How does a sump pump empty?

Set the float on a submersible pump

The submersible pump must switch off before the pumped medium is emptied

Submersible pumps (€ 29.50 at Amazon *) are, as the name suggests, pumps that are immersed in the medium to be conveyed. However, it is not desirable for the pump to run continuously. In particular, it must be ensured that a submersible pump only pumps until a sufficiently high level ensures that the pump cannot run dry.

Switching on and off manually is not recommended

You can now switch the pump on and off manually, but this could have fatal consequences if you forget the pump - both when switching it on and off. Therefore float switches are used that switch the submersible pump depending on the liquid level. There are different systems here:

  • a float pushing a rod up or down a pipe
  • a float with a magnet that triggers a Hall sensor or reed contact
  • A float with a ball that actuates a switch depending on its position
  • Level switch

The float at a submersible pump

In the case of the submersible pump, the principle most commonly used is a ball that actuates a lever. Depending on the inclination of the encapsulated housing, the ball actuates or releases a lever attached to the side with the switch at the other end. The ball runs back and forth on a predetermined path.

The second part is a weight that sits on the power line of the switch cord. By moving the cable length to the float switch is shortened or lengthened. The cable hangs here in the container, for example a pump sump or a pump shaft.

How the float works for a submersible pump

The feeding cable is held at a certain depth in the fluid due to the sliding weight. The encapsulated float switch hangs down in the container. If the level now rises, the float lies flat in the fluid until the set cable length is reached.

The elongated capsule, which lies horizontally in the water like a boat, is now forced to stand vertically upwards as the water level continues to rise, because it is attached to the cable, which no longer gives way. The air in the housing is buoyant, however, so that the elongated capsule has to stand upright. The ball now rolls down and activates the switch.

Adjusting the float of a submersible pump

You can therefore set the float switch using two settings:

  • how far the cable extends into the pump sump before the weight comes
  • based on the cable length between the weight and the float capsule

In this way you can determine exactly the height at which the float should switch on the pump.

Note that such floats can work "both ways". That means, depending on the connection of the float switch to the submersible pump, it is switched on or off when standing upright. If the float sinks and is vertical in the opposite direction, it can also be switched on or off depending on the connection. This is important if you want to retrofit the float. When installing this type of installation, make sure that the housing is appropriately earthed, as otherwise the fluid may be live in the event of damage.

Author: Tom Hess

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