This article reports on an investigation [7] into the influence on the characteristic curve of a pump of changing the pipe diameters and the positions of the measuring planes. The aim was to find out how the characteristic curves are affected by the flow conditions as well as the measuring planes on the suction and discharge side of a horizontal, single stage volute casing pump. The results show very clearly, that no single Q-H curve exists and they confirm that the determination of the correct Q-H curve requires strict attention to the existing ISO codes for acceptance tests. Using the set-up of measuring instruments in accordance with e.g. ISO 3555, the required strict attention to an international accepted code can be achieved. But what happens when the static pressure head is not measured as a mean value in accordance to ISO within at least 2DN from the discharge branch, or when it is measured anywhere in the region of the discharge branch, e.g. at different positions of the circumference of the discharge branch or at the manometer connection which is normally placed below the flange of the discharge branch?