Abstract: An apparatus and related method for obtaining samples from within a multiple liquid container, e.g. a tank within an oil tanker, at various levels in such container and for determining the level of the interface of the different liquids in such container. The apparatus operates in cooperation with a sampling bob which can obtain liquid samples from within the tank, at whatever level it is located. The apparatus is attachable to a hatch which leads into the tank, and initially fixes the level at which the sampling bob takes a sample, usually at the tank bottom. Then, the apparatus enables the sampling bob to be shifted to a different level by movement of a positioning slide through a measured interval, so that the sampling bob can take a sample at this second level. Continued sampling at different levels enables the user to determine the location of the interface of two liquids, such as oil and water, in such a container.