A new donor explosive, ROWANEX 3601 has been selected for use in the large scale gap test (LSGT). It was necessary to calibrate the peak pressure recorded on the central axis of the PMMA gap using calibrated piezo-resistive pressure transducers as a function of the gap thickness. The stress history within the PMMA gap was measured and the peak pressure plotted against gap thickness as a calibration. Further effects were noted relating to the curvature of front exiting the donor charge and the validity of the measurement of the transmitted shock at small gap thicknesses.