Monitoring Temperature in a Greenhouse

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Kristen

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Re: Monitoring Temperature in a Greenhouse
« Reply #30 on: February 25, 2015, 17:20 »
If there's any merit in them it's just the fun of seeing what's going on temp wise.  Is that wrong?

I think the merit is:

Weather forecast, say, 5C minimum.  Lets assuem that's what you get (but of course by logging it you can see if you are consistently some degrees above (in town) or below (in country)

Then the thing, for me, of interest is how quickly / slowly the greenhouse cools down compared to outside (and if you try insulating it with bubble-wrap how much that helps and so on). Once you have a base-line recorded then any changes you make you can easily compare to see how much improvement they make. Lets say you've got a few fence panels lying around, perhaps waiting to go up for a fence elsewhere.  Erect them temporarily to see if reducing the wind flow over the greenhouse makes much/any difference.

Lastly, knowing how low the temperature gets, how much lag compare to outside temperature, and thus how long, each night, it stays at that low.  Maybe you want to maintain 10C minimum but in practice the overnight temperature is only getting to 9C for an hour, and maybe that is just fine.

So, for me, the data empowers me to make decisions about "when" I can move plants out to the greenhouse, or if an insulation experiment I tried made any difference, or how well my greenhouse heating is performing - of if it is over heating the greenhouse and wasting my money!

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cadalot

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Re: Monitoring Temperature in a Greenhouse
« Reply #31 on: February 25, 2015, 17:52 »
Readings 6:30 tonight

   9.8C  Outside

10.98C Inside average over 4 readings in the greenhouse

  1.18C Difference with outside 

 11.8C Inside the first level Propagator

  2.0C Difference between inside first level propagator and outside Greenhouse

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jrko

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Re: Monitoring Temperature in a Greenhouse
« Reply #32 on: February 26, 2015, 11:40 »
Any enclosed space will have some lag re the outside temperature as the air masses are separated.  The differences are down to the amount of energy put in (by the sun in the case of a greenhouse.  Any thermal storage will be minimal in the colder months.  So in winter the differences will be small.  At the height of summer the input from the sun is immense and so we actually have to cool our greenhouses using vents, fans or leaving doors open.

A greenhouse is a very small sealed airmass fighting the outside trend when it's cold and running with it when it's hot.  Think of it as an ice cube in a drink.  At first the cubes melt quickly (winter heat loss quickly & temps struggle to out do ambient) but as the drink cools the melting slows and you can have ice in your drink for quite a long time (summer temps maintained for long periods)

The greenhouses 'buffering ability' (insulation) defines it's ability to defy outside temps

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Kristen

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Re: Monitoring Temperature in a Greenhouse
« Reply #33 on: February 26, 2015, 11:59 »
The greenhouses 'buffering ability' (insulation) defines it's ability to defy outside temps

Indeed. It seems to keep coming back to Insulation (e.g. bubblewrap and blocking-up any gaps) and increasing the Thermal Mass - concrete floor / water butts inside the greenhouse (although a thread somewhere recently [possibly this one, I've not read back thorough it ...] said that a 250L water container in a greenhouse with a Max/Min thermometer in it had only moved a degree or so over (from memory) a whole week.
« Last Edit: February 26, 2015, 11:59 by Kristen »

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jrko

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Re: Monitoring Temperature in a Greenhouse
« Reply #34 on: February 26, 2015, 13:38 »
Heres a bit of further reading re thermal masses etc

1) a guide to thermal mass usage/design
http://sustainabilityworkshop.autodesk.com/buildings/thermal-mass

2) a list of materials thermal properties
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

3) some research I did on greenhouse build/insulation
http://chat.allotment-garden.org/index.php?topic=90038.msg1002496#msg1002496

 :ohmy:

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JayG

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Re: Monitoring Temperature in a Greenhouse
« Reply #35 on: February 26, 2015, 15:56 »
It seems to keep coming back to Insulation (e.g. bubblewrap and blocking-up any gaps) and increasing the Thermal Mass - concrete floor / water butts inside the greenhouse (although a thread somewhere recently [possibly this one, I've not read back thorough it ...] said that a 250L water container in a greenhouse with a Max/Min thermometer in it had only moved a degree or so over (from memory) a whole week.

