The often-excellent QC Lab blog from Kolin Powick, director of quality for Black Diamond, has delivered another set of fascinating test data. This time, Powick and his cohorts looked at a question that must spring to mind every time a climber burns his hand on a blistering-hot belay device after a long rappel: Could that thing melt stuff? Specifically, could it melt the slings attaching me to the anchor?
The answer, Powick found, is yes, but it's extremely unlikely under real-world circumstances.
The full report is a must-read, but here are a few important conclusions:
• Faster rappels and heavier climbers yield hotter temperatures. The weight of the climber (and the pack or haul bag he's carrying) has a particularly large impact.
• During long, free-hanging rappels and a simulated multi-pitch rappel (six rappels in a row), the testers couldn't get their ATC devices hotter than about 135°C. In lab tests, even a 10mm Dynex sling (the material with the lowest melting point) wasn't damaged by a hot belay device pushed against the tensioned sling until the temperature reached 250°C. Other materials survived even higher temperatures. That's a pretty good margin of safety.
• If you want to minimize risk, the basic best practices for rappelling and anchor setups apply: Rappel slowly, especially if you have a haul bag hanging from your harness; use nylon slings at anchors instead of Dyneema or Dynex slings; and anchor with two slings—never rely on a single attachment point.
Oh, and one more thing: The old "spit test" for belay devices is useful for telling you if a device is hot enough to burn your skin (ca. 100°C), but not much more. The BD engineers determined that spit sizzles on a hot device at about 120°C, far lower than the temperature needed to melt any sling.