A horizontal cylindrical vessel used on offshore platforms for the separation of gas and water from crude oil and is identical to a production separator except that it is used to process the contents of a single well. The oil enters the separator through a manifold. The reduction of pressure causes the release of dissolved gases, which are removed from the top of the vessel. The water and oil separate by virtue of being immiscible and having different densities. The oil and condensate is separated from the water by overflowing a weir. The results from the well test separator are used to provide information on the performance of the well.