Mackay et al. (1995) have presented an approximate technique to determine the elongation viscosity from pressure drop measurements in a simple stagnation flow device. In the present paper we describe experiments using a high viscosity Newtonian oil, aimed at probing some of the assumptions made by Mackay et al. We find that Trouton ratios calculated using the original analysis are well above the value of three expected for Newtonian fluids. Finite element simulations of the flow field show this is due to the net pressure drop having a substantial shear contribution, which should be corrected for before the Trouton ratios are evaluated. Interestingly, most of the shear correction is due to shear on the inside of the orifice near the exit from the central flow region. The shear contribution to the pressure drop occurs for all flow rates, however, at large flow rates there is also an inertial correction to the pressure drop. In this paper we describe an approximate method that corrects for both shear and inertial effects. With these effects recognised and corrected for, the measured Trouton ratios are reduced to around three.