How accurate is advertised rope weight?

My new Mammut Infinity 60 m dry duodess weighs in at 65 g/m on my kitchen scale. This is about 10% heavier than the advertised weight of 59 g/m. Mammut responded to me with pretty reasonable explanation that delivered weight can be greater than production weight because they add 2.5 m to ropes to account for shrinkage over the life of the rope and because of absorption of atmospheric moisture.

Still, I was expecting a lighter rope, and I'm wondering if this discrepancy between delivered weight and advertised weight is common across models and manufacturers. How does the weight of your new rope compare to what was advertised?

SteveF wrote:My new Mammut Infinity 60 m dry duodess weighs in at 65 g/m on my kitchen scale. This is about 10% heavier than the advertised weight of 59 g/m. Mammut responded to me with pretty reasonable explanation that delivered weight can be greater than production weight because they add 2.5 m to ropes to account for shrinkage over the life of the rope and because of absorption of atmospheric moisture. Still, I was expecting a lighter rope, and I'm wondering if this discrepancy between delivered weight and advertised weight is common across models and manufacturers. How does the weight of your new rope compare to what was advertised?

2.5m would make the rope 61.4g/m, not 65. I weighed my brand new, never used Maxim Pinnacle 9.5mm and it is only 0.75g/m heavier than its advertised weight of 61. It's sat on a shelf in a city known for humidity for a year. I also weighed a well-used Maxim Pinnacle (after wash and dry) and it clocks in at 2.5g/m heavier than advertised.

I have no way of measuring my rope to make sure that's its 62.5 m, but I'm guessing that it is.

It seems like the remaining weight difference is still quite a lot, especially in Colorado which is generally pretty dry, but I'm not really sure how much atmospheric moisture should increase the rope weight to begin with.

The advertised rope weight is a per meter weight, not a total weight. You can't know if that weight is off or by how much if you only know your rope's total weight and don't know its length.

1. Scales. There are no absolutely precise scales. This is the 1st effect to consider.

2. Weighting the scales. Some scales are sensitive to weight positioning. This is the 2nd effect to consider.

3. Water. There are a decent amount of water in the air around us. Ropes are somewhat hygroscopic (even double dry ones). Air humidity changes could affect textiles weight. This is the 3rd effect to consider.

4. Ropes materials variations. Yet another effect to consider.

5. Ropes technology variations. Merchandises descriptions are almost always include a string (printed in the smallest type size available) like "Manufacturer reserves the right to change blah-blah-blah without prior notice". Yet another effect to consider.

6, 7, 8, 9, 10, etc.

Greg D wrote: Wow. You are really good at math.

Not sure why that was warranted... the OP seemed concerned that his rope weight was off, but also seemed to be calculating it by ";guessing"; that it's 62.5 meters. Just pointing out that it may be the length that's off rather than the weight, and that worrying about the weight being off doesn't make much sense if you don't even know yet whether it is. If that's not a helpful observation, feel free to ignore it.

Em Cos wrote:The advertised rope weight is a per meter weight, not a total weight. You can't know if that weight is off or by how much if you only know your rope's total weight and don't know its length.

The manufacturer told me they cut their ropes 2.5 m longer, and the weight would still be greater than the advertised weight either way.

I was kinda hoping more people would weigh their ropes so we could see what the differences commonly are between the advertised weight and the delivered weight for different models and manufacturers.

FWIW here's what I get with a consumer-grade digital scale. These ropes have all seen moderate usage.

Mammut Twilight 7.5mm 70m: 41 g/m (manufacturer claims 38 g/m)
Edelweiss Oxygen 8.2mm 70m: 48 g/m (manufacturer claims 45 g/m)
Beal Rando 8.0mm 20m: 41 g/m (manufacturer claims 37 g/m)

So all about 3-4 g/m heavier than advertised.

My guess would be that it's combination of ropes being longer than stated, water absorption and measurement error.

