9 Aug 12 19:12
You need pump-up operation
A single dual pole relay where one set of contacts is used for latching action is needed for pump-up action.
As others noted, pump-up action requires a latching action to keep the pump on after the low point switch trips as the level rises above it.
The sketch below shows pump-up control with 2 level switches and a double pole relay. The level switch contacts are closed below the trip setpoint and open when the level rises above the switch trip point. The relay contacts need to be capable of handling the pump load.
This action is called "pump up" and it requires latching action to keep the pump on when the low level trips as the level rises above its trip point.
At 'empty' (below low level), both LS1 and LS2 are 'made' or closed, energizing the relay coil K so the pump turns on. At the same time, the latching contact K is made. As the level rises above the lower level switch point, LS 1 opens, but the motor continues to run because the latching contact K holds the relay coil in.
As the level rises, eventually LS2 trips and opens, dropping out power to the relay coil, stopping the pump motor and opening the latching contact K. At that moment LS1 is open, LS2 is open and the latching contact K is open.
The level doesn't have to fall too far (deadband or hysteresis) before LS2 makes, but the relay's coil stays unenergized until the level falls below LS1, at which time LS1 makes and energizes the relay coil and the cycle repeats.