Monitoring Temperature in a Greenhouse

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cadalot

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Monitoring Temperature in a Greenhouse
« on: January 23, 2015, 09:01 »
I now have two greenhouses one on the allotment given to me by my brother-in-law who has upsized and a Space Saver in the back garden that was purchased for me. I'm waiting to plant onions that will need somewhere to go after germination, but I'm holding back because of the Frost and low temperatures.

This got me thinking about what actual temperature difference there is during the day.

Yesterday during the day I took 3 reading and this morning so far one, has anyone actually monitored a whole day?   

Time     Outside      Inside    Difference
12.30        5.3         11.1        5.8       
  3:30        4.4          5.7        1.3
  4:30        2.9          2.2        0.7

  8:30       -3.5         -2.7       0.8

Obviously I'm more interested in the night time difference, and trying to work out when it's really practical to use an unheated greenhouse.

Has anyone actually monitored a whole day?   

I'm guessing it would also be interesting to look at difference on fleece and bubble wrap covered seedlings. I may have to pop to Lidl and pick up a couple of more of their Digital Termometers

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JayG

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Re: Monitoring Temperature in a Greenhouse
« Reply #1 on: January 23, 2015, 11:21 »
I haven't compared inside with outside temperatures like you have, but your figures look very much as I would expect, and shows that standard greenhouses are pretty poor insulators, and the smaller they are the quicker they will follow any changes in the ambient temperature.

Some people still manage to grow some hardy salad leaves in winter, but mine gets very little sun in winter (even when there is some) so after a few attempts I gave it up as a bad job where I am.
Sow your seeds, plant your plants. What's the difference? A couple of weeks or more when answering possible queries!

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #2 on: January 23, 2015, 14:07 »
There's another debate going on about a passive solar heater to try and delay the temperature fluctuation and last year adr123 put water butts to act as a heat sink.

Even in the weak winter sun, the temperature in my polytunnel has been up to 20C this winter but I don't think much can be done to keep a greenhouse or polytunnel at a working temperature without artificial heat.

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adri

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Re: Monitoring Temperature in a Greenhouse
« Reply #3 on: January 23, 2015, 17:10 »
The water butts (8x225 litres sprayed black) really worked I think. Can't remember when I put them in but even early in the year they helped maintain an even temp transition from day to night. We were giving cukes away and even composting them....

The polytunnel is a big one though so I could easily fit them in.
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Yana

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Re: Monitoring Temperature in a Greenhouse
« Reply #4 on: January 23, 2015, 19:31 »
I have a max/min thermometer for the air temp, and also have soil temp and moisture level sensors. The sensors are driven by a solar panel that is hard wired to the sensors that wirelessly transmit the data to a console that sits in the kitchen.
I have three sensors; one in a glass greenhouse; a wood framed greenhouse; and one outside in a raised bed.
I read somewhere that is it not necessarily the air temp that is the problem, it is the soil temp that needs to be above 7 degrees c (I think) for germination.
I know all this sounds high tech, but it was bought for my birthday last year by my OH and I LURVE IT!
The glass greenhouse air registered -5.6 the other night yet the soil was 2 degrees. The seedlings/plants were covered in fleece and so far are coping fine.
Hopefully this all makes sense?
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davetoddy

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Re: Monitoring Temperature in a Greenhouse
« Reply #5 on: January 24, 2015, 21:47 »
I have been collecting the bricks from storage heaters to act as a heat store in the polytunnel, get them free from electricians who are glad they don't have to get shot of them , they weigh a ton . And have been looking at temperature data loggers to monitor temps , any one used one they could recommend

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #6 on: January 25, 2015, 16:10 »
The water butts (8x225 litres sprayed black) really worked I think. Can't remember when I put them in but even early in the year they helped maintain an even temp transition from day to night. We were giving cukes away and even composting them....

The polytunnel is a big one though so I could easily fit them in.

Did you measure the max/min temperatures of the water or anything? I thought that if the temperature of the water varied then one could calculate the amount of heat that was dissipated and justify (or otherwise) the space taken up.

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adri123

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Re: Monitoring Temperature in a Greenhouse
« Reply #7 on: January 25, 2015, 19:48 »
HeadGardener22...Nothing so scientific but we live in the Peak District where some smoothing out of extremes of temperatures would be helpful and I'm sure it worked.  If it only took off 1 or maybe 2 degrees of cold at night that's a biggie when our last frost date can be as late as late May.  And I also think it had an effect on the max daytime temps though not as much.

Can't really see any way of testing it by measurement as I'd need to be able to put in and take out the barrels for comparison and that's not feasible.

I'm getting hold of another batch of barrels anyway as I have faith in the system and I think if our local farms will donate the barrels then I'll go all the length of the PT under the staging.









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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #8 on: January 26, 2015, 14:21 »

Can't really see any way of testing it by measurement as I'd need to be able to put in and take out the barrels for comparison and that's not feasible.


My thoughts were simpler than that. I agree that having large amounts of water in the polytunnel should smooth the temperature variation, I just struggle to work out how much smoothing occurs.

Any change in the temperature of the water in the barrels, shows that heat is being absorbed and dissipated (therefore smoothing the temperature variation a bit). So I thought this could be used to calculate how much heat was being put into the greenhouse and what the equivalent heater would be.

