 Ahhhh Thanks JB so now we are in to real calorific values rather than the percieved energy that a warm liquid provides..... Well spotted John..
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 it would depend how much sugar you put in the coffee!
Well, yes, there's the dark Muscovado, but there's also the Glenmorangie ...
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 Breaking Trail. Why is it that my dog moves with the greatest of ease on top of the snow while I continualy break the crust and sink down to my knees. I always thought that it must be a four legged thing with weight spread out over a bigger area howver, I have made a few calculations and I am exerting a much lower lb/sq" than the dog At 14 stone (boots rucksack the lot) I am exerting 5lb per square inch on one leg .
The dog at 5st2lb while on two paws is exerting 6.3 lb per square inch. So how come he breaks the surface far less frequently than me?
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 What a good question - can't wait to see some informed answers. I'm between dogs at the moment so I haven't been able to check on this, but from my memory of a dog's foot I wonder whether it spreads out when weight comes on much more than a booted human foot does. Reiver, your calculations must have included a figure for the load bearing area of a paw. Did you calculate/measure that from a dog's foot without weight on it or from a footprint - and is there enough difference between the two to matter anyway?
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Measurements were taken from paw prints in the snow, and if anything the size exaggerated. My own thoughts were that a boot has such a definite square edge and is more likely to break the surface of the snow, where as a dogs paw is much softer and rounder and is possibly less likely to break the surface. Must try it in bare feet, or at least with some big thick socks on (no boots)
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My dog squeals if I stand on his foot. If he stands on mine I don't squeal. Assuming squealing is an indicator of pain, either I have a higher pain threshold than my dog, or he is a lot lighter on his paws than me.
It is worth noting that a dog's paw distributes weight fairly evenly across the paw pads. The foot falls differently to a human's, coming in toes first. When my foot strikes the ground, it does so heel first, concentrating the weight considerably more than a whole foot placement would do.
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Rather than resurrect the "Paramo - is it still recommended" thread where Nick Brown chimed in, I'll post here. Does Paramo really "pump"? From what we've been told, Paramo works by capillary depression. The cohesive forces between water molecules is greater than the adhesive forces between the water and fabric. So, most often, you get a situation like this (| is skin, < capillary, and 0 is water drop) |<0 The water drop will pull any water inside the capillary to itself because, again, the water molecules are more attracted to themselves than to the walls of the capillary. BUT, what happens if a drop of condensation forms inside the capillary, or a drop is forced into the capillary, and no drop on the outside is there to pull the water out? The drop inside the capillary will want to form into a sphere as best it can, but what causes it to move away from the body? It must have something to do with the radius of the capillary getting larger. But, the capillary isn't resisting the water, or pushing it away--in fact, it still has a weak attraction to the water, but the water is more attracted to itself. So what is causing the water drop to be pumped out?
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Bloody hell mate.... I mean I always thought the pump bit was just a metaphore - which it is. Surely heat differential needs to entered in to the equation somewhere?
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When my foot strikes the ground, it does so heel first When traversing across a snowfield where the surface is continually breaking allowing one leg to go knee deep in a joint jarring fashion, I tend to be gently placing my feet down sole first. Anyway, just bought some snow shoes on ebay I may get to try them out tonight. I misread the ad and thought it said "revolutionary design from Austria" On delivery I realize it say "Australia" What do Australians know about snow ???? could be a big waste of £30 ... They are Yowies if anyone has ever heard of them!
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Yowies are brilliant bits of kit. Much easier to use than standard snowshoes. If you don't like 'em, let me know.
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 > BUT, what happens if a drop of condensation forms inside the capillary, or a drop is forced into the capillary, and no drop on the outside is there to pull the water out?
Capillary action isn't between the water and itself; it's between the water and the walls of the capillary tube. So, if a drop of water appears in the (roughly coniform) capillary created by the pile of the pump liner, there will be a capillary force to push it towards the open (wider) end of the tube.
The force is the capillary depression, generated by the (erm... electrostatic, I think) reaction between the water and the proofed fibre.
