Abstract: The invention relates to a valve incorporating a flexible diaphragm, and improves upon known diaphragm valves to permit the use of a diaphragm-controlled valve in systems for which such valves were not previously suitable or utilized. There is provided a valve which includes a housing (2, 3, 34) having a hollow interior within an outer housing surface (31), a diaphragm (1a, 1b) extending across the interior and creating a diaphragm control chamber (10) within the interior, a valve seat (4) against which a part (1a) of the diaphragm can seal, a first conduit (5) leading around at least part of the valve seat, a second conduit (6) leading to within the valve seat, and a diaphragm control means (8, 9, 91, 92, 101, 111, 112), wherein the diaphragm control chamber is isolated from the first and second conduits. A valve unit and a lower control system are also disclosed, utilizing one or more valves as herein described.