Author Topic: Talking Thermodynamics  (Read 194630 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #960 on: June 11, 2018, 02:09:54 PM »
Hi MJM. yes that is the engine that i based mine on ..and a put these wheels on it so it could be moved about on a table to also add interest for a small boy....Tubal Canes model used Meccanno gears but mine were cut out of bought gears. Some of the pictures on google show this lubricator  with a solid cap and others with some sort of screw valve arrangement on the top ?? I haves many books that i forget which ones are the relevent tomes !!
willy
PS...I had another smoothie today that was ice cold...i asked them to put some very hot water in it to warm it up !! but the hot water just stayed on top and would not mix with it ??!!!
« Last Edit: June 11, 2018, 11:42:50 PM by steam guy willy »

Offline MJM460

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Re: Talking Thermodynamics
« Reply #961 on: June 12, 2018, 12:57:59 PM »
Hi Willy, the similarity between your crane and the one in the book looked like more than coincidence.  The advantage of a small library is that I did not have to look very far to find it.  You are way ahead of me, in that you actually made the model.  I bought the book many years ago, before I bought the lathe, but never got back to building it.  I must admit that I found the number of tiny holes requiring very accurate placement had me a bit intimidated.  Perhaps I should have another look at it.  Using gears other than Meccano, is mentioned in the instructions, as is varying the jib construction.  It looks like you freelanced the boiler firing as well, is that the terminal box and controller for the electric elements?

I wonder if the intermittent operation of the crane is a factor in how that lubricator works.

Hot water has lower density than cold, especially as the smoothie is thickened up with whatever they put in them, which possibly further increased the density, so in principal the hot water could tend to float on top.  The viscosity of the smoothie would also tend to inhibit mixing, which might explain the difference between water in the smoothie and milk added to a cup of tea or coffee.     Do you know what ingredients the smoothie contains?  So while it is not intuitive that the two would not mix, I would assume the heat still had the desired effect by conduction.  Was the hot water poured gently on top?  That would not encourage mixing, but I assume it would mix if stirred with a spoon.  I think we need pictures!  (I don't think I will risk spoiling my iced coffee by adding hot water to see of it does the same.)

The next step in the boiler testing requires insulation to be added to the outside, under the planks, to reduce heat loss.  That will take me a few days at the speed I progress.  To many household projects and social engagements still taking too much time to allow any continuity.  So I will continue to check in each day (too many wonderful threads that I don't want to miss) and will continue the discussion on any questions that are posted, they are always very welcome.  However, I will take a break from trying to add something new until that boiler insulation is complete.

Thanks for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #962 on: June 12, 2018, 02:40:20 PM »
Hi MJM,
<a href="https://www.youtube.com/watch?v=2EkjRslFgSM" target="_blank">http://www.youtube.com/watch?v=2EkjRslFgSM</a>

here is a video of the steam crane at work...witha dissapearig trick at the end. The electrics in this engine were analogue  relays and things rather than modern solid state components !!

Offline MJM460

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Re: Talking Thermodynamics
« Reply #963 on: June 15, 2018, 01:07:00 PM »
Hi Willy, thanks for posting that video, wonderful to see it actually running.

Now the boy has grown into a young man, you might be able to do some real engineering tests on the crane with him.

First, how much can it lift at different pressures?  A small bucket with water makes an easily adjusted and calibrated load.  Might need temporary counterweights clamped on so the crane does not overturn.  You will both soon see why building cranes have those big concrete weights.

Second, how does the power vary with pressure?  And how much power is achieved at maximum pressure?  Power is simply calculated if you time how long it takes to lift a known weight through a given distance.  Work is force times distance, power is rate of doing work, or work per unit time, so power is force times distance divided by time.  Might need to lengthen the rope and place the crane near the edge of the table and lift from the floor, so you lift the weight perhaps 0.6 or 0.8 meters for a more practical time period.  Use the stop watch function on the iPad to time the lift, screen time will be an added attraction.

Might need to max out a bit below the safety valve setting.  By the way, do you need to lift it to make sure it is not stuck?  I was getting a bit worried that it did not lift at full pressure gauge reading.

Nothing wrong with those relays and stuff, especially with a heating element as it is a simple resistive load.  And always useful to actually hearing it switch on and off.

Thanks for looking in,

MJM460

The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #964 on: June 16, 2018, 12:55:24 AM »
Hi MJM, glad you liked it and it was posted quite a while ago when the technology was a bit primitive  !! Also i am glad that i don't have the monopoly on the questions :mischief: :mischief: Yes relays are cool except when in 1964 my trade tester in electronics decided to slip a piece of paper under the contacts !!! it sounded ok with the unmistakable click ...but...nothing happened  !! I have not run this engine for sometime so not sure if it is still ok, also there is not much room on the spindle for any length of 'rope', Still very busy in the allotment at the mo...Ok a question If you have a cubic foot box full of ice level with the top how full will it be when it has all melted ?

