Wet or Dry rope SRTing ???

[Ed W]

Surely the crucial thing is the TEMPERATURE of the descender in contact with the rope.  THis is very different from HEAT, which is a form of energy.  Temperature is a physical property of an object which is related to how heat energy has excited the particles within it (for a better description see http://en.wikipedia.org/wiki/Temperature).

Though abseiling fast would result in a higher proportion of the initial Potential Energy (Mass * Height of pitch * acceleration due to gravity) of the caver to Kinetic Energy (1/2 * mass * velocity squared), hence a smaller proprtion of said potential energy being converted into heat energy.  The temperature of the descender will depend not only upon the total heat input, but also on the rate at which that heat is generated by friction converting the potential/kinetic energy.  Think about trying to hard boil and egg over a candle flame for several hours vice a few minutes over a gas flame.  The total heat energy input may be the same, but the temperature of the water is much different.

The reason for this is in the way that heat energy flows from hot to cooler mediums (conduction, convection and radiation).  I was no genius with thermodynamics (and in writing this post have discovered that I have thrown out my text books on the subject), but I am sure that conduction will be by far the most efficient way of transferring heat energy from the descender into the environment (i.e. via the rope).  There will undoubtedly also be a small amount of heat transfer to the air via radiation and convection, though I would guess these to be small.

Therefore my guess (also based on many years of observation) is that the faster one descends, the hotter the descender will get.  The rate at which heat energy is input into the descender will be higher, but the rate at which it is transferred to the ambient environment (via conduction to the rope combined with radiation and convection into the air) will remain constant.  The laws of heat flow are analogous to fluid flow, and a simple demonstration would be to take a litre of water and drip it slowly into a cup with a small hole.  The slow input drip will mean that little water accumulates in the cup.  Now try and upend your 1 litre container into said cup, and watch the water overflow.  The principal is the same.

If conduction of heat energy into the rope is the primary transfer mechanism, then I would suggest that a descender will be cooler at the bottom of a pitch (all else being equal) since the efficiency of that conduction will be improved.  It is also worth remembering (someone else mentioned it) that heating of water is a very efficient mechnism for removing heat energy.  Every ml of water soaking up 4.2J per 1 Deg C (or Kelvin to be really pedantic) rise in temperature.  For reference (if I have read this link http://www.kayelaby.npl.co.uk/general_physics/2_3/2_3_6.htmlcorrectly Nylon has a specific heat about half that of water at the sort of temperatures we are dealing with, thus I would fully expect the descender to reach a higher temperature on a dry rope than wet rope as the heat outflow by conduction will be about half that compared to water in a wet rope.  Again, from years of experience, descenders stay cooler if the rope is wet.

The only times I have seen sheath glazing on my ropes is when they have been dry!

Finally I find wet ropes much easier to handle and knot than dry ropes.

 

[jarvist]

The only times I have seen sheath glazing on my ropes is when they have been dry!

If we have our standard 81.5kg caver, he/she will dissipate 800J of energy per metre of descent. The vast majority of this will be into heat by friction physically generated at the wear surface of the abseil device.

Heat of vaporisation of water is absolutely massive -- 2.3MJ/kg (i.e. equivalent to heating the same mass of liquid water 500C). This means that you only need to boil a third of a millilitre of water per metre of abseil (i.e. a drop) to get rid of all the heat generated by friction.
Therefore just a slight wetting of the surface of the ropes is sufficient to protect it from getting above >100C, let alone near the melting point of Nylon around 200C.
I also find that having my bobbin start sizzling and a tendril of steam flying passed my eyes is very persuasive in telling me to slow down the abseil.

Similarly, it's perfectly safe to abseil at infinite speed on dry ropes -- the heat is just turned into the 'glaze' of melted nylon @ 800J per metre. The danger comes if / when you stop & then your 200C bobbin wheels neatly crimp their whole way through.  :

 

[Fulk]

Ed W:

Therefore my guess (also based on many years of observation) is that the faster one descends, the hotter the descender will get.  The rate at which heat energy is input into the descender will be higher, but the rate at which it is transferred to the ambient environment (via conduction to the rope combined with radiation and convection into the air) will remain constant.

I was under the impression that the rate of transfer o fheat depends on the temperature difference between the hot and the cold body ? the higher the temperature of the former, the greater the temperature difference, and the higher the rate of transfer of heat / energy?

 

[Alex]

I think all this arguing is pointless, I think only a practical experiment is required. The hard part would be to eliminate all the variables.

Attach a descender to the rope and take a temperature reading (Not sure of the best method to do this however). This will be your base reading.

Descend a 50ft pitch slowly. Take a temperature reading at the bottom.

