A hydrostatic pressure testing system for hydrostatic pressure testing of pipe or other vessels. The closure element 34 is attached to the closure cap 14 and movable to an intermediate position in which the closure cap 14 seals against the rim 18 of the container body 12. A motor assembly at the surface rotates the rods to drive the pump. The sets of reflection signals are used to generate an image of a profile of the sample and an image representative of acoustic impedance features in the interior of the sample. When the authentication data is input, a list of terminals associated with the verification data matching the authentication data is displayed. The stages define two end groups and at least an intermediate group of switches, the two end groups comprising switches of each cell belonging respectively to the first and last stage connected in series, the intermediate group comprising pairs of switches of cells belonging to two neighbouring stages connected in series.