Hi everyone,

I'm building a small environment chamber for testing of a small PCB over a temperature range. I'm hoping for a -20C to 100C range and to be able to sweep this range over a 24 hour period.

I bought two 75W peltiers from digikey that ideally create a 69C differential and a water cooling kit for a PC. I want to run these in parallel in order to increase the cooling/heating power of the unit. I tested a single module in open air and was able to pull -25C to 100C from ambient (24C). I'm planning on using mineral oil in the chamber to keep the heat even across the PCB and to couple it well with the peltiers, which are dipped in a small aluminum heatsink.

I'm trying to figure out the enclosure design. The inside area needs to be 6"x6"x3". I bought some styrofoam containers, but I don't really like them from a structural point of view. I read that polypropylene and teflon have very low thermal conductivity, which seem like the way to go. Should I wrap the interior of the chamber with aluminum foil? Can someone recommend a way to do this or have any advice?

I already did all of the electronics with a simple 16A h-bridge with PWM interfaced with the computer, so I am not worried about this part of it.

Steve

 

Hi Steve,
Funny, I've been thinking about doing something very much like that for several years. Which Peltiers did you order?

Down at a big orange hardware store (a big hardware/building supply chain in the States) they sell 4'x8' slabs of 2" thick styrofoam insulation. I've used the stuff for insulating my garage door so I could use A/C out there in the summer.

It's actually fairly strong stuff. I think that for an environmental chamber, I'd make it two or three layers thick. The thicker your insulation, the better your thermal isolation.

One item for consideration is the ESD problem. Styrofoam is zap-happy. Covering all surfaces with aluminum foil initially sounds attractive, but I feel that a resistive covering would be better, like a closed-cell conductive foam.

Another would be routing signal and power cables. Copper conducts heat as well as electricity. You might think about using thinner-gauge cables with terminations on the inside and outside of the enclosure to minimize heat exchanges that way. Here in the States, you can get a low-expansion foam in a can that's designed to seal voids around windows and doors; that could be handy for sealing up seams & the like.

 

I found this one at Tanner's. I'm thinking seriously about putting it inside my computer case to cool things down, and duct the hot air out side with a slot fan. It's rated at 12V at 4A. BTW, http://www.physorg.com/ had an article several months ago about a minor breakthrough in TECs (another name for the same device), where they increased the thermal isolation while increasing the electrical conductivity, the end effect would be to decrease the heat generated internally and isolate the temperature differentials, which would increase the efficiency of it substantially.

 

I have a little experience with Styrofoam and expanded polypropylene (EPP) for construction. Styrofoam's only advantages are that it can be attached with various adhesives and is cheap. Aside from that Styrofoam dents and some versions can be quite friable. It has low chemical resistance... even beer dissolves Styrofoam in cups.

It is tough to make things stick to EPP and/or polyethylene(PE). Pressure sensitive tapes work well enough for model airplanes. I have also used heat from a hot-air blower, sort of like welding and Pliobond. A low-temp hot melt should also work. It is quite chemically inert. One of its best properties is that it is available in an anti-static (pink) form. (I am not 100% sure EPP is available anti-static; PE certainly is.) We used to get laboratory equipment packed in a rigid anti-static foam all of the time, because it doesn't crumble.

McMaster-Carr has several other foams available and a table of properties. Try: http://www.mcmaster.com/ and search on foam or EPP foam. Read "about foam."

Good luck. John

 

Thanks for all of your replies! This has been a bit of a head-scratcher, so your valuable input is welcomed.

Wookie,

I ordered from Digikey, not sure of the part number, but they're the 4cm square by 3.8mm tall. I'm considering getting the higher density pink foam or some thick polypropylene to make the chamber. One thing about the polypropylene that I am worried about is the thermal mass of plastic. I don't really understand the correlation between thermal mass and conductivity. It seems that the plastic has a great deal of thermal resistance, but wouldn't it take some time to bring the surface temperature up and down?

For now, I'm thinking of using the foam insulation, then wrapping with aluminum foil. It will help with any sort of radiation-mode thermal coupling. So, it will keep it out on the outside and in on the inside. For the feedthroughs, I was thinking the same thing. We call this sort of thing "Great Stuff" in Canada.. I need to run three cables though, which is definitely a concern... One 37-pin, another 50-pin flat, and the last a 6-conductor 22awg stranded cable.

Bill,

Nice find! I didn't know that this technology was still developing... You should watch out if you use peltiers, since they can generate frost and can cause a lot of condensation buildup. People have had huge failures in their PCs.. I recommend water cooling, since it is proven and works really well. Peltiers are only really useful when trying to get something below ambient. The cooling power that I am getting with the el-cheapo water cooling system is remarkable.

John,

Thanks a lot! I am going to have to check into what you suggest for the best option. I really don't like how flimsy the styrofoam products are, but they're really cheap and will work well at insulating. I have read that polyethylene is 'weldable' via solvent, but, for added strength, a hot glue like technique is used for reinforcement. I don't have any special equipment, so I was hoping for a simple adhesive I could get locally.

I will let you guys know how things go. I was looking for any red flags or concerns, but it looks like I am good to go for now

Steve

 

Bill,
My local techno-junkie shop, SkyCraft Surplus, has those same Peltiers for around $29/each. What did they run at Tanners? Apparently, they came out of portable refridgerators.

Steve,
All I can think of is that the lower the density of the material with a high closed-cell count, the less energy it will take to heat or cool the surface. Yes, some aluminum foil probably wouldn't hurt to block IR radiation. You might want to put shiny-side in on the inside, and shiny-side out on the outside. Connect the two layers electrically with some 220K or so resistors for ESD protection.

For your heat sinks, consider using copper instead of aluminum. They're nearly twice as good at conducting heat.

 

Actually they came out of Colman coolers (same thing), ice chests with a power cord. I paid $20. I'm not sure, but I think BG Micro still has them, Tanners and BG go together to buy stock, since they don't directly compete (same stock, different methods of outlet).

I had thought about the condensation issue. Basically the unit will be on the bottom of the case, using the case fans to circulate air over the cooling side (top). The bottom will be the hot side, and will suck air out of the case, which will also evaporate the moisture. Besides rust, there shouldn't be any issues. I'll use aluminum metal duct tape and styrofoam to insulate the hot side a bit.

My CPU runs about 60 C, which is a bit high, but normal for high speed Athlon XP types. I figure if I can get the case temperature down it will do nothing but good things.