MadSci Network: Engineering
Query:

Re: How to fabricate a bimetallic spring

Date: Tue Jun 21 08:27:46 2005
Posted By: Jeff Yap, Physics Teacher
Area of science: Engineering
ID: 1118019409.Eg
Message:

Hi Dean,

That's a really good idea. If you get it to work, patent the thing and retire.

Just to get everyone else caught up: A "bimetal" thermometer is made by sticking together two strips of metal (like Dean said, commonly brass and steel, but any two metals will work) with different coefficients of thermal expansion. As the metals are heated, they will expand at different rates and cause the object to curl.

Dean, my first suggestion would be to buy or cannibalize an existing bimetal thermostat for the parts. Factory made thermostats will be more accurate, and you can use house thermostats which have the electrical or mechanical on/off switches already built in. All you would have to do is wire the thermostat to your power supply for the heating element. If your smoker has a gas burner, then you could get a gas throttle and adjust it that way. All these parts should be available from the HVAC section of a hardware store. For different smoking temperatures (more on this later) you could also get oven thermometers and take them apart for the coil.

If you're set on building your own bimetal thermometer (which I totally dig), I would suggest coiling both strips of metal into the desired shape before attaching them together, and fastening them at the ends, and periodically along the coil. Factory made coils will be fastened all along the strip, but if you plan to use glue or tack welds, more fastening will make the strips bend less and place more stress on the welds. However, more fastening will make the coil keep it's shape better. A higher number of loops in the coil will make the coil more precise to the nearest degree.

The simplest model would be two long straight strips tacked just at the ends. When heated, the bar curves and touches an electrical contact. You can then move the contact around to modify the set temperature.

Ok, here's the rub: Thickness of the metal WILL affect the response time, torque, and sensitivity. So will the length of the metal strip, the shape of the metal (coiled, uncoiled), the two metals you choose, and the number and kinds of welds. You can make reasonable guesses and calculations about all of these, but the bottom line is that you'll have to build one to test it out. I'd suggest using a thermometer and a heat gun to calibrate it or to see if it works at all.

I hope this helps, and good luck with your project. It sounds like a lot of fun.

Jeff Yap
Mad Scientist

On a side note, you only need a thermostat if you are using a "hot" smoker. Alton Brown's cardboard box smoker - Good Eats "Cold" smokers use a separate smoke chamber that pipes room temperature smoke into a cooled compartment where you store the meat. It takes longer, but has a different taste.


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