What is in automatic vent openers used in greenhouses

Most of the descriptions say it has some kind of wax in a cylinder. When t emp 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 ki nd 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 o f leaving me to goof off everyday.
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not sure exactly what's inside as the magic substance, but here's a cut-away diagram of one
http://www.rostravernatherm.com/thermal-actuators.htm
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On 2/13/14, 3:46 PM, Cydrome Leader wrote:

The 'actuators' of automotive thermostats are based on wax expansion as well.... have been for eons.
More here:
http://en.wikipedia.org/wiki/Wax_thermostatic_element
Be sure to look through the extremal links at the end.
Erik
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The dang things don't work for shite. One you build will likely be worse.
get a used furnace fan and thermostat. Free if you can find an HVAC guy that keep the dead furnaces he changes out.
karl
P.S. My tomatoes just came up today. Broccoli yesterday. that's in the basement under the grow lights.
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Curious... how much are the Tomatoes & Broccoli going to cost you... factoring in the increased electric bill and all?
Erik
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Until Milady decided it was too much work, she sold about $2K in produce for about $1K in expense.
Now, we just trade the entire neighborhood fresh produce for other favors. We'll keep the bills down to $500, mostly heat.
Karl
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Karl Townsend wrote:

So far the kale and iceberg lettuce have made an appearance . Oh , and just in , the cauliflower and onions are starting to peep thru too . It's all on a table top installed as a window sill in a SE facing window . This stuff has all been planted starting on 2/8 , and I'm pumped !
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    I have no experience with these, but I suspect that there is either a spring or a weight which pushes the rod out the 18" or so, and the wax simply locks the rod in place as long as the temperature is low enough. When it melts, it does not *push*, but rather *allows* the rod to move.
    Are those temperatures in F or C? 55 Sounds rather low to be opening a greenhouse -- but I don't know what you are growing. And 55 C is equivalent to 131 F which is a point at which I would expect many plants to be uncomfortable. 70 C is 158 F, which seems even more uncomfortable for most plants.
    Anyway -- once you know what kind temperature you are looking at, then experiment with Paraffin (used for sealing "canned" foods) and other waxes (I save a lot of wax from Gouda cheese which melts at lower temperatures than the Paraffin does. Find out what temperature you get while there are still significant chunks of solid swimming around in the melted wax. If you find two, and one is higher than you need, and the other is lower -- experiment with mixing them in various proportions (mix well while molten) and see what melting point you get as a result.
    At a suggestion, I would make the rod have a number of grooves in the area which is in the wax holding cylinder so it is less likely to slip if bumped when cold. Once the wax melts, it should move out freely. But -- you would have to re-heat it to reset it.
    O.K. -- looking at this web site:
    <http://www.littlegreenhouse.com/accessory/vent2.shtml
I see that their openers work between 60 F and 75 F (apparently adjustable), and they don't say "wax" but rather "a mineral which expands when heated". *And* they self reset when it cools off. Perhaps the design converts a high force from the expansion to long travel via levers. Then again -- wax does expand significantly when it melts. The Gouda wax which I save I melt to about 7/8 full in a soup can, and when it cools (at the outside first) it leaves a cone (funnel shape) in the top surface where the level retreats as it solidifies. So you could get a significant amount of force as it melts. But the real range shown by the web site 60-75F is rather low as a melting point for the waxes which I have experience with. Hmm ... what does beeswax melt at?
    Do I assume that you have a lathe? That would be helpful in making the cylinder in which the rod operates and grooving the rod, along making with the spring or weight and linkage to provide the operating force. Since you are posting in rec.crafts.metalworking, I consider it a reasonable thing to expect you to have at a minimum.
    And later in the web page it does say that it uses wax and some of the cylinders to have your 55-70F range, so ignore my conversion from C above.
    So -- find a wax which melts at the right temperature, and enclose it in a cylinder with a piston. The larger the diameter, the more force you will get, and you can use leverage to convert that force to travel.
    O.K. The adjustment of temperature is simply an adjustment of how far the piston can move before it starts moving the vent. Download the instruction sheet from them to get an idea as to what the linkage looks like.
    Good luck,         DoN.
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And how does it *close again* under your suspicion?
If you ever cast wax candles, you have seen the considerable shrinkage of wax as it changes from liquid to solid. That's all that's going on here, and it provides plenty of force in both directions (in a well-designed system, anyway.)
They are simple hydraulic mechanisms, the only part that's voodoo is a wax that melts at ~70F rather than the more easily obtainable waxes that melt around 100-120F, and good (or bad, more often) basic hydraulic/mechanical design under the somewhat different parameters where the volume of the cylinder is important to the distance of actuation. Assembly without trapping air pockets will also help.
One possible source for this type of wax is refills for "wax cooling vests" which have a similar melting/freezing temperature. Not exactly cheap; but accessible; read the MSDS if you want to try the "direct ordering from a chemical factory" route, but that's generally not all that cheap or easy in small quantities without an established account.
Piston area is still directly related to force/pressure. The total volume of wax in the cylinder is what sets how far it will actuate, as the expansion when it melts and contraction when it freezes is the "motor" in this system. Providing the cylinder with fins or heat pipes can improve response time to temperature changes. Not overloading the thing will improve service life...
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    There was no mention of it closing again in the original posting, so I did not cover that at the start. I was figuring manual reset, and the vent opening to be a catastrophe prevention measure.
    He asked how it could possibly work, and I threw out one possibility -- before going on to do more searching.

    Did you not read all of what I posted? I described that very shrinkage -- and did a web search for the term to see what some makers might say about it.

    Someone else posted a pointer to a site which included a cut-away view which showed how it was made. A bit more complex than I first thought. Wax cylinder with a flexible diaphragm at one end, which bulges and presses on hydraulic oil, which then pushes the piston. The return force for closing (based on the first site I found) comes from springs in the mounting, not from the shrinking of the cooling wax. The shrinkage simply *allows* the springs to re-close the vents.
    Enjoy,         DoN.
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replying to Ecnerwal, Sid S. wrote: Jojoba oil (a plant product) is not actually an oil, it is a low melting point wax that melts at appx. 44-54℉. Paraffin Gulf Wax melts, depending on exact mix, around 99-120℉. Beeswax melts even higher. If one was to mix in the right amount, around 50/50 more or less, one of the latter two mentioned waxes with Jojoba oil(wax) you may hit the sweet spot you want where the expansion/phase shift occurs at your desired temperature range. I have heard people in the greenhouse industry sheepishly allude to the use of both beeswax & natural plant oils/waxes inside their greenhouse vent products. Some say an oil, some say a wax. The fact that the actual ingredients are skirted around when mentioned leads me to believe the process is not patentable nor hard. Jojoba plant wax in the beeswax or paraffin may be the "ingredient that Shan‘t be named". IDK, would have to try. Maybe even Vaseline would work in the mix.
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It is oil. I have one on my Greenhouse - had two but a young installer took apart the second one - the one he was doing - and leaked out the oil. It naturally didn't work.
Palmgreen is a source.
Martin - learn to insert CR/LF or press returns after 80 columns...
On 2/13/2014 4:33 PM, Butter wrote:

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On 2/14/2014 0:33, Butter wrote:

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.
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