In this article, we describe an instrument for measuring the mean stress of large-area membranes for ion projection lithography (IPL) stencil masks. The apparatus incorporates two improvements over the conventional bulge technique, where the deflection at the center of a membrane is measured as a function of differential pressure across it, that provide a hundred-fold improvement in the precision of the measurement. The first is to simply place the membrane, without an o-ring or mechanical clamp, on an optically flat silicon ring separating the two pressure chambers. This. eliminates the in-plane mounting forces that previously resulted in 1-2 MPa errors. The close contact between the integral ring supporting the mask. and the silicon ring provides an adequate vacuum seal between the chambers. The second improvement is to provide an isothermal environment to reduce stress errors due to temperature variations in the membrane. As a result, the measured stress is independent of temperature and the standard deviation (1sigma) of measured stress is 0.02 MPa.