Martin le Roux wrote:FWIW here's what I get with a consumer-grade digital scale. These ropes have all seen moderate usage. Mammut Twilight 7.5mm 70m: 41 g/m (manufacturer claims 38 g/m) Edelweiss Oxygen 8.2mm 70m: 48 g/m (manufacturer claims 45 g/m) Beal Rando 8.0mm 20m: 41 g/m (manufacturer claims 37 g/m) So all about 3-4 g/m heavier than advertised. My guess would be that it's combination of ropes being longer than stated, water absorption and measurement error.

If they are used they are going to contain dirt and aluminum. You need to wash them and let them dry completely before obtaining an accurate measurement. I suspect if you do that the weight will drop slightly (as it did in my case).

You have to weigh the rope while you are hanging on a full rope length of it.

Mammut claims static elongation of 6.8%.

So your "62.5m" rope is really 66.75m long. Thus it really is only 58.4g/m.

PS: this is why they call it "weighting the rope"

PPS: I was only kidding but the math works out (and the units are right, so the legions of armchair physics experts should be convinced...)

You have to weigh the rope while you are hanging on a full rope length of it. Mammut claims static elongation of 6.8%. So your "62.5m" rope is really 66.75m long. quote>
No it's not. Static elongation is the stretch of the rope with 80kg hanging on it. This has nothing to do with weight anyway. Stretching the rope does not make it lighter or heavier. Yes, the rope would be 66.75m long with 80kg on it, but that's irrelevant as weight is not measured per meter when the rope is weighted. It's per meter of unweighted, unloaded, unused rope.

mbk wrote:You have to weigh the rope while you are hanging on a full rope length of it. Mammut claims static elongation of 6.8%. So your "62.5m" rope is really 66.75m long. Thus it really is only 58.4g/m.

clever

20 kN wrote:You have to weigh the rope while you are hanging on a full rope length of it. Mammut claims static elongation of 6.8%. So your "62.5m" rope is really 66.75m long. quote> No it's not. Static elongation is the stretch of the rope with 80kg hanging on it. This has nothing to do with weight anyway. Stretching the rope does not make it lighter or heavier. Yes, the rope would be 66.75m long with 80kg on it, but that's irrelevant as weight is not measured per meter when the rope is weighted. It's per meter of unweighted, unloaded, unused rope.

But at what temperature and relative humidity?

PRRose wrote: But at what temperature and relative humidity?

At least one more important parameter to consider: pressure.

It looks like a few other ropes also have advertised weights that are greater than their delivered weight. I guess it's good to know that mine isn't a complete anomaly.

After this I'm wondering if I should even consider weight as an important factor when choosing a rope because I'm unlikely to get what I expect. If there was a consistent bias across ropes then it would still make sense to choose a rope based on weight, just expect 3-4 grams over whats advertised. But, if the differences are more variable and possibly due to more than just extra length and moisture, as some have suggested, then maybe deciding between ropes based on small weight differences is pointless. Or maybe there should be some more well defined standards for advertising rope weight since the recent craze over thinner and supposedly lighter ropes suggests that weight is very important to a lot of people.

I believe that I once read that the weight per meter is calculated as follows:

1) The rope is hung vertically
2) a 10 kg weight is attached to the bottom. (Yes, Virginia, that means there's some stretch placed in the rope; but this is done "so as to make sure all ropes are treated alike." DUH !)
3) An exact 1 meter length is marked and cut.
4) The cut piece is weighed, reported to the nearest gram per meter. (i.e. 58.7 is 59g/m ; 58.4 is 58g/m

You can probably find the exact method used in the UIAA regulations on rope weight. Certainly, somewhere in the millions of pages of EC regulations the test is also given.

A far larger problem, in my opinion, is rope shrinkage during use, and what factors affect it and which ropes are more prone to it. My 70m is now 62m long.