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #965 on: June 16, 2018, 03:40:59 AM »
Willie....

This...."a cubic foot box full of ice level with the top how full will it be when it has all melted ?" ...is a trick question :shrug:

If the level of the ice is at the top of the box, it [the water]  has already expanded & grown in height level to the top of the box ...so when it melts...the water level must be the lower ...however if the box was full of water, then frozen.....the level of the ice rises above the top of the box & so when it melts the water level assumes the original height in the box

[from 1962 Science at Corrimal High School  :stickpoke:]

Derek
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Derek L Warner - Honorary Secretary [Retired]
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Offline MJM460

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Re: Talking Thermodynamics
« Reply #966 on: June 16, 2018, 01:42:39 PM »
Hi Willy, I reckon that crane will still work, or is at least well worth restoring.  But not while you have suitable weather for the allotment.  Rumour has it that you people never have to wait long for a rainy day, so there will be opportunities.  There is not much on the tele these days.  Remove, inspect and do some electrical insulation tests on the heaters and control system, or replace them.  A pressure test on the boiler, ease the safety valve by hand to ensure it is not stuck before you test its lift pressure and adjust if necessary.  It really should lift within the range of the pressure gauge.  Then some work on the engine and gears to get them all moving, touch up the paint and it will be in going order again.  Not much, of you say it quick.  So long as the boiler was originally silver soldered.  I think I would probably not be so keen of it was soft soldered.  Even if it then only joins others on the shelf, it should surely do so as a fully working model.

Regarding the ice question, I see Derek has already answered that one.  But keep the questions coming, they are always interesting and most welcome.

Hi Derek, good to hear from you again.  Thanks for providing an answer to that one.  Always encouraging to have contributions from others.

I wonder of you grew up listening as I did, to Professor Julius Sumner Miller.  I used to set my alarm early so I could listen to him on the radio before I got up in the morning, with his trademark question, "Why is it so?"  Perhaps it is his influence that is responsible for my somewhat long winded answers.  Often the "why" is so much more interesting than the plain facts of the answer. 

Anyone who has ever put an ice block in a drink, or seen ice flows on a river in winter knows that ice floats.  And you don't have to visit Antarctic waters to know that most of the ice block remains below the water surface, only the "tip of the iceberg" is visible above the surrounding water.  Ice floats because it's density is less that that of water at the same temperature.  In other words, water expands on freezing to occupy more volume than the liquid phase from which it forms.

Now we have to turn to the history books to find that a character named Archimedes is credited with discovering that a floating object displaces its weight in water exactly, no more and no less.  Supposedly, he was in the bath, and cried "Eureka, when the principal occurred to him.  If you push the object down, so that it is below the water surface, it displaces more water, so there is an upward force opposing that downward push, and the object will return to the initial floating position as soon as you stop pushing.  Alternatively, if you lift it a bit, the object will no longer displace its weight, and as soon as you stop lifting, the object drops back to its floating position.  This is the reason a boat lifts at every wave to stay afloat, rather than be swamped by each passing wave.  It is quite dramatic to be sitting in a small boat, watching big waves approach you from behind, and always, just in time, the stern rises and the boat is carried safely up and over the wave, only to descend into the trough before the next one, particularly in the dark!  Of course, the dynamic forces involved when waves break in a fierce storm are another matter.  It is also the reason a boat floats upright, despite the centre of gravity being above the water level and centre of buoyancy, but that is a different question.

So it is because the density of ice is lower than that of water, meaning the volume of the ice is more than the volume of liquid formed when it melts, that the liquid level behaves just as you described.

I have been fitting two layers of 3 mm thick cork to the centre-flue boiler to try and reduce the heat loss to the outside.  It seems a pity to have achieved the necessary heat transfer to the water, then to let it be lost through the thin layer of wood strip cladding I had originally fitted.  I actually thought it might have been enough, but my infrared thermometer was indicating an outside surface temperature of the timber of over 95 degrees.  Apart from the heat loss, this is hot enough to burn your skin if you accidentally touch it, so from the safety point of view, more insulation is needed. 

