3 Sigma

THREE SIGMA RATING SYSTEM
Make it a point to know as much as possible about your equipment. The strength ratings on climbing gear are there to give you clear and conservative information, but the meanings of ratings given by different companies can vary depending on how manufacturers interpret their data. To rate our climbing gear with a meticulous degree of accuracy, Black Diamond uses a rigorous rating standard called Three Sigma. Here's how Three Sigma works.

Example: Carabiner Ratings
Engineers test a random sample of carabiners under international standards using a machine called a tensile tester, which literally stretches the carabiners until they break. They determine the average, or mean, breaking strength of the batch. From this data they also calculate the average deviation from the mean, called the "standard deviation," symbolized by the Greek letter sigma. The standard deviation represents the variation of breaking strengths within a given group of samples. If the biners are rated three standard deviations less than the average breaking strength, 99.87% of the biners will be stronger than the rating.

So let's say we break 20 biners with an average strength of 6000 lb, and with a standard deviation of 100 lb. The Three Sigma rating would be 6000 - (3 x 100), or 5700 lb. The Three Sigma standard tells us that only 2 in a batch of 1000 biners might not meet the 5700 lb rating.

What Is a Kilonewton?
When you check BD biners for their rated strength you will find it expressed in metric units of force called kilonewtons (kN).

Force (N) = Mass (kg) x Acceleration (m/sec2) 1000 N = 1 kN 1kN = 224.8 lb force

The kN rating measures the amount of force it would take to break a piece of equipment, as opposed to a static weight. A biner rated at 25kN (5620 lbf) could be expected to break if the load of a fall equaled this value. All BD biners are stamped with closed, open-gate and minor axis strengths.

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