Automated Reactor Sampling (ARS) systems deliver precise, on-line sampling of bioreactor contents while ensuring complete sanitary isolation. Over flow top reactor sampling system extracts a liquid sample from reactors and vessels via a dip pipe. The sample is representative, fixed volume and cannot overflow the sample bottle. Its large valve ports and minimal dead space prevent plugging. All wetted parts are made from corrosion resistant materials. The dip pipe is purged using nitrogen. Vacuum is applied and sample is drawn through the stand pipe which overflows and fills the sample chamber. Nitrogen is applied and excess sample in the sample chamber is drained off. The fixed volume of sample in the standpipe is transferred to the sample bottle.