Water cooling is mostly an indirect cooling of the resistors where the heat loss forming in the resistors is transferred to the water through an insulated intermediate carrier. Due to its high specific thermal absorptivity, water is an ideal heat carrier. The heat transfer between resistor and water is up to 10 times higher than air in steady water and up to 100 times higher than air in flowing water depending on the speed and turbulence. However, it must be available in sufficient quantities or cooled again in suitable reflux coolers, which in turn result in higher costs. Direct water cooling is only suitable for relatively small voltages due to the poor insulation properties of non de-ionized water. At higher voltages, the water must be de-mineralized and de-ionized and the residual conductivity must be continuously monitored. A current flow in case of an excessive residual conductivity in the water will result in dissociation and thus in the formation of oxyhydrogen gas.