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Sunday, February 27, 2011

DIY Thermal Test Chamber (low budget)

This post is sort of a update on something 'quick and dirty' that I built for my day job. We needed to have a thermal chamber that we could flow gas through at an elevated temperature with the chamber holding samples of various materials. The cost of this project was to be kept to a minimum and there was no time during the work day to build it so I did it at home. Generally I don't like to work 'offline' on my own time but this seemed like a fun project so I built it. I'm putting this up online as reference only and I don't recommend that you build one unless you know what you are doing! (not to imply that I do).
There are a few things to consider after realizing that this isn't a big budget project:
  • How to make a gas tight enclosed chamber that is thermally robust and chemically inert?
  • How to heat that chamber to a predetermined temperature and keep it there?
  • How can you see what is going on inside the chamber without opening it up and letting the gas out?

D.I.Y. Thermal Test Chamber
Fortunately we had several types and sizes of stainless steel Sanitary Fittings laying around and a temperature controller isn't that expensive (but it did consume the entire budget!). Sanitary Fittings are used mostly in the food industry as pipes to flow potable liquids (like milk, beer, water etc..) because they don't connect with threads and are easy to keep clean. They have flat flanged face seals that are held together with clamps so you can easily take them apart and clean them.

You can see in the above and below pictures (click on them for a better view!) a 4" stainless steel Tee fitting with all three ends capped off. I believe that this is a Alfa Laval fitting Tri-Clover part and the caps are sealed with Teflon Gaskets that you can see HERE. The 'end caps' are held in place with clamps that have thumb screws so you can take them off and put them back on quickly. The clamps squeeze the perimeter of the cap against the Tee fitting flange with a Teflon gasket in between the two. Two of the ends are capped with stainless steel plates and the third is a polycarbonate window that is sold as a sight glass so you can see your favorite beverage inside the pipe. In this application you can see whatever is inside the Tee fitting. The window's temperature rating is -40F to +240F so it has plenty of range for the temperatures that this thing will see.

D.I.Y Sanitary Fitting Thermal Chamber
To make this thing get hot I wrapped the outside with three Omega FGR-60 Rope Heaters. The Rope Heaters are just what they sound like, flexible heating elements that look like rope! They are each three feet long, 250 watts and 120 volts AC. Using three of them was a bit overkill but because they are wired in parallel and connected to a terminal strip it's easy to take one or two of them out of the circuit if it gets too hot too fast. The rope heaters are the white rope looking things wrapped around the ends of the fitting on the left and right. In the picture below you can see a small rectangular piece of white Delrin under the Tee fitting. The Delrin has a pipe clamp screwed to it holding the Tee fitting to the base plate and has a green ground wire attached to it. The Delrin provides some thermal insulation between the Tee and the aluminum base plate. The ground wire is there just in case one of the rope heaters somehow shorts to the Tee. Without the ground wire the Delrin is electrically isolating the Tee from the facility ground and if there were a short the Tee could become electrically 'hot'.
To control the temperature I used a Omega DP7000 Temperature Controller that I mounted in a home made aluminum box. The box was some scrap aluminum sheet metal that was in the junk pile and I bent it up between a couple of angle iron pieces held together with C-Clamps. I put a power switch and a power on indicator lamp. I painted the outside black to make it look high-tech and screwed it down to a 3/8" thick aluminum plate that was also in the junk pile.
Temperature Controller Box
In the above picture you can see the temperature controller mounted in the home made box held together with some strangely placed sheet metal screws. On top of the box is a small set of quick instructions for programming the controller. The temperature controller is connected to an Omega TC-J-1/8NPT thermocouple probe that is essentially a stainless steel pipe plug with a thermocouple sticking through it. I drilled a hole in one of the metal caps on the Tee fitting and tapped it to 1/8 NPT and screwed in the probe. That probe gives the temperature of the inside of the Tee fitting (actually a balance of the inside temp and the Tee itself) for the controller.
The last thing to do was provide a way for the gas to get in and out of the Tee fitting chamber. I did the same thing as I did for the temperature probe and drilled a couple of NPT tapped holes in the metal plates. One hole has a stainless steel tube that sticks all the way into the Tee almost touching the opposing metal cap. The gas enters that tube and travels all the way to the end of the tube then flows out and fills the Tee. The outlet is on the other side so the gas circulates across everything inside the Tee to get out. If that doesn't make sense leave a comment or email me at and I'll be happy to explain it!
This little project turned out pretty good and it seems to work. The entire cost is in the rope heaters and the temperature controller because most everything else was in the junk pile. Check out the Site Index of Projects link at the top to see other more interesting things I have built!

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