Author Topic: This looks like a nice abseil  (Read 2208 times)

Offline Fulk

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Re: This looks like a nice abseil
« Reply #50 on: March 13, 2019, 11:58:54 am »
Just to clarify things, the bit I like is
Quote
to be honest I wanted the rack to burst into flames
.

Offline mikem

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Re: This looks like a nice abseil
« Reply #51 on: March 13, 2019, 11:59:31 am »
Apparently the autoignition temperature of wood is over 180'c, so I'm not surprised you couldn't make it light up....

Would be interesting to know if fulk & langcliffe ropes were also new, or likely to have damp patches!
« Last Edit: March 13, 2019, 12:10:58 pm by mikem »

Offline langcliffe

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Re: This looks like a nice abseil
« Reply #52 on: March 13, 2019, 12:12:27 pm »
Would be interesting to know if fulk & langcliffe ropes were also new, or likely to have damp patches!

Sorry - it was a long time ago, and I find it difficult to remember what I had for breakfast these days.

Offline Fulk

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Re: This looks like a nice abseil
« Reply #53 on: March 13, 2019, 12:49:47 pm »
I can't remember really, but I suspect that the thinner rope was relatively new – and I guess it could have got a little bit wet in Alum Pot.

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #54 on: March 13, 2019, 02:53:38 pm »
In an earlier post I referred to some work I did measuring temperature rise in a STOP.
I just dug out the results, from January 2013.

I glued 2 thermocouples to the inside of the hollow bottom bobbin of an old STOP, drilling a hole in the back for the wires.
One was on the lower curve, close to the hole that can be used for a krab to remove the autolock function, the other was directly opposite that, inside the pointy bit that nips the rope on autolock.
The temperatures were read off on a 2-input digital thermometer.
I have a photo, but no clue about how to post it here.

I was given an old, stiff, retired 50m length of 10.5mm Mammut and used it on the 45m surface pitch of Titan.
The pitch was dry (as it almost always is) and the rope had been kept knitted up in our living room for days so it was also dry. I had considered using the TSG drying room, which has a dehumidifier, but decided against it.

The rope went through a braking krab, which was never used.
I think each drop took less than a minute, but I didn't time them.
I could read one channel of the thermometer on the way down.

On the first drop I pressed the handle fully in the prescribed manner and easily controlled the descent by hand.
At the start both temperatures were 4C and at the end the lower thermocouple read 110C, the upper one 65C.
Half a minute later both read 65C.

On the second drop I controlled the descent using the handle, as this seemed likely to generate most heat at the site of the upper thermocouple.
At the start both read 16C, and at the end they read 82C (lower) and 86C (upper).
Half a minute later both were down to 55 - 60C.

Neither the body of the STOP nor the handle were anything like too hot to touch at the end of either drop.
The rope was not glazed at all.

The mass of the bottom bobbin plus backplate and handle is 110 grams, so the bobbin itself (steel) would make up most of that. The rest is aluminium alloy.

My mass of about 70kg means that about 30,000J of heat would be produced on each drop.

A very simple and simplistic calculation, using 100g as the mass of the bobbin, says the temperature rise that would produce in the bobbin is 600C.

That's daft because some will be taken by the top bobbin and some will escape via convection, forced or otherwise, but to my mind it would take some extreme assumptions to believe that all (or even most) of the heat goes into the descender.

Apart from the odd occasion,  of course.


Offline mikem

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Re: This looks like a nice abseil
« Reply #55 on: March 13, 2019, 04:13:04 pm »
Some heat will have been taken up by the stretch in the rope, as well as by friction into the sheath (although it is an inefficient transfer there is a lot of material - c. 3kg of rope, compared with 0.1kg of descender) & some by the humidity in the cave.

Another discussion that may be of interest:
https://www.ukclimbing.com/forums/gear/heat_generated_during_long_abseils-455860
« Last Edit: March 13, 2019, 04:29:00 pm by mikem »

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #56 on: March 14, 2019, 08:23:22 am »
Some heat will have been taken up by the stretch in the rope, as well as by friction into the sheath (although it is an inefficient transfer there is a lot of material - c. 3kg of rope, compared with 0.1kg of descender) & some by the humidity in the cave.

Another discussion that may be of interest:
https://www.ukclimbing.com/forums/gear/heat_generated_during_long_abseils-455860

Yes, there are quite a few places the heat can go apart from into the descender.

Re rope stretch, only about 2% can go there (the maximum energy available for that is mg*rope stretch, and the static stretch of the rope with 70kg on the end is about 2%).

The rope itself is a much better candidate: the SHC of nylon is about 1600J/kgK, so with 3kg of rope half of the total energy would give a temperature rise of about 3C - not something anybody would notice, and difficult to find anyway, even if you are looking for it. But rope is a poor conductor of heat and the sheath would stay warmer than that for a while.
A fairly short pitch would do for a trial.

That reference is interesting. There's nothing in most of it that hasn't appeared here at various times, apart from a mention in the last-but-one post of glass transition temperature by a chap who has made other sensible contributions on ukclimbing.
Perhaps that's where Kenilworth got it from.
Doesn't mean it's wrong of course - I still want to know why our descenders in Nettle got too hot to touch under normal use and wrecked a rope, whereas my attempt (second drop) on a similar length pitch to provoke a similar outcome failed dismally.

Offline mikem

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Re: This looks like a nice abseil
« Reply #57 on: March 14, 2019, 09:16:56 am »
Can you remember the weather conditions on that day?

Offline Chocolate fireguard

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Re: This looks like a nice abseil
« Reply #58 on: March 14, 2019, 11:56:42 am »
Not when we did Nettle.
On the Titan abseils it was cold and not raining or snowing.