separates a latching fill/empty relay from another relay for a high or low alarm. A third sensor input therefore does not automatically mean a third normal pump-control stage. Read the input and output assignments together. Flowline LC4X
Distinguish the float signal from the pump command
A control float can report a liquid-level condition to a panel. SJE’s Type 113 manual illustrates this role with separate pump-control and high-level alarm functions. The controller then applies its configured logic. A float contact designation alone does not explain the complete operating sequence. SJE Type 113 manual, pages 1–2
Before comparing controllers, write down the tank’s fill or empty duty, the normal start and stop levels, the expected behavior between them, and any separate alarm point. Match those requirements to the exact controller’s documented functions. The examples here describe a normal cycle; power-up and fault behavior require the selected system’s documentation.
For the liquid, temperature, mounting and contact-load checks on the sensor itself, see high-temperature float switch selection.