The purpose of the test was to demonstrate how the carabiner might behave when a common tree climbing configuration was applied. The test configuration, in this instance, loaded the DMM Sentinel carabiner in such a way that it gave way at 21kN instead of the rated strength of 24kN.
Mid-June Treetools blogged about the importance of configuration and how it effects the rated strength of a carabiner. We tested our theories by loading the configuration to breaking point.
International standards require carabiners to be individually tested (see Grivel testing video below). Each carabiner is set between two opposing 12mm steel pins and stressed to a prescribed level.
Any migration of load away from the spine of the carabiner (towards the gate) will weaken the carabiner - hence the reason for a 21kN break on the test Sentinel. Expecting 24kN in this configuration is unrealistic.
The Hitch Climber set-up, so common now in tree climbing, was a direct result of similar tests conducted by the Treemagineers. The Hitch Climber configuration ensures the carabiners are loaded correctly, eliminating any chance of the load migrating towards the gate.
If you are in Auckland on Friday August 5 2011 you will have the opportunity to discuss these carabiner tests further with the Treemagineers themselves - they will be in store all afternoon and into the evening! We'll post more information soon.