The conduction equation tells us that with the same temperature difference between the boiler and the surrounding air, thicker insulation results in less heat transfer.  When the conduction equation is used in conjunction with the convection equations for heat transfer between the timber surface and the air, we find that the timber surface will have a lower temperature.  So a lower outside surface temperature, and less heat loss.  I hope to be able to demonstrate just how much difference it makes with more test runs when I get the insulation refitted.  Two layers gives 6 mm on the radius, or 12 mm on diameter, and Pi times 12, or 38 mm on the circumference.  I will need about four extra wood strips to complete the cladding over the cork.

Not such a big job, just many interruptions.  Had the whole family to dinner this evening, two of us, our three children with their spouses, and all seven grand children.  It will be some years before we can do this again, as our daughter and her family are returning north to Darwin in about a weeks time.  Nearly 4000 km from here, so don't get to visit often, let alone have everyone together.  A wonderful occasion.

Thanks to all for looking in,

MJM460


The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #967 on: June 17, 2018, 01:56:09 AM »
Hi MJM , actually the question about the ice was to ask what the measurement of the reduction in the level of the water once the ice had melted was.? Could this be worked out with a particular formula ? measuring it with a ruler might be a bit difficult as the meniscus would have to be taken into account ! Also why does the volume increase when water freezes and what does the increase consist of ?.......Thanks....

Willy

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #968 on: June 17, 2018, 02:47:50 AM »
I seem to remember Julius Sumner Miller  :headscratch: on ABC television every Thursday night @ 7:30 PM...maybe 1st or 2nd year @ highschool so....1962/1963?

In those days...there was only 7, 9 & ABC2.....I was allowed to watch Sumner Miller ...but then sent to my room as The Mavis Bramston Show was on 7

Obviously my parents thought it was in appropriate viewing for me   :lolb:

Who was the other American Uni Lecturer on TV?......always had a cigar butt in his mouth & was a mad keen water speedboat fanatic

edit......had an American type Crewcut....first name Don......?

 
« Last Edit: June 17, 2018, 04:49:10 AM by derekwarner_decoy »
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www.ils.org.au

Offline MJM460

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Re: Talking Thermodynamics
« Reply #969 on: June 17, 2018, 01:16:59 PM »
Hi Willy, those are two very good questions.  So not intended as a trick question after all.  Water is one of very few substances which expands as it freezes.  I believe there is one other, some sort of metal alloy, I think used in type setting, though I don't know it's composition.  Most substances contract on freezing.  This is because the molecules slow down as they cool, and the attractive forces between the molecules hold them closer together until they are only vibrating through a very small amplitude, and are effectively quite closely packed in a regular array, rather like marbles in a tin.  Some molecules collapse into a cubic array, some a cubic array but one in the middle of each cube, called body centred cubic, some with with one in the middle of each face of the cube, called face centred cubic.   I hope the names make sense.  The molecules are still moving randomly, but within the confines of those molecular forces, so they stay in a more or less fixed position in that regular array.  And the total volume is smaller than when the molecules are moving further and more freely in liquid phase.

But water freezes into a hexagonal matrix, not flat but three dimensional, but basically the same three axes of symmetry, and the basis for the hexagonal snow flakes.  What's more, instead of fitting really close together, those hexagons each have an empty space in the middle, so the lattice has a lot of empty volume.  Part of it is due to the molecular structure, each molecule having one oxygen atom and two hydrogens.  And the two hydrogens are at approximately 105 degrees apart, but other molecules do not end up sitting to fill that v between the hydrogens.   It is not clear to me just why it does this, that is the field of molecular physics.   It is easy to see how a slightly different arrangement of the same atoms could more completely fill the volume, so the whole space would be more fully occupied.  But it is that empty volume in the middle of each hexagon that is the reason for the extra volume.  It is made up of empty space.

When the ice melts, the molecules increase their movement, and in the process, the atoms do encroach into that space which is empty in the ice matrix, so do a better job of filling all the space, so the volume of the whole contracts.  Very mysterious.

Now, as to how much it expands on freezing, the thermodynamics book I am using at the moment, as well as having property tables for the liquid and vapour regions of the water phase diagram, also has the table for the solid-vapour region of the diagram.  This table has a column for the specific volume of the solid, equivalent to the specific volume of liquid column in the steam tables.

From this we can use the accurate table model for the behaviour of ice.  At the triple point, the only point where you can have liquid vapour and solid in equilibrium, 0.01 deg C, and 0.6113 pPa, the specific volume of ice is 0.0010908 m^3/kg, or a density of 1090.8 kg/m^3.  This compares with 0.001000 for liquid water at those conditions, or a density of 1000 kg/m^3.    (The figure is so exact because that is the definition of the mass of 1 kg.). So water expands by 9.08% when it freezes.

