Recent work on fully opened rubber-lined diaphragm valves showed that due to the lack of geometric similarity, dynamic similarity could not be established. The laminar flow loss coefficient constant therefore becomes diameter dependent as is the case of turbulent flow loss coefficients. The purpose of this work was to establish if this is the case for all types of diaphragm valves, by testing diaphragm valves from a different manufacturer. Accurate loss coefficient data is critical for energy efficient hydraulic design. Saunders type straight-through diaphragm valves ranging from 40mm to 100 mm were tested in the fully open, 75%, 50% and 25% open positions, using a range of Newtonian and non-Newtonian fluids. It was found that the laminar flow loss coefficient constant suggested by Hooper (1981) is sufficient for all valve diameters at Reynolds numbers below 10. However, for transitional and turbulent flow the same loss coefficients cannot be applied for more accurate designs for diaphragm valves from different manufacturers. A new correlation has therefore been developed to predict the loss coefficients for straight-through Saunders diaphragm valves at various openings from laminar to turbulent flow regimes. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.