Conductivity Probe Method:

Below figure describes a conductivity probe level detection system. It holds of one or more level detectors, an operating relay, and a controller. While the liquid makes contact along with any of the electrodes, an electric current will flow among the electrode and ground. A current energizes a relay which causes the relay contacts to open or close depending on the state of the process involved. A relay in turn will actuate a pump, a control valve, an alarm, or all three. A classical system has three probes:  a high level probe, a low level probe, and a high level alarm probe.

Figure: Conductivity Probe Level Detection System