A) New Edelrid Aramid 6mm cord (black); tied off on all strands.
B) Cycled Edelrid Aramid 6mm cord (green); tied off on all strands
The important value in these results is the Std Dev (Standard Deviation) The three Sigma result is calculated as 3 Times the standard deviation subtracted from the average, it is the same statistical method DMM Engineering use to ensure new products meet the required or stated kN rating for them.
If we look at the values for new Edelrid Aramid Cord 6mm first, the standard deviation figures are 2.27 for the single strand tie-off and 0.56 for the three-strand tie-off. This gives us final kN ratings of 25.03kN for the Single strand tie-off and 31.89kN for the three-strand tie-off.
If we look at the figures for the cycled Edelrid Aramid Cord 6mm, the standard deviation figures are 2.73 for the single strand tie-off and 1.39 for the three-strand tie-off. This gives us final kN ratings of 19.27kN for the single strand tie-off and 23.47kN for the three-strand tie off. Bear in mind that this cord is being tested at the point of retirement and has been used on several training courses and exercises.
The final calculations are quite different to those published by Sterling Rope, drawing a conclusion from our figures would suggest the opposite to the results from Sterling, our lowest Standard deviations were found on the Three Strand tie off method rather than the single strand method.
If we were to look at our figures in isolation it would suggest that the three-strand method is more consistent and gives a higher 3-Sigma kN rating on both the new cord and the retired cycled cord.
Interestingly the lowest 3-sigma figure for either tie off method in our results was 19.27kN on cycled (now retired) cord, considering that when building cord anchors we rate them at 20kN per leg, our current advice appears to be supported by our test data.
So which tie off method should we recommend? It should probably be the one which the end user can confidently and most important, consistently tie off.
What other conclusions can we draw?
More testing is needed. There could be differences between individuals tying knots so a larger sample of end users could generate better results. We are comparing figures for Edelrid Aramid Cord 6mm against Sterling 5.9mm Powercord, back to back testing should be conducted with the same cords on the same test machines, something we will aim to do this year.