Let the descender cool to the original temperature reading and descend quickly this time. Again take reading at the bottom.

The problem variables, should be slight, but there may be Fluxuations in air temperature and wear on rope and descender from the first experiment. To allivate the latter take a separate identical rope and descender. Also we would need to decide and control how fast is slowly and quickly.

 

[Ed W]

Fulk,  you are quite right, the rate of heat transfer is indeed dependant upon the difference in temperature.  Thus a hotter descender will transfer heat energy rather quicker into the environment.  However, I suspect that this is far outweighed by the difference in the rate of heat generation between a really fast descent and a slow one.  Again, experience suggests to me at least, that a slow descent results in a cooler descender.

I can't remember the formulae for heat transfer, but I seem to recall it is proportional to the temperature difference in degrees Kelvin (i.e. approximately the temperature in degrees Celcius plus 273).  Given that the ambient cave temperature will be c.280K, even a temperature difference of 50 degrees C between our hot and cold descender isn't going to have a dramatic effect on the transfer rate (say our cool descender is 30 Deg above ambient and our hot one 80, then the transfer rates are 310/280 against 360/280,  which I make to be about 16% greater in the hotter case.

However, for the fast descent, let us assume a pitch of 100m, with a weighty caver of 100kg and approximate the acceleration due to gravity as 10 m/s/s.  This results in a potential energy of (by my dodgy maths) 100kJ (100 * 100 * 10).  If our slow descent is at 1 m/s, then the caver has a kinetic energy of 50J (0.5 * 100 * 1squared).  Leaving 99.95kJ to be turned into heat energy via friction.  I.e. the kinetic energy is almost negligible.  This heat is generated over a time period of 100s, supplying just under 1kW to the environment (power being energy divided by time, J/s).

If we now take a fast abseil of 2 m/s (I seem to recall this figure quoted on the distructions for a Stop many years ago as a "do not exceed figure"), then our caver gains kinetic energy of 200J (four times as much as it is proportional to velocity squared), leaving 99.80kJ to be converted to heat via friction.  As can be seen, although doubling the speed of descent increases the kinetic energy four fold, the kinetic energy remains negligible compared to the potential energy that needs to be converted to heat.  By my dodgy maths, this results in a mighty 0.2% reduction in the heat generated.  However, this heat is now introduced into our system in only 50s, half the time.

Therefore this is equivalent to very slightly less than 2kW power, i.e. we have doubled the rate of heat input.  Therefore the temperature of our descender is going to rise faster than in the slow scenario, even allowing for say 20% increase in heat transfer rate from the hotter descender.

I now await the holes in my logic to be cruelly exposed!

 

[Fulk]

Hmmmm,

I now await the holes in my logic to be cruelly exposed!

I think that here's a hole in your logic ? I think that the rate of transfer of heat depends on actual temperature difference, not its relationship to deg K . . . so

say our cool descender is 30 Deg above ambient and our hot one 80, then the transfer rates are 310/280 against 360/280,  which I make to be about 16% greater in the hotter case.

, then, I think that in the latter case the rate of transfer is 80/30, = 2.7 times . . . so 170% more.

 

[Peter Burgess]

Fulk is correct.

Q = U.A.dT

Q = rate of heat transfer (J/s or W)
U = Coefficient of heat transfer
A = Area through which heat is being transferred (sq.m)
dT = temperature difference (K)

 

[Fulk]

So ? what does all this stuff boil down to?

On a short abseil, it probably doesn't matter whether your rope is wet or dry.

On a long descent, then either (1) wet your rope before you rig it (assuming it's not a wet pitch) or (2) abseil slowly.  I've done, e.g. Alum / 220ft descent on a dry rope with no problems, but I've also had a rope 'glazed' in Bar Pot at less than halft hat distance.

If in doubt, wet it!

 

[Peter Burgess]

There are similarities with drilling steel plate. If you don't lubricate the drill, something will get hot and normally it's your drill bit that suffers. Keep it fed with lubricant and the bit stays cool. If the steel plate is thin, you will probably get away with it.

 

[Ed W]

Oh well, I did say I was crap at Thermodynamics!  I still remain convinced from experience that the descender gets hotter with a faster descent.  I am also pretty confident in the statements about rate of heat flow into the descender, i.e. that the rate of heat flow (power) is proportional to the speed of descent even if the total heat energy is the same for a given drop.  I also however accept that I cocked up on the heat flow out of the descender, it being proportional to the temperature difference between the descender and ambient not in relation to absolute zero (it was a nice try though).