Taking as an example a theoretical example (if I have my physics and maths right - and I'm quite prepared to be wrong):

A 250 litre water butt falling in temperature by 1C would generate:

250 x 1000 x 4.19 = 1,047,500 j. (4.19 is the specific heat capacity of water)

Putting it into context a 60w bulb left on for 10 hours generates:

60 x 10 x 3600 = 2,160,000 j

So two 250 ltr waterbutts with the temperature of the water falling by 1C would be the equivalent of leaving a 60w lightbulb on overnight.

I have no idea how much the water in the waterbutt actually falls by overnight but I do know that the temperature in my greenhouse varies between 20C (in the day) and -2C (overnight) and that a 2kw greenhouse heater doesn't keep it frost free and, however much I want to, I can't get my brain to justify the waterbutts.   :)

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beesrus

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Re: Monitoring Temperature in a Greenhouse
« Reply #9 on: January 26, 2015, 16:26 »
I've monitored and compared inside and outside temperatures, and the bad news is a cold greenhouse at night barely makes 1 degree difference, even the poly carbonate twin walled sort I have. It's the early season daytime sun enhanced temps, and light levels, where greenhouses score. It isn't all temperature of course, as wind protection and even wind chill are a consideration.
Some people will tell you it is pointless trusting a cold greenhouse early in the year, and you should heat it. Well from experience, I know they're wrong. But what you have to do is double insulate close to the plants. Below is a pic of my present onion seedlings in the cold greenhouse a couple of days ago.That double insulation gives another 4 degrees at night.... all the difference in the world, and almost comparable to small heaters in greenhouses. If the forecast is armageddon, you can always bring them home for that odd night.

As for Lidl's digi thermometers, be careful how you use them. If you leave them too close to the soil and moisture, you can ruin them, as I did. Wilkos do a dearer one at 8 at present that don't seem to suffer the same problem. None of these digi thermometers really like too humid a condition.
onion seedlings2015.JPG
« Last Edit: January 26, 2015, 16:35 by beesrus »

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #10 on: January 26, 2015, 17:12 »
I think it depends upon what you're growing. Its probably excellent for onions but if you're trying to grow tomatoes, they don't like it below 10C and a temperature difference of 4C still isn't enough.

In the past I have bubblewrapped my polytunnel and then constructed a greenhouse within that with spare bubblewrap on a bench. Whilst the result feels warmer in the daytime than both the outside and the inside of the polytunnel, the minimum temperatures still can get down to freezing (maybe I need to buy some more max/min thermometers to compare multiple places).

Despite my scepticism, I think what I'm going to try as well is to put a waterbutt underneath the bench so that any warmth rises to the underside of the inner greenhouse. (I had thought of putting the greenhouse heater inside the inner greenhouse but as you point out it gets very damp).

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beesrus

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Re: Monitoring Temperature in a Greenhouse
« Reply #11 on: January 31, 2015, 08:52 »
Yes, I certainly wouldn't do it for tomatoes when it's freezing, but it's still quite useful for toms/peppers come March, as most nights give a 9 or 10 degree temp inside the domes. I still have to take them indoors on the odd night, but the extra light is well worth it....no other option really apart from sowing much later. I wouldn't want to put a heater in my wooden greenhouse up on the plot as that's just asking for trouble from the local young lads.
Am thinking of making a mini supplementary double insulated cold frame in the garden.

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cadalot

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Re: Monitoring Temperature in a Greenhouse
« Reply #12 on: January 31, 2015, 10:46 »
Some interesting post, this morning following light snow there is only 0.1C difference between inside and out side temperatures. I will try monitoring a setup like the one Beesrus post to see if I can achieve that 4 degrees, but I think the reality is wait for the weather to improve and sow with the temperature in mind.   

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Kristen

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Re: Monitoring Temperature in a Greenhouse
« Reply #13 on: January 31, 2015, 15:23 »
I've monitored and compared inside and outside temperatures, and the bad news is a cold greenhouse at night barely makes 1 degree difference

Although ... :)

I have found that there is a lag between the falling outside temperature and the inside one, so inside perhaps minimum temperature is only help for an hour whereas outside it might be 2 or 3 hours, so the plants are only subjected to minimum cold for a brief period before temperature climbs when the sun rises.

On a cold night it is usually cloud-free, so sunny the next day.  Of course if it is overcast, and cold, then the greenhouse temperature will equilibrate with outside.  On ice-days, where outside temperature does not climb above freezing, inside the greenhouse may do the same, which is bad news of course!

But if you find that inside temperature is lagging behind outside then I suppose one bit of good news is that if you choose to heat it then its only for "part" of the cold of the night.  I think it also benefits in Spring as the days are warmer, often sunny, the "thermal mass" of the greenhouse heats up, so the lag, at night, is even greater, so on a cold frosty night at the point when minimum outside temperature is reached it is still falling rapidly, rather than having bottomed out, and then starts climbing as the sun comes up; the inside temperature is falling too, but with considerable lag, which means its usually several degrees warmer than outside, and it starts warming with sunrise at the same time as outside does :)

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Headgardener22

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Re: Monitoring Temperature in a Greenhouse
« Reply #14 on: February 01, 2015, 11:46 »
And the lag is even longer if you insulate the greenhouse with bubblewrap.



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