Flow into or out of two opposed nozzles immersed in a liquid offers a means of measuring an extension‐dominated response of a liquid up to extension rates as high as 1000 s−1and more. We evaluate the concept as it applies to Newtonian liquids, by solving the Navier–Stokes system for complete fields of stress and extension rate. We examine possibilities of inferring nearly extensional stress over some internal area from a measured force (torque, actually) on one nozzle assembly, of resolving that stress into its pressure and viscous parts, and of having nearly uniform extension rate over the same area so that an extensional viscosity can be reliably deduced; the choice of the area and the control volume of which it is part proves pivotal. This examination leads to an instrument design optimization problem which we solve approximately for the near‐best nozzle bore, separation, and exterior shape, and for the near‐best choice of control volume. Results are compared with experimental findings and implications for measurements of non‐Newtonian liquids are touched on.