20 kN wrote: I dont know what there is to standardize. The process of weighing a rope is literately so simple a kid could do it. I doubt relative humidity matters that much. Maybe the difference between 0% and 100% is noteworthy, but going from 40% in the factory to 70% in your house is not going to do much. If you want to find out, grab a bunch of clean, unused clothes from your drawer, weigh them, then put them in the dryer to remove any humidity and weigh them again. That would give you an idea, although nylon will absorb much less water than nylon, so the effect would be amplified with cotton clothes. As far as the rope being longer, if it is longer the entire rope is heavier but the weight per meter is not greater than if the rope were shorter. If the numbers are really off, I think the most obvious conclusion would be that the manufacturer is not providing truthful data. That's not exactly a stretch considering considering how often companies pad the abilities of their products in order to make the product sounder better than it actually is.

Nylons would absorb approximately 2% by weight of water between 40% and 70% humidity.

Robert Hall wrote:I believe that I once read that the weight per meter is calculated as follows: 1) The rope is hung vertically 2) a 10 kg weight is attached to the bottom. (Yes, Virginia, that means there's some stretch placed in the rope; but this is done "so as to make sure all ropes are treated alike." DUH !) 3) An exact 1 meter length is marked and cut. 4) The cut piece is weighed, reported to the nearest gram per meter. (i.e. 58.7 is 59g/m ; 58.4 is 58g/m You can probably find the exact method used in the UIAA regulations on rope weight. Certainly, somewhere in the millions of pages of EC regulations the test is also given. A far larger problem, in my opinion, is rope shrinkage during use, and what factors affect it and which ropes are more prone to it. My 70m is now 62m long.

You're right according to one of the standards for mountaineering safety equipment (EN 892:2012). They weight a test sample of the rope with a 10 kg mass (for single ropes) for one minute, mark and cut out a 1 m section and weigh it to the nearest 0.1 g.

Robert Hall wrote:I believe that I once read that the weight per meter is calculated as follows: 1) The rope is hung vertically 2) a 10 kg weight is attached to the bottom. (Yes, Virginia, that means there's some stretch placed in the rope; but this is done "so as to make sure all ropes are treated alike." DUH !) 3) An exact 1 meter length is marked and cut. 4) The cut piece is weighed, reported to the nearest gram per meter. (i.e. 58.7 is 59g/m ; 58.4 is 58g/m You can probably find the exact method used in the UIAA regulations on rope weight. Certainly, somewhere in the millions of pages of EC regulations the test is also given. A far larger problem, in my opinion, is rope shrinkage during use, and what factors affect it and which ropes are more prone to it. My 70m is now 62m long.

The routine is quite strange. Should they mark and cut the top "meter" or the bottom "meter"?

I'm not sure...but given it's an EC standard I'm sure it's defined in the test!

The point I was trying to make is that with the stretch induced by the 10kg (22 lbs) it is difficult to simply take this "manufacture's number" of Xg/m and multiply by the length of the rope in meters. A 60m rope ( 60m as measured 'on the ground/on the cliff') will weigh more than 60 x Xg/m; and that is even if the rope is exactly 60m, and not cut long, as some/most manufacturers do, because the 1 meter of "un-stretched" rope weighs more than the 1 meter of "stretched" rope did in the test.

"I dont know what there is to standardize. The process of weighing a rope is literately so simple a kid could do it. I doubt relative humidity matters that much. Maybe the difference between 0% and 100% is noteworthy, but going from 40% in the factory to 70% in your house is not going to do much. If you want to find out, grab a bunch of clean, unused clothes from your drawer, weigh them, then put them in the dryer to remove any humidity and weigh them again. That would give you an idea, although nylon will absorb much less water than nylon, so the effect would be amplified with cotton clothes. As far as the rope being longer, if it is longer the entire rope is heavier but the weight per meter is not greater than if the rope were shorter. If the numbers are really off, I think the most obvious conclusion would be that the manufacturer is not providing truthful data. That's not exactly a stretch considering considering how often companies pad the abilities of their products in order to make the product sounder better than it actually is."

Not suprised you deleted that before you looked like a complete fool:-)
The standard weight measurement is taken with a rope conditioned for 72 hrs at 65% humidity. And the measurement is taken as part of the certification, not by the manufacturer.
A brand new 10mm rope stretches 5.4% tested as required by the standard (but at 74% humidy in my workshop and 12°C which will reduce the stetch a bit).