Once frozen, ice then behaves more normally in that it contracts on further cooling   So at -10 C the specific volume is 0.0010891, and at -40 C, the end of the table, it is 0.0010841 m^3/kg.  At least that will cover the temperature range experienced by most forum members, though I am not sure if perhaps Admiral Dk experiences lower temperatures in winter.  And of course any forum members wintering over in northern Canada, Siberia or the Antarctic.

Hi Derek, I must have seen Julius Sumner Miller on the Tele, but I was actually remembering him from radio, again ABC, he had a five minute segment each morning, or was it weekly?  Come to think of it, while TV was introduced in this country in 1956 for the Melbourne Olympics, my family did not have one for five years or so after that.  But late fifties or early sixties.  I don't remember the guy with the cigar, but there have been and still are a few of those characters that present science in an interesting way to the youngsters.  Pity book shops don't follow through with anything just a little more advanced.  They seem to think the budding scientists progress in one invisible step from primary school "safe" experiments to astrophysics, and don't need books in between!  Better stop griping before I incur the wrath.  So our Mavis was suitable for your parents, but not for you!

Thanks to all for looking in,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #970 on: June 18, 2018, 01:05:42 AM »
Hi MJM, wow that is a pretty comprehensive reply...Thanks...Liquid Vapour !!! that is something to think about!!... So 9.08 % presumably that is a volumetric measurement  so in a container 12" cube how high is the top of the water ?  It is a bit late here and my brain has gone to sleep.. is it about 11" ?  waiting for some rain so i can get in the WKSP.... I was looking in a large tome on Thermodynamics and in the front was a dedication    "to my wife" !! so lucky lady ;D

Willy,

Offline MJM460

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Re: Talking Thermodynamics
« Reply #971 on: June 18, 2018, 02:19:09 PM »
Hi Willy, thanks.  It was another double banger question, but pleased to be able to answer it.  That description of the physical structure if ice and what happens when liquid freezes is well covered in Rochard Feynmann's "Six Easy Pieces".  It is quite easy to read despite him being a Nobel Laureate in Physics.  Apparently he was known to do so much clowning around that people didn't know whether or not to treat him seriously.  Then he won the prize.  Brilliant.

If that ice is in a cube box, and to avoid any tricky implications, assume it has been shaved off level with the top of the cube, then when it melts the height would reduce to about 0.916 times the height of the cube, (1/1.091 times the height) as the sides of the cube are assumed to be unchanged. 

If, however,my out could somehow contain the water with some sort of elastic membrane so it remained a cube, then the length of each side increases by 1.03 times its original length as the liquid freezes.  You can easily check that 1.03 cu ed equals 1.09.

Hmmm, liquid vapour, or solid vapour!  Not very precise terminology for the mischievous mind.  The solid to vapour phase change is called sublimation.  It was taught along with some obscure chemical example when I was at school, an example I have long since forgotten.  But the process really came home to me when I was living in Toronto, when I noticed that while usually the blanket of snow over the city would melt to streams of running water and plenty of mud, sometimes, it just dissapear end overnight, with the roads and pavement stayed quite dry.  Seemed to be not enough heat in the air to explain in terms of melting the ice and then evaporating the water, the ice moved straight from ice to vapour. 

However, properties do not depend on the process, only the end status, so the same amount of energy has to be involved, whether the ice melts to liquid then evaporates, or just sublimes directly to vapour.  And sure enough, the solid-vapour tables show the enthalpy for the change from saturated ice to saturated vapour is about 2800 KJ/kg, so the explanation lies elsewhere.  Still trying to get my head around it, but basically if there is a low enough vapour pressure of water on the air, then as the temperature rises, water changes directly from solid to vapour.  I remember that the air was cold, but can't remember now whether we got a warm breeze that supplied the heat or just what happened. It is quite a long time ago now.  Perhaps someone from northern climates where this process can be observed will come in and help with more explanation.  We don't get much snow in Melbourne, so no opportunities to check the observations.

Hope the garden gets the necessary rain soon for you,

Thanks for following along,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #972 on: June 21, 2018, 01:15:28 AM »
Hi MJM, Just "four easy questions" !!.....when ice is very cold is it less slippery ?? and if you have a quantity of water with a very narrow top opening, does the water evaporate slower than with a very large opening ??  is this useful in various applications ?? and why are questions so easy ??.....Thanks

Willy.....

Offline MJM460

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Re: Talking Thermodynamics
« Reply #973 on: June 21, 2018, 12:57:21 PM »
Hi Willy, good to have you back asking questions again.  Great questions and a fun excuse sometimes just to remind myself of stuff.  Other times, to look up and clarify things I know I should know, but put on the spot, I like to do a bit of checking.  Unfortunately as a student, and usually also at work, you don't have the time to look at things in so much detail, so it is quite rewarding to do it now.  Of course, my work probably involved more of this than for most, but that is where we each collect and contribute different knowledge and experience.

