On 2/14/2014 0:33, Butter wrote:
Most of the descriptions say it has some kind of wax in a cylinder. When temp gets to about 55 to 70 degrees it melts forcing rod against greenhouse vent. I'm wondering how to make something instead of paying $60 ea. What kind of wax could this be. Doesn't seem like melting would make much pressure. Some of these automatic vents push out 18in
I need to automatically open vents when it gets too hot. Forgetting to do it manually just once will be the end of all the plants in greenhouse.
I discovered that after retiring I can't afford to spend money anymore. I'm building a greenhouse so wife, who is still working, will not get tired of leaving me to goof off everyday.

Such thermal expansion devices have a very short stroke but LOTS of
force. So you need some lever-arrangement or such that transforms the
short stroke to the stroke needed.

I would start with a stainless-steel bellow, weld a couple of valves
for filling/ deairing, and fill with something. Oil? The bellow will
extend/retract based on temperature. Then the levers from flat steel.
Make an arrangement that moves freely more when the bellow reaches
"fully closed" and "fully open" positions. Otherwise, after that it
will break the system..

To be honest, those ready-made things start at around 20usd, so there
really is no reason to build one.. If I were to make one, I could make
it cheaper with "Arduino uno" MCU and electric actuator, and then it
would be a precise control. Of course, totally not necessary, but there
are propably a few gotchas in making the mechanic one work in all
situations. The electric one would be simple to test.