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I'm going to build a greenhouse

Do-it-yourself projects such as greenhouse or shadehouse builds and related topics.

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Postby daiv » Tue May 19, 2009 7:38 pm

OK, I told you I was going to clean up the greenhouse and surroundings when I was finished.

So here is the outside:
Image
(Note: the board on the ground is supposed to be there. It levels the large pot that will be going in that spot.)

Here is a look inside. Lots of cacti that are glad that winter is over!
Image

Here is a pic of my thermometer. I had a red-liquid-in-a-tube style thermometer and it apparently was too cold for it this winter and it was ruined. So I bought this bi-metal spring type with hydrogemeter. The problem with this one is that the temp varies widely based on where it is in the greenhouse. It has a black housing. If the sun is on it just a little bit, the temp goes way up. I put that white piece of plastic behind it to shade it from the sun.
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It makes me wonder. What is the best place to measure the temp in the greenhouse? Certainly directly above the surface of the benches and pots is going to be hotter than the empty spaces further away. Also the bottom is considerably cooler than the top. What do you other greenhouse owners do?
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Postby Tony » Tue May 19, 2009 7:44 pm

Looking good Daiv... I didnt know it could get too cold for a thermometer. :lol:
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Postby Arjen » Tue May 19, 2009 7:50 pm

I think it looks good! wish I had the room in my garden to build one there, my wife is already complaining about the amount of cacti inside the house.
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Postby CoronaCactus » Tue May 19, 2009 7:57 pm

Looking good Daiv!

I've been thinking of the same thing (re: thermometer in the GH) My quick solution was to use 3 thermometers at 3 different levels. Low to the ground, bench height and up high...take the average and thats the temp. I have a few $6-7 thermometers from Home Depot that work well and are small and light enough to hang on a string from the GH roof.

But i'd certainly like to hear from others. As i've been wanting one with a remote wired sensor and min/max display capability. I'd like to know just how hot it gets during the day and how cool at night.
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Postby daiv » Tue May 19, 2009 9:20 pm

CoronaCactus wrote:But i'd certainly like to hear from others. As i've been wanting one with a remote wired sensor and min/max display capability. I'd like to know just how hot it gets during the day and how cool at night.


I want to get one of those too. Actually, I'd like one that I could call with my cell phone and check the themp. That would be really cool. Probably does exist, but is expensive.

I get some more cheap ones and place them in various spots like you do. How many degrees difference have you noticed from the lowest temp to the highest?
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Postby Buck Hemenway » Tue May 19, 2009 9:22 pm

CoronaCactus wrote: As i've been wanting one with a remote wired sensor and min/max display capability. I'd like to know just how hot it gets during the day and how cool at night.



Nice work Daiv.

I don't know the best answer re temps. We've rolled up the sides on our green house. The opening is about 18" abve bench height. There's obviously a huge difference in temp between bench height and head height (another 18" or so above the rolled up sides. Big mistake that I did not build the freaming to allow a vent in the top end. Maybe next year.
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Postby daiv » Tue May 19, 2009 9:30 pm

It's funny because last year with the white therm. mounted at chest height, the temp seemed to jive with how it felt. I mean, you can tell it is wrong if the temp is actually 80, but the thermometer reads 95 or 100.
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Postby CoronaCactus » Tue May 19, 2009 11:07 pm

Daiv,
I haven't actually done it yet, that was just *the plan*.

I'll be getting them today or tomorrow and will be able to answer at the end of the week.

Buck,
Oh no! No roof vents :shock: Would the large size of the GH help to keep the hot air far enough away from the plants? With the sides rolled up it should keep the bench plants at a reasonable temp.
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Postby iann » Tue May 19, 2009 11:43 pm

Taking the temperature, who'd have thought it could be so difficult?

The "official" temperature is the temperature of the air, away from solid objects which could heat or cool it so strictly the temperature of the air at about head height. To measure this in your greenhouse, you'll have to protect the thermometer from the heating effect of the sun. It will need to go into a shaded location and not one right next to another object that will be heated by the sun. I have mine inside a white polystyrene enclosure. You could test your enclosure by taking the temperature in the sun out in the middle of your yard. If you get something close to the official air temperature for your location then the enclosure is working, usually it will read quite a bit warmer.