...which confirms that the concept of thermal storage works - the tank is clearly quite good at buffering its own temperature variations, but unfortunately cannot store enough heat energy to effectively warm the greenhouse because the ambient temperatures are at low levels for far longer than they are at higher ones at this time of year, although it's presumably also true that the variations in the greenhouse temperature would be even greater without the water container.

Interesting topic, although I have to say that an 8 X 6 greenhouse like mine isn't really big enough to accommodate a large enough water container to make much difference without losing too much valuable growing space. It does have a solid concrete floor, and the staging supports are concrete breeze blocks (long story  ::)) which I suspect is all the thermal storage capacity it's ever going to get!
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jrko

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Re: Monitoring Temperature in a Greenhouse
« Reply #36 on: February 26, 2015, 16:19 »
Yes for thermal storage.....no for water  :tongue2:

You need a dense mass that heats up slowly and releases slowly ie stone, concrete etc.  A bunch of bricks from storage heaters would be my best bet (or maybe heavyweight concrete blocks) Paint em black and place them in the sunniest position et viola!

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Kristen

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Re: Monitoring Temperature in a Greenhouse
« Reply #37 on: February 26, 2015, 16:21 »
the staging supports are concrete breeze blocks (long story  ::)) ...

Dense blocks would have been a good choice :)

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JayG

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Re: Monitoring Temperature in a Greenhouse
« Reply #38 on: February 26, 2015, 16:28 »
the staging supports are concrete breeze blocks (long story  ::)) ...

Dense blocks would have been a good choice :)

They are extremely heavy, solid concrete, breeze blocks (still call 'em breeze blocks round these parts, although there are lighter types too.)

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Kristen

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Re: Monitoring Temperature in a Greenhouse
« Reply #39 on: February 26, 2015, 17:05 »
I don't know the difference, but our extension was build with dense blocks (including for internal walls) to increase the thermal mass. I know that the brickies cursed the blocks being so heavy :)

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #40 on: February 26, 2015, 17:22 »
Surely the whole thing depends upon the ability of the "dense mass" to absorb the heat (as well as radiate it). The benefit of water (might be) that it would form convection currents from the side of the butt and therefore raise the whole mass rather than just the top few centimetres in a concrete block. Even better (perhaps) if one created a solar panel? This might have been why my water butt didn't seem to change temperature much, I was measuring the temperature in the middle of the butt, maybe I should measure it again at the sunny edge. However, that would imply that the total amount of heat absorbed/radiated is even less than a full butt's worth.

Insulation seems the best answer.

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jrko

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Re: Monitoring Temperature in a Greenhouse
« Reply #41 on: February 26, 2015, 17:30 »
Check the list in link 2 of my post above. 

You can store more thermal energy in the block than the equivalent amount of water and it holds onto it for longer

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #42 on: February 26, 2015, 17:37 »
I don't disagree that a block can store and radiate the thermal energy. What I wasn't sure about was whether a concrete block (or equivalent) would absorb as much heat as the equivalent weight of water in the short but sunny days of early spring.

(and I've no way of working it out).

My problem (even now) is temperature variation. The polytunnel has already been up to 30C even though its still down to 3-4C overnight.

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jrko

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Re: Monitoring Temperature in a Greenhouse
« Reply #43 on: February 26, 2015, 17:54 »
That's where density and surface area come in to play.

the lower density of water and its higher surface area (for a given mass) means that even if it could absorb more heat it would still be losing it faster rather than it can store it.

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adri123

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Re: Monitoring Temperature in a Greenhouse
« Reply #44 on: February 26, 2015, 18:09 »
@jrko

So my 8 x 225 litre water filled black barrels in the PT are actually doing harm?  I don't think so.  As far as I'm concerned they provide a buffering of temperature extremes (as much when it's v hot as when cold).

It'll take some persuading for me to believe otherwise.





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