Abstract: A contamination monitoring system for monitoring contaminants in a liquid, comprising a first electrical conductivity sensor, a second electrical conductivity sensor, a filter and a pump, the filter being disposed between the first sensor and the second sensor, the pump being arranged to circulate the liquid, the first electrical conductivity sensor providing an indication of dissolved ionic contamination levels in the liquid prior to filtration of the liquid by the filter, the second electrical conductivity sensor providing an indication of dissolved ionic contamination levels in the liquid after filtering the liquid by the filter, the conductivity measured by the second electrical conductivity sensor being compared against a value to determine whether the liquid should be retained or discarded.