UKC

Just bought a 50 m Beal Opera Goldendry rope and it weighs in at 2749g for a measured 54 m.  Seems strange to get that much more rope than stated on the packet and even taking that into account the rope weighs just less than 51g per metre which to my mind is a long shot from the advertised 48g and takes a lot from the point of such a rope.  Has anyone else experienced similar with these?

How confident are you of the error bounds on your length and mass readings? The discrepancy you're concerned about is only around 6%, which seems like it could plausibly be put down to measurement error, especially as it's the combination of two results.

The extra few metres is normal.  Nylon ropes may shrink over time, so manufacturers will add a margin on top of the advertised length to make sure it’s at least the length advertised after shrinkage and by the time you pick it up to buy it and eventually use it.  

I’m not totally sure about the details but the rope weight measurement (g/m) is standardised by measuring under a given tension/load which would elongate things and reduce the g/m.  The rope diameter is also measured along these lines.  

“> How confident are you of the error bounds on your length and mass readings? The discrepancy you're concerned about is only around 6%, which seems like it could plausibly be put down to measurement error, especially as it's the combination of two results.”

No certainty at all really, measured it against a metre rule 3 times and weighed on what I think are an accurate enough kitchen scale (same set up for accurate weighing of 3 x 1 kg weights) and the length I measured was consistent and the weight same at2750 +/- 3g at most as far as I remember.  Very far from standardised or scientific in approach or kit I happily admit.  Still think that the results I get are not consistent with the advertised values and this could be considered relevant when ropes are seemingly sold on similarly narrow margins of weight difference.

not sure what you mean by combination of 2 results though Luke?

Thanks Innes, I did not know that rope manufacturers nowerdays give more than the advertised length, sounds a good idea and an improvement on past times. I cannot find any standard(ised)method for arriving at advertised weight measurement on the net (I am not that clever at my searches though so I don’t doubt its out there).  Still think this one is heavier than I would have hoped but there you go, still easy enough to get to the rags and looks likely to stop falls well. Probably too cowardly to test that out much though!

What I meant about a combination of two readings was that your estimate of weight per metre contains two measurement errors*. Any error in your measurement of the overall weight will combine with any error in your measurement of the length, giving an overall uncertainty that's greater than the uncertainty in either measurement individually.

Repeating measurements and finding that they're consistent with each other is a good way of establishing that some kinds of error weren't too large but doesn't do anything to eliminate the possibility of a systematic error which consistently biases the results in a particular direction.

I assumed the weight per metre was your real gripe. If getting more length than you paid for is what's bothering you then it's an easy fix.

*Error isn't meant to imply that you did anything wrong. The best labs in the world don't make perfect measurements.

check my thread from a while back ...

https://www.ukclimbing.com/forums/gear/roca_rope-607798

Good link and I think Ian whitehouse’s contribution about the review work he did on these type of issues for climber is particularly revealing. Thanks for you reply lithos