Article: 3-Sigma Rating

Obviously all climbing hardware available in the UK is fully compliant with all basic European safety standards - CE mark. However if you are looking for an extra stamp of assured safety/quality look out for hardware displaying the 3-Sigma rated stamp of approval.

Below are two short mini-articles explaining how two of the most reputable gear manufacturers (DMM and Black Diamond) instigate the 3-sigma rating, and a rough outline of what it offers over and above the standard CE mark.


Black Diamond: The 3-Sigma Rating

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.



DMM: The 3-Sigma Rating

What is 3-Sigma?
In 1985 DMM developed a system of testing designed to bring real consistency and clarity to what was meant by the "rated strength" of products. The result was the 3 Sigma Rating System and this is now one of the cornerstones of the ISO 9001 registered Quality System. DMM were the first to implement this system and have been campaigning hard to make it an industry standard, with increasing success, it is now used by both Wild Country and Black Diamond.

The outdated and most commonly used method of strength rating is simply to rate a product at an arbitrary safety margin of 10% less than the mean of its test results. With this system, taking into account the full spread of results from an entire batch and the resultant variability, there is a strong chance that a product could actually fail at less than its rated strength, and indeed, this has been known to happen.

3-Sigma Explained...
Every sample of test results, whether for karabiner strengths or heights of people, shows a normal distribution. The standard deviation is the figure which expresses the statistical variability from the sample or range of samples tested. In simple terms, the Three Sigma method multiplies the mean result from the sample by 3 standard deviations to predict, with 99.9% statistical accuracy, the spread of results which we would get if we were to destruction test the entire batch. This exceptionally high level of confidence is your guarantee of safety.

* Note: 3-Sigma ratings are the forces the product is expected to break at or above, NOT the safe working load.