The problem is of course that we have a non-steady system.  In order for the heat flow out of the descender to increase, it needs to be at a higher temperature.  Therefore we run into a cause and effect situation, which is that in order to get higher heat transfer out of the descender we need to put more in.  At no point will we actually reach a heat flow equilibrium - especially when a non-steady descent is factored in.

In reality does it matter?  In terms of speed of abseil I can think of lots more reasons why a slow descent is preferable than heating your descender, not least being to appreciate the view on the way down.  As stated before, I try and wet my ropes every time.  Given that they have to be strong enough when wet anyway, I find that the rope handles better in this condition.

 

[TheBitterEnd]

Are we sure that glazing is caused (entirely) by heat? Could there be other factors involved? Some materials, such as rock polish without melting. 

 

[owd git]

Time for my 2 penne'th, similar to Faulk i have decended 240' without glazing a dry rope, and didn't notice a worrying heat build-up or temperature rise (.K) in my stop. it was a slow and exploratory sort of decent, i feel the time the stop was in contact with the rope 'at rest' allowed a heat transfer which wouldn't be the case in a continual decent. as i don't regularly wear gloves 'on rope' i know a heat or temperature rise would have been immediately noticeable.
In contrast on descending Elisabeth @ Nettle recently i found a good  build up, and deliberately did a dead fly so's to open my stop immediately. 
This was on a stiffer rope tho' of the same diameter.
Oh, i shall wet a rope for a biggun henceforth, if there's some water nearby/ on the route
Owd Git. 

 

[Fulk]

TheBitterEnd (as in 'the last drop of beer'?):

Are we sure that glazing is caused (entirely) by heat? Could there be other factors involved? Some materials, such as rock polish without melting.

Very interesting . . . but in my experience ropes only get glazed as a result of a fast descent; which doesn't prove that there isn't some other factor at work, but tends to support the hypothesis that it is heat ? thus  melting ?  that is the culprit.

 

[Chocolate fireguard]

I think Ed W is right when he says we will not reach a heat flow equilibrium if conduction, convection and radiation are the only ways of removing heat from the descender, because such an equilibrium will require it to be at a temperature high enough to do worse than glaze the rope.
Sensible estimates for the rate of heat production are about 0.5kW, with half of that (at least) going into the descender, so equilibrium will be reached when the descender is getting rid of heat at the same rate.
Air is a very poor conductor of heat. normal convection and forced convection won`t do either as normal convection takes heat upwards and forced convection takes it in the direction of the airstream, also upwards relative to the caver, and someone would surely have noticed if they were leaving a "slipstream" of warm air (I made a few simplifying assumptions about the cross sectional area of this stream and I reckon it would need to be several tens of degrees warmer than the cave air to take the heat away).  At ambient temperatures radiative heat loss becomes significant only when the hot object is far hotter than any of us would like their descender to get, and anyway shiny things don`t radiate well.
It would have to be a pretty deep pitch though!
Thank goodness for water. It has a high specific heat capacity and a specific latent heat of vapourisation that kicks in at a low enough temperature to protect our ropes.

 

[cap n chris]

Hypothetically, ;-) , what kind of heat would be generated on, say, a dry 10mm rope descent of a 30m pitch in, say, a "Hollywood" abseil taking 5 seconds?

 

[owd git]

Hypothetically, ;-) , what kind of heat would be generated on, say, a dry 10mm rope descent of a 30m pitch in, say, a "Hollywood" abseil taking 5 seconds?

None they are too cool
O. G.

 

[Bob Mehew]

My eyes glaze over with this question as the physics is extremely complex; see the last issue of Speleology for some of the parameters.  The basic conduction equation is d^2T/dx^2 = (1/K)dT/dt where T is temperature, x is distance, K thermal conductivity and t time.  (Sorry I can't find a superscript to make the first differential term clearly 2nd order.)  We have two materials of substantial different thermal conductivity (metal and nylon) in contact with heat being presumably generated at their point of contact at a rate proportionate to the rate of descent.  Also both materials have different thermal capacities, so for a given amount of heat put into the system, one will find each material reaches a different temperature.  However whilst the metal material has a "fixed" point of contact, the nylon point of contact is continuously being "renewed".  I am not at all clear as to how to relate the heat being generated into a temperature since amongst other things, I don't know if the heat is being generated at the interface and is then conducted away; if so what is the temperature of the interface?  (Or is the heat being raised at different proportions in each material which sets different temperatures on either side of the interface.  Though I suspect this is wrong.)  We also have the metal being a reasonably easily modelled block, but how does one model the nylon in the form of a kernmantle rope construction?  And talking of modelling, I don't see how to transform the equation into the 3 dimensional reality of say a rope passing over a bar on a rack. 

