A new type of valve is introduced, thesquare‐rootvalve, in which the valve stem radius is machined to vary as the square root of the turn numberN. The open area then varies in direct proportion to the turn number, and the measured flow rateQs(N) was found to be adequate to allow this valve to be used in an automatic feedback control system. As the flow‐rate function will generally depend on the source and load impedances, a systematic method to empirically obtain precise linear (or other) flow‐rate functions is outlined. ©1995 American Institute of Physics.