Peltier Coolers

[Dave Liquorice]

Need a couple of chunky peltier coolers, one around 120 W another above 60 W. Recommended are 12709 and 12710, plenty of suppliers on Amazon/eBay but they all seem to be Far East based.

CPC etc have such devices but at £15 to £20 each, where as the Far Eastern ones are less than a fiver delivered. Can anyone recommend (or not) a Far Eastern supplier?

The lad wants to build a cloud chamber as a Christmas present for himself. Old ATX PSU for the peltiers, hefty CPU heatsink, sealable clear bowl, sponge soaked in isopropanol and Americium-241 from old smoke detector as a source. Peltiers are a bit more convenient than dry ice, though might not work quite as well.

[Gordon Henderson]

Oddly enough been looking at them myself, but I'd just go with the Farnell offerings as I'm too lazy to spent time looking at the imports...

Saw something on the box recently about this - A BBC article on weighing a cloud - apparently there are ultrasonic devices you can buy in garden centres that make water vapour out of cold water - designed for garden water features, etc.

Google for garden fog maker - eg

Gordon

[Martin Brown]

Chances are you are only going to use it a few times so I'd suggest buying in a block of dry cold after you have built the contraption. I used methylated spirits in mine way back. You need really good black velvet and a powerful compact light source.

It is actually quite hard to maintain large temperature differences on Peltiers and you will probably need the hot side heat sink fan cooled with forced air ventilation (or possibly even water cooled). Otherwise you will end up with the cold side -80C below the hot side which will end up about 50C or maybe even warmer.

I have long suspected although never actually tried it that you can almost get to cloud chamber conditions in the gap above a cup of freshly made coffee in a cold room.

The old design in Scientific American by C. L. Stong is pretty robust and relatively easy to get working. Beware of frostbite!

Book out of print CD version also available but no idea what it is like. (BEWARE that US edition includes some vivisection experiments that have been illegal in the UK for several decades)

I have a friend who built half the atom smasher - the Van de Graph.

[Andy Burns]

Vantablack would be the modern choice ;-)

[Dave Liquorice]

This is probably true but *IF* it works it should just work provided there is enough propanol in the chamber. It could be left running...

The blacking I've yet to really consider, depends on what chamber we can get. Powerful light sources are easy these days with 5 W LED tourches...

Air cooled, f'ing great CPU cooler, 120 mm fan, supposed to be able to handle 180 W. Heat pipes transfer the heat from the CPU contact area into the heatsink fins. Hum, they'll actually be upside down, so might no work as well as they might. ie the condensed heatpipe "stuff" won't be running back to the hot bit, hum....

We shall see... Needs to get down to around -25 C, so provided the heat sink doesn't get much above 40 C it should work.

Is that based on dry ice? I sort of looked at using dry ice but the places I could find only wanted to sell rather large quantities, 10 kg or more.

[Dave Liquorice]

Different sort of cloud. B-)

This is to show ionising radiation from a source (the Americium-241) or just the background "cosmic rays".

[jkn]

FWIW I have the 'Amateur Scientist' CD-ROM, bought a few years ago. IIRC it is a bit annoying in that it wants to run an application which you then use to navigate to the year, issue etc. that you are interested in. I think the sc ans are in TIFF or similar form as well. A little background project is going t o be to try and extract all of the 'meat' of the CD and create a series of PDFs, which is really what I wanted in the first place...

Jon N

[Andrew Gabriel]

I bought a couple of the 40mm square ones from a radio fair many years ago - not sure such things happen much anymore.

One possible source might be the small pelier fridges, which might appear in the sales after (or even just before) Christmas. (I was buying them to fix two such fridges 30 years ago.)

Note they come in sealed and not-sealed types. If you will be running one side below the dew point, you need sealed ones, or they will have a short life due to internal moisture and corrosion. (That's what had killed the ones in the fridges, although I managed to fix one of the peltiers afterwards by bypassing the first junction.)

[dennis]

The last cloud chamber I saw worked at room temp without cooling other than that produced by reducing the pressure with a hand operated pump (bike pump with washers reversed type).

[Martin Brown]

Sounds about right. 10kg block of dry ice will keep nicely for a week or so inside one of the thick polystyrene boxes with a bit of additional polystyrene around it (you don't want a gas tight seal). It will keep for a few days in the bag it is usually supplied in.

