In the Plowshare Program many calculations of the effects of underground explosions are made. Usually these are done on high-speed digital computers. The effects are calculated for ranges up to hundreds of meters from the explosion; at these ranges pressures become less than one kilobar. In order to make these calculations, information about the properties of the materials involved is required. Benedick [13] has developed a low-pressure, plane-wave lens using nitroguanidine. It was decided to use a similar lens in the Plowshare Program. A number of lenses were built using Benedick's technique. This report is of a study of simultaneity and pressure uniformity of these lenses, with some attempts at development of a reflection pressure vs particle velocity curve for them.