So as Alex suggested an experiment is the simpler way of settling this argument but I would not suggest using a live person to do it.  Nor can I see how to easily do it since the temperature profile of the descender will be highly variable even for a slow descent. 

But Jarvist was mostly correct though one point worth making is that it is not the melting point which is of concern, it is the deflection temperature which for nylon is around 160C.   

And for Chris, exactly the same as for taking 10 or 20 or even 30 seconds.  The basic equation is energy released equals mass * g (as in gravity) * height.  The challenge of this question is that as Ed was trying to say, the speed of transfer of heat away from the point of contact is dependent upon the temperature difference, so there is a sort of feed back mechanism going on.

 

[Ed W]

I hated thermodynamics, and now you lot are dragging me into thinking about it in my spare time.  What a strange past time caving is.

Also, why is it that whenever you post you think of a better way pf putting it an hour later.  The more I think about this, the more I convince myself that all else being equal, the faster descent will inevitably lead to the descender reaching a higher temperature.  Though Bob is entirely correct that in practice we are discussing a very complex thermodynamic system, I think that we can reduce it sufficiently to show that a fast descent will result in a hotter descender.

As posted previously, the rate at which heat energy is input into the descender does appear to be proportional to speed of descent (though total energy input over the descent is to all intents and purposes the same as the amount of KE generated by the caver is insignificant compared to the PE for all practical and safe descent speeds).  The temperature of the descender will increase with heat energy input according to the equation supplied by Peter.  However, this ignores the heat flow out of the descender due largely to conduction (but also to a littl;e radiation and convection).  The rate of heat flow out of the descender is related to the difference in temperature between the descender and the ambient environment.

Clearly in the faster descent, the descender will heat up more quickly, as heat is being introduced into the system faster than in the slow descent.  The question then is how is this offset by outflow, which is also dependant upon that temperature increase?  If we reduce the problem to a constant speed descent, in which the heat flow reaches equilibrium (i.e. steady temperature due to heat in equalling heat out), then for a higher rate of descent the descender must release heat at a higher rate to maintain constant temperature.  The only way that heat can be released at a higher rate, is if the descender temperature rises.  Thus, in this case the descender used on the faster descent must be at a higher temperature than that in the slower descent.  The fact that the rate of heat flow out has possibly increased faster than the heat flow in does not affect this argument, it merely suggests that the steady state temperature achieved will not be directly proportional to the speed of descent.  Though a true descent is an unsteady and much more complex situation, I would have thought that the general principal still holds.

Going back to my previous illustration using the litre of water and a cup.  This could be compared to a situation where the hole in the cup gets larger as the depth of water in the cup increases.  If we start to fill the cup at a given flow rate, the depth of water in the cup will increase until the hole in the bottom enlarges sufficiently that the outflow equals inflow.  The resulting depth of water is them analagous to the temperature in our descender.  The faster we fill the cup, the deeper the water, despite the rate of outflow being higher.

I'm off for a coffee and back to making ships - they are much easier!

 

[ChrisJC]

In our experience, likelyhood of rope glazing is proportional to weight being descended, whether it's 'Fatman' or me with four rope bags.
Which seems logical given PE=mgh.

Has glazed rope ever been tested?

Chris.

 

[Roger W]

As posted previously, the rate at which heat energy is input into the descender does appear to be proportional to speed of descent (though total energy input over the descent is to all intents and purposes the same as the amount of KE generated by the caver is insignificant compared to the PE for all practical and safe descent speeds).

For any given descent (caver X weighing Y kg descending Z metres) the total amount of potential energy to be lost will be a constant.  This will be converted into kinetic energy as X drops down the pitch, resulting in a lot of KE to be explosively dissipated at the bottom if he goes down under free fall or gradually converted into heat energy through friction if he uses a rope and descender.  Again, the total heat generated in the descent should be constant, however quickly or slowly he descends, but the speed of his descent will affect the time it takes him to reach the bottom (obviously!)  and a fast descent will mean less time for the heat generated in the descender to be transferred into the rope. This, I think, will mean a hotter descender (less time for the heat to be transferred out of it into the rope) and also a hotter rope surface (hotter metal in contact, less time for the heat to dissipate into the depths of the rope).

And, of course, if X decides to drop like a stone for the first hundred metres or so and then slow down to a safe landing speed in the last few metres, there will be an awful lot of kinetic energy to be converted into heat in a very short time and distance.... 

So far, we have been thinking about heat generation and the rope getting glazed due to getting too hot.  How about other aspects?  What about mechanical damage to the rope?  Is a wet (and muddy) rope more likely to suffer abrasion damage than a dry muddy rope?  And then there's the issue of handleability...