Cutting it up is very noisy (wear ear defenders noisy). Beware of cold metal that has been used to cut it up or crush it.

You might be able to cadge some for a nominal sum from your local university physics department if you explained what you want it for (as opposed to dodgy Jekyll&Hyde/Pangalactic gargleblaster drink additive).

It is annual Xmas lecture season and we were out supporting the RSC last night in Newcastle. Saw two new variants of old tricks by Nick Barker of Warwick University. One was elephants toothpaste with realistic pigment based stripes in it and the other was a large cloud made by dropping bulk crushed dry ice into a large conical flask 1/3 full of boiling water (stand well clear of the back draft).

[Dave Liquorice]

Americium-241)

I've certainly seen ones in museums etc that are just there working and didn't appear to have any cooling (ie not covered in frost)...

Maybe this is technology for technologies sake?

[Dave Liquorice]

They do but I doubt they have quite the range of "useful bits" that they did in the late 70's early 80's.

It would have to be a very cheap (not powerful enough?) fridge to cost less than CPC et al, and VERY cheap to beat the Far East. Also as I'm stacking them I need two different power ratings.

Yes, I've picked up on that, the Far East ones tend not to mention sealed or not, though pictures show solid edges not open. Could one get away with DIY sealing with a bead of silicone sealant?

[Dave Liquorice]

That long. Longevity in storeage wasn't really mentioned but making

2+2=5 I got the impression it sublimed fairly quickly as almost every where effectively made to order.

I wonder why that is? B-)

I don't know anyone in the local University Physics Departments.

Unless ...

[Andrew Gabriel]

I don't know if the acetic acid might damage the junctions or PCB tracks. It would be a question of finding a sealant which doesn't harm these during curing or after set.

[Martin Brown]

I think acetic acid would be decidedly bad for them or at least for the metallic interconnects.

Epoxy glue or acrylic car body filler should be OK provided you never want to take it apart again.

BTW if your two stage cooler has two different sized devices you will need to interpose a thin ~2mm aluminium plate between them. This trick is common in Peltier cooled cameras where a large device pumps the output of a smaller device attached to the back of the small CCD.

Not quite so relevant for a cloud chamber since you are trying to make a fairly large region very cold.

[Dave Liquorice]

mention

Non acid cure silicone ...

There is going to be considerable thermal cycling, hopefully not far short of 50 C (-25 to +20). I think something with a bit of "give" is in order.

I think they are both 40 mm square so will just be "stuck" together with a thinlayer of thermal compound.

Finding a suitable chamber is looking to be the hard bit. All the plastic food storeage containers I've looked at so far are milky not clear. There is a glass biscuit jar in Tesco for £2 that has flatish sides but is quite large and glass is relatively poor conductor of heat, so getting the inside cold enough might be tricky.

Oh and the low pressure devices seem to be a repeatable single shot sort of thing. That is you have a sealed chamber cotaining your saturated vapour then rapidly reduce the pressure this lowers the temp and super saturates the inside enabling the tracks become visible. For a while? Presumably it soon warms back up and you have to repeat the pressure drop? On the DIY version I found there is also a "clearing" 150 V DC charge involved. The Peltier thing I've looked at uses a poly cup or balloon. I might try as many series connected PP3's as I can find...

[Dave Liquorice]

TK Marx may have come up trumps with a "Secret de Gourmet Boite de Conservation Hermetique 0.4l" £2.99. Google/TK Marx site (nasty horrible javascript only thing) seems remarkably reluctant to produce an image or anything that looks similar. It's clear, round, flat bottomed(*) with a lid that seals with a lever action.

(*) Apart from some slightly raised markings for recyling (7 SAN, styrene acrylonitrile) and food safe logos etc. They won't last long under the influence of fine sand paper and block...

[Martin Brown]

Probably a better choice.

Epoxy can be made a little bit rubbery if you deliberately alter the proportions to I think be resin rich. It isn't as strong then but it does have a bit of give. Exeriment on scrap first.

The Japanese store Muji have some low profile water clear acrylic boxes that would probably be ideal for this. ISTR Mine was a glass petri dish.

I wouldn't worry about clearing it. The chances are you won't have a source strong enough to cause too many tracks. They appear and rain out over a period of 5-10s thick ones for alpha and thin ones for beta.