An apparatus and method of determining the level of a liquid in a reservoir having curved surfaces is described, where the measuring device comprises a support having a radius of curvature conformable to the curvature of the reservoir. A float assembly is captivated to the support so that it may move freely in the vertical direction, but is constrained in rotation with respect to the support by compatible asymmetrical structural features. The float assembly includes an element that can be sensed by a sensor assembly that is positioned along the support over the length of the support encompassing the range of fluid levels to be measured.