A thermometer in the sun only tells you how hot the thermometer is and that's fairly pointless because it isn't the same temperature as anything else like you or a plant. It isn't even the same temperature as another thermometer right next to it which tells you how pointless it is. One possibility is to place the thermometer not in the shade but in a strong stream of air such as in front of a fan. If there is enough air movement, then the thermometer will read fairly close to the actual air temperature even if it is in the sun. Remember not to place it in an air stream coming from outside if you want to know the temperature of the air inside the greenhouse ;)

The next question is whether you actually want to measure the temperature of the air. After all, the temperature of the plants and soil is what really matters and it could be quite a lot higher than the air. Or lower. The effects of heat directly from the sun and reflected from objects including the greenhouse glazing are what is really damaging in a greenhouse and you can scorch plants even when the air is quite cool. For example, air at 50C but without any sun would be quite tolerable for most cacti, but air at 50C with the sun shining through glass would kill almost any cactus quite quickly. Still air is more harmful than moving air even at the same temperature because moving air takes heat away from objects sitting in the sun. And objects in the sun under glass heat up more than objects in the open, because the sky is cold believe it or not (read up on radiative cooling if you want to know more, its why your car windscreen freezes over before anything else and before the air temperature drops to freezing, its also why the sun feels much hotter through glass even when the air is the same temperature). You could experiment with probes in pots or even in plants if you like, but generally I think it is impractical to measure "the temperature in the sun" in any useful way. Worth thinking about though.

So you measure the temperature of the air, but where in the greenhouse? Does it matter? After all, do you really know the temperature at which your plants will be damaged in the conditions of your greenouse? And it will vary wildly depending on the amount of air movement and the strength of the sun anyway. Play around measuring temperatures in different parts of the greenhouse to see how much variation there is and then pick a spot and leave it there. Call that the temperature even if bears no relation to anything else, because at least it will be consistent for you and you'll be able to compare it with "temperatures" the plants have experienced before.

Taking the temperature, who'd have thought it could be so difficult?
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Postby daiv » Wed May 20, 2009 12:24 am

iann wrote:Taking the temperature, who'd have thought it could be so difficult?


I certainly didn't think it would be until I tried it. I actually like Darryls idea of taking temps at three different spots and then like Ian says, leave them in that spot and then you will have consistent measurements for reference at least.
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Postby GeneS » Wed May 20, 2009 5:05 pm

There are, as iann says, many variables in the temp game. Even a small greenhouse will have definate microclimates arising from size, orientation to sun and wind, even material selection for framing. benches and floor.

A large hydroponic grower of roses nearby gave me a tour that was a real eye-opener on climate control. They have 4 fully controlled gh's each covering 2 acres. To permit roof drainage, the highest end is 3' higher then the lower. Despite full computerized ventilation control, water piped heating at plant level, automatic misting systems, and a bunch of other sophisticated stuff like CO2 injection(recycled from the boiler used to heat the heating water), automatic curtain walls and sliding upper covers to adjust the volume of the spaces, the high end averages 10F warmer then the low end just due to warm air rising. Microclimates are inevitable so use them to your advantage!

I put several hi-lo type thermometers around my space and move both them and plants untill everything seems happy. This varies seasonally as well. Air movement is the main key to avoiding problem areas both in climate and pest control.

Cheers ... GeneS
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Postby Andy_CT » Wed May 20, 2009 5:18 pm

Greenhouse looks nice Daiv! I wait for the flower pics that will come in the next few months 8)
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Postby daiv » Thu May 21, 2009 3:55 am

Good word Gene!

Andy - you know I will be posting them!
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Postby John C » Thu May 21, 2009 2:15 pm

Nice Daiv! It looks great!
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Postby Dmyerswny » Sun Jun 20, 2010 4:16 am

Did you use this GH throughout the winter? I saw snow in some of the pics...If you did use it in winter how did it fair?
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