Misalignment effects using blast pencil probes

This report investigates the use of blast pencil probes and the effect on the pressure-time history when having a non-optimized alignment of the pressure gauges, the so-called misalignment effect. This may be of special importance when we are dealing with a non-static blast source, several blast sources or with a complex blast environment having several reflected waves impinging the blast sensors. We first address the pressure-time history changes when having misaligned pencil probes and compare the data to correctly aligned pencil probes. Both simulations and experimental data are considered. Next, we investigate different procedures to compensate for the misalignment effect. Both a filtering procedure and a "by hand"-method are proposed. The pencil probe tested is the PCB 137A23. The overestimation of peak overpressure, due to misalignment, seems to be slightly dependent on the charge size. However, when looking at the mean value for all charges tested, we see that a misalignment between -10° to +10° results in an error of less than 13 %. The maximum acceptable misalignment angle is of course dependent on the required accuracy. If the proposed filtering procedure or "by hand"-method is used to correct the peak overpressure, the error of the pencil probe is less than 13 % for smaller charges, while still 15-30 % for larger charges. Whether this observed increase in overestimation is a physical effect or just a stochastic variation in our limited data set is not known, hence no procedure is outlined to compensate for this effect.