Calibration of an air cannon for safety penetration tests

The testing of machine safety guards useful to mitigate the ejection risk of small parts or tool tips are provided by standards such as the one described in Annex B of ISO 14120. The inadequacy/suitability of the tested materials for machine guards is obtained by the perforation/no perforation of the guard in a test at the maximum foreseeable speed of a standardized projectile. The test is usually performed on a specific compressed gas cannon whose impact velocity must be known with little error, typically 5m/s, to be valid. Because gas cannon is a simple testing machine, usually the velocity is measured near the exit of the barrel, internally or externally, and the velocity measure is taken as it is. The measure of velocity near to the target is not possible because an undesired post impact between the bouncing projectile and the velocity probe is very probable. Even if the internal pressure/volume of the reservoir is not so big (usually V ≤ 25 dm3 and p ≤ 2MPa) there are some effects to be taken into account: a residual short space between the measure point inside the barrel and the exit of the projectile from the barrel in which the projectile continues to accelerate, a residual acceleration during the projectile free flight due to the air that continues to push the bullet out of the barrel. The acceleration law and the relation between pressure and velocity will be presented in the paper using three different models, isothermal process, adiabatic process and constant acceleration with a constant energy loss. Measures of the effects in a real gas cannon will be presented. Calibration of the parameters will be shown.