Ice is usually slippery because it is not really very cold, and a little heat from our hand, or from friction is enough to melt some surfaces dot make a thin film of water (say under an ice skate blade or toboggan skid, or snow skis,).  The ice then slides on that film of water which produces minimum friction.  It is called hydrodynamic lubrication, or thin film lubrication.  Of course, if you stop, the thin film quickly freezes, and your sled, or finger becomes stuck to the ice.  If the ice is much colder, the heat from your hand is not enough to melt any ice, and your skin has enough moisture to quickly freeze to the ice.  Not a good idea to test this!  We lived in Canada for a bit over three years and we had to teach the kids not to put their tongues on cold objects.  I feel a bit out of place talking about what winter is like in Canada, but we had a very steep learning curve for some of these things, that most Canadians probably take for granted.  But Toronto seemed to have a lot of negative temperatures when we were there.  But a dry metal object does not slide so well either if the contact pressure and friction are not enough to warm up and melt a thin film of water.  In really cold weather, you have to "kick" your skis forward to start them sliding, and preferably have a film of wax to stop them sticking to the snow.

Evaporation, yes, you are quite right, if your container has a small surface area, it slows evaporation, while a large surface area facilitates evaporation.  Evaporation takes place at the surface, unless you mean in a boiler with a really strong heat input, and continues until the water vapour pressure close above the surface is equal to the equilibrium pressure for that temperature, then it slows down or even stops.  The vapour is easily confined near the surface in vessels with a narrow neck, filled so the only surface is the area of that neck, so evaporation is slow.  On the other hand if you have a very large surface area, not only do you get more evaporation over that large area, but if there is any breeze, it easily sweeps the vapour away, so lowering the vapour pressure, and more has to evaporate to replace it to restore that equilibrium vapour pressure, so evaporation is greatly accelerated.  In a bottle only partly filled, so it has a larger surface area, it is not so obvious, as the vapour is well confined near the surface rather than being  easily carried away by any air currents.

Use of the large area to increase evaporation is useful if you want to use solar radiation to remove the water from salt for salt production, or from brown coal, which has to be dried before burning it results in much excess heat.  Or other similar drying processes.  In fact, if you do your laundry in a washing machine, or a tub, wring it out by hand then leave it all piled up in a basket, it will usually go mouldy before it ever drys.  However, if you hang it on an outside line, or drape it over the furniture so the biggest possible area is exposed to the air, or even tumble it in a tumble drier, the greater surface area exposed to air results in much quicker drying.  Unless of course, the room is closed off, so the humidity rises to the point where evaporation stops.  It is necessary to have a well ventilated area, not just heat alone.

On the other hand, in dryer parts of this country, farmers put floating blankets of plastic balls on the surface of dams (they have to have fences to keep the cows out, and pump the water into a trough for the stock), or people put floating blankets to cover most of the exposed surface of a swimming pool when it is not being used, to reduce evaporation.  It retains the heat in a pool as well, as the evaporation carries away all that latent heat.

So those are just a few examples of that principal being useful.

I think when ever people are both observant and curious, there will always be many opportunities to ask, as Julius Sumner Miller did, "Why is it so?"  And the joy of science studies is in finding the explanation for so many interesting things we see each day.  So yes, the questions come easily, and the answers are often not too difficult if the relevant science is understood.  Of course, sometimes the science is a bit more complex, so the answers don't come so easily.  The science behind most engineering problems is quite well understood these days, which is why we can create such wonderful structures and machines, we just have to include well taught science in schooling, along side the other subjects that are so well represented in bookshops.

And of course not all the science is easy.  Look at that 7.5 hp 30,000 rpm engine in a different thread on this forum, for example.  Even with high powered computers, computational fluid dynamics and so on, there is really complex science behind that and similar designs, and even then some brilliantly informed trial and error.  I have to admit that I would not know where to start, let alone have the skill to make it.  It's really inspiring stuff.

I hope that answers your questions adequately.

Thanks for following along,

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #974 on: June 22, 2018, 02:11:21 AM »
Hi MJM, thanks for the info....I have seen those covers on swimming pools but just thought they were to keep leaves off the water ..a bit like those wrong sort of leaves that fall on the railway lines here in Blighty !!... so regarding evaporation.. if the water is warming up but the evaporation is slowed down with a small opening is there some sort of complicated dissipation of energy doing something mysterious ??

Willy

 

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