Oil discriminating pumps are designed to separate oil from water and pump out the water from a containment or bunded area 12. They are commonly used in transformer bunds to remove rainwater that has mixed with oil leaking from the transformer 2. The pumps use advanced circuitry and micro-controller technology to discriminate between oil and water, keeping oil contained and expelling water from the containment or bunded area 1. The submersible stainless steel sensor unit monitors the different liquid levels via a series of conductive probes.