A new experimental "similarity" vent-sizing tool (A/V ratio) for non-tempered chemical systems was developed. It combines the advantages of both the Design Institute for Emergency Relief Systems (DIERS) method (laboratory scale) and UN method (less overconservative). This tool is based on the VSP2 adiabatic calorimeter. It uses a 0.11 test cell and allows blowdown experiments to be carried out in the 0.9 x 10(-3) m(-1) < A/V < 4.0 x 10(-3) m(-1) range. A simple additional set-up allows for real-time vented mass measurements. Similarity limitations are discussed. Sizing experiments for an organic peroxide (cumene hydroperoxide 30% w/w) using both the "similarity tool" and the UN 101 reactor are compared. The similarity tool is on the safe side (0-50% oversizing). Heat losses are the main limits of the new tool. (C) 2008 Elsevier Ltd. All rights reserved.