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| Edited: 21/01/08 17:36 |
Captain, that was my initial guess as well--that capillary depression puts an opposite force on the water from capillary wall. This is not the case. In fact, hydrophobic materials still are attracted by weak forces to the water! This attraction, however, cannot overcome the cohesive forces of the water. Hydrophobic capillaries are really just neutral. I've come a little closer to an answer. From the equations showing the capillary pressure, everything is dependent upon radius. The force the body of water exerts against the atmosphere is more at the large end than it is in the small end. Because of this pressure differential, the water will move towards the larger end. But how does the radius change the pressure? You'd have to tell me. The opposite is true for wicking. In a conical capillary, the smaller radius will produce the greater force against the atmosphere, and the drop will move toward the smaller end of the capillary. This must be a fairly strong pressure difference, as it breaks the adhesive hydrogen bonds of the water to the capillary wall. But again, how the radius affects these, I'd like to know. How can a small radius produce more pressure in one situation, and a large radius produce more in the other? I'm guessing it has something to do with shape: in Paramo and capillary depression, the water forms convex meniscus, while in hydrophillic and wicking capillaries, the water forms concave meniscus. We need the Paramo gurus to answer this one I think.
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 Pete, The shape of the meniscus is relatively easy to explain. Left to free-fall (in the absence of drag) a water droplet will become spherical as the surface tension acts on the fluid. The sphere is the least-energy configuration. Add in a contact surface and you have an additional force. If the capillary surface is attractive to the water molecules, this is a little like a pulling outwards on the edges of a sheet - the surface formed is concave, somewhat like a catenary in two dimensions. Conversely, if the surface repels water, then the lowest energy configuration will have least contact between water and capillary. This is a bit like 'squeezing' the water from all directions and results in the convex shape you describe. At the capillary size level, these forces are more significant than gravity that acts on the bulk of a fluid. It is for this reason that a capillary will fill with liquid when stood vertically in water. More info here: http://en.wikipedia.org/wiki/Capillary_action It is therefore the relation between the forces (attractive or repulsive) of contact between the water and the fabric that determine what happens to it.
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Yowies are brilliant bits of kit Hope you are right, I,m about to head off into the Rannoch area for a moonlit walk, doubt i will get to try the Yowies as there has been no new snow for a while and I guess the tops will be reasonable neve.
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> From the equations showing the capillary pressure, everything is dependent upon radius.
You've missed the most important bit, and that's the contact angle. A positive contact angle produces a wicking fabric (and a positive meniscus in a conventional parallel-sided tube) and a negative contact angle produces an 'anti-wicking' fabric (and a negative meniscus).
TxDirect cause the contact angle to be negative, thus forcing water towards the lower energy state (as you say, the larger radius of capillary).
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I think I haven't been clear. I understand the contact angles. What I don't understand is how the changing radius creates changing pressure. When I talk about pressure, I do not mean between the walls and the water; I mean the pressure difference between the inside of the curved meniscus and just outside. It is this pressure difference that drives capillary forces. No matter what the contact angle, there will never be repulsive forces, or anything like"squeezing." Hydrophobic fabrics still attract water, but with very weak forces. I drew a picture of hydrophobic and hydrophillic capillaries and the net forces on both as red arrows: http://img177.imageshack.us/my.php?image=conicalcapillariesmn2.png On both images, the red arrow at the smaller end is pushing harder than the red arrow at the larger end. The pressure is directly related to surface tension, and inversely related to radius, so something like P = T/R. Obviously, the smaller the radius, the greater the pressure. I'm trying to figure out why radius has this affect.
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 Deleted. I'll try to explain it better later, mabe.
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| Edited: 22/01/08 10:10 |
Alright here's a new one that I've been wondering about: What's the difference between brushed polyester and fleece?
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 thinking about the diabetic wild camp thread i was wondering if the eating regime hedley has worked out for himself is actually a general better strategy for eating whilst out?
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In this thread http://www.outdoorsmagic.com/forum/forummessages/mps/dt/1/UTN/21950/V/8/SP/ the science of heat exchange masks came up, as is usually the way, one thing led to another with Parky asking these questions: perhaps we need threads along the lines of "outdoor wisdom - caffeine - fact or blox; or does it matter" "outdoor wisdom - hydrostatic head - fact or blox; or does it matter" I reckon this would be the place to expand on those ideas if anyone has any comments of additional 'outdoors dogma' challenges....  John
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