Ovenizing vs. compensation

On a sunny day (Thu, 04 Jun 2015 21:35:00 +1000) it happened Chris Jones wrote in :

In these day, where the modern TV needs longer to boot than the old tube TV, I think nobody cares about time to usability? I mean 15 minutes till your oven gives the sign; 'Well done' ? :-)

Woah... Awesome.. I totally owe you a diner if I ever get out to SF.

Yeah and a bodged together circuit. (But later..)

George H.

At high temperature diamond has the same heat capacity as everything else. (3 kT per atom... each degree of freedom can soak up 1/2 kT.) What it has is a high Debye temperature.. the temperature at which all it's atoms/ degree's of freedom/ lattice vibrations are activated.

George H.

Use a bunch of FET's. Measure body diode for temperature and then use it as a pulsed heater. It would take a lot of wires though.

George H.

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nsfer is remarkably fast. The vapour pressure of the vapour in the chamber is set by the coolest point, and the working fluid condenses there and wick s away to the hotter points.

at transfer through that would probably be the limiting factor, but the vap our is is usually condensing into a woven copper cloth wicking layer.

ine when I encouraged a vacuum distillation process with a hot air gun.

vapour flow a lot. At Iasys we used to test our heat pipes for rapid heat transfer at close to room temperature to make sure that the manufacturer ha d got all the air out before putting in the water - initially we were sendi ng back quite a few, but they got better.

differentials close to room temperature, but that was only true of badly ma de heat pipes.

That's interesting. I've never used a heat pipe. Why does a containment gas slow things down?

George H.

I built a number of XOs and things on heated aluminum slabs, pictured elsewhere, mounted on bigger PCBs, with a deep-drawn metal cover over each brick, to keep air flow off. I experimented with various forms of insulation inside the cover; styrafoam, fiberglass, stuff like that. The best insulation was no isulation. In a flat, confined space, there is little opportuity for convection and, lacking transport, air is a better thermal insulator than anything else except vacuum.

Well, I guess I could have pumped it full of radon.

Nice! Was your write-up aimed at internal use?

I've done that with quilted aluminum foil, annealed, anodized and then wrapped with a flex containing a whole bunch of heating resistors and thermistors. If you maintain the preload, you get pretty good thermal contact, and the whole thing is pretty fast--as quick as several tenths of a hertz. (This was for that downhole stabilized laser gizmo I've talked about before. The company ran out of money for other reasons before I got the miniaturized version finished, but the tabletop version worked great.)

It's speed that you really want for an oven. Mass is a reasonable substitute, in the same way that a two-ton optical table on air springs is a reasonable substitute for an active vibration isolation system.

Cheers

Phil Hobbs

Xenon is cheaper. (Wow, xenon is cheaper than _something_, anyway. Not to mention the other benefits of not using radon.)

At room temperature there are other gases that are usefully better than air while not costing like xenon. CO2 and argon come in at about 0.016 W/m/K (40% lower than air), and small HCFCs are about 60% better (0.01 W/m/K), which is competitive with krypton.

You do have to worry about radiative coupling if you don't use opaque insulation, which makes a big big difference if you aren't careful.

Cheers

Phil Hobbs

Cool.txt? Yes, but I should pretty it up in Word and check it and add a bunch of stuff and make it more public. Maybe one rainy weekend, if it ever rains.

I got the proto board that has various resistors with various heatsinking. I need to measure that and add it in.

I could use our IR imager, or use thinfilm RTDs as the heaters+sensors.

A hi-res thermal image of surfmount resistors would be interesting, especially the thermal effect of laser trim cuts.

This. Yes, to be perfectly specific, low room temperature heat capacity.

Tim

It might be RoHS when shipped, but not by the time the customer gets it. :)

SF6 comes to mind, or is that worse due to extra degrees of freedom?

Tim

Hmm I was going to suggest SF6, but it's not as good as one might expect.

I wonder why that is? It's density is higher than Kr.

George H.

What a weird figure-of-merit that turns out to be. It's called "specific heat" and it's quite non-intuitive.

Nylon beats aluminum by a huge factor.

Roughly speaking, the denser the material is, the less heat it stores per kilogram. I guess there's something fundamental there that I don't recall (or never knew)..

Wiki says "the heat capacity at room temperature approaches 3R = 24.94

heat capacity.

Volumetric heat capacity behaves more intuitively with dense metals near the the top of the solids, but still water beats copper.

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ransfer is remarkably fast. The vapour pressure of the vapour in the chambe r is set by the coolest point, and the working fluid condenses there and wi cks away to the hotter points.

heat transfer through that would probably be the limiting factor, but the v apour is is usually condensing into a woven copper cloth wicking layer.

line when I encouraged a vacuum distillation process with a hot air gun.

he vapour flow a lot. At Iasys we used to test our heat pipes for rapid hea t transfer at close to room temperature to make sure that the manufacturer had got all the air out before putting in the water - initially we were sen ding back quite a few, but they got better.

t differentials close to room temperature, but that was only true of badly made heat pipes.

I'm guessing that anything that doesn't take part in the vapor/liquid proce ss just gets in the way with out doing any work

-Lasse

Basically it's how many atoms there are in a block of the stuff. Which, since all atoms happen to be the same size[1], is basically independent of material. So you win by going light.

Beryllium places at 1827 J/kg/K. While placing higher than aluminum (and nylon), it still falls short compared to its density advantage, probably due to its low atomic weight and packing. It's no diamond, but at 16.4, the molar heat capacity is considerably lower than 24.94. Boron is also light and hard, and even lower at 11.1 (diamond is a paltry 6.15).

Water is anomalously much more active (in terms of both specific and latent heat) than pretty much all others, apparently because of its abundant hydrogen bonding.

Tim

IIRC it's a bit disappointing due to the low frequency of the vibrational modes. (Hydrogen is less massive than fluorine, so the mass-spring resonances are lower by the square root of the ratio.)

Cheers

Phil Hobbs

That's not a very technical description. I don't remember all the details of Physical Chemistry, however I do recall that partial pressures do not significantly interact. But if you have air inside with a pressure of 1 atm, for the liquid to vaporize the internal pressure would be 1+ atm. The higher the pressure the less vaporization achieved and so a slower transfer. But I won't bet any money this is a better description of what is going on than what Lasse said.

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transfer is remarkably fast. The vapour pressure of the vapour in the cham ber is set by the coolest point, and the working fluid condenses there and wicks away to the hotter points.

, heat transfer through that would probably be the limiting factor, but the vapour is is usually condensing into a woven copper cloth wicking layer.

um line when I encouraged a vacuum distillation process with a hot air gun.

the vapour flow a lot. At Iasys we used to test our heat pipes for rapid h eat transfer at close to room temperature to make sure that the manufacture r had got all the air out before putting in the water - initially we were s ending back quite a few, but they got better.

eat differentials close to room temperature, but that was only true of badl y made heat pipes.

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Yeah, like buffer gas in an optical atomic cell, you'd think that would only have an effect where the two pressures were about equal, which sound like a lot of containment gas. (I don't know the vapor pressure of water.)

George H.