Allotment Gardening Advice Help Chat

Growing => Growing in Greenhouses & Polytunnels => Topic started by: cadalot on January 23, 2015, 09:01

Title: Monitoring Temperature in a Greenhouse
Post by: cadalot 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
Title: Re: Monitoring Temperature in a Greenhouse
Post by: JayG 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Yana 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?
Title: Re: Monitoring Temperature in a Greenhouse
Post by: davetoddy 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
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 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.








Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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.   :)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: beesrus 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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).
Title: Re: Monitoring Temperature in a Greenhouse
Post by: beesrus 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot 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.   
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen 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 :)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 on February 01, 2015, 11:46
And the lag is even longer if you insulate the greenhouse with bubblewrap.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: beesrus on February 05, 2015, 19:43
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.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 :)
:D Actually Kristen, I agree with your lag theory. We are a pair of theorists.  :)
With that lag in mind, and if my early cusp Spring days are not too fully booked, I make sure that my visits to the greenhouse are early morning, rather than at dusk. That way, any end of day stored heat in the greenhouse is not wasted by my opening of the door. Early morning visits are de rigeur.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 on February 22, 2015, 17:55
My experiment  proved nothing  :(.

I put a max/min thermometer inside a black waterbutt which was stood in the polytunnel in the sun for over a week. The difference between max and min was one degree, regardless of the fact that the temperature in the polytunnel ranged between -2C and 25C and it was sunny for some of the time.

Time to think of something else.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on February 23, 2015, 13:52
That's a good idea - particularly as it turned out that water did not heat up much. I've been trying to think of how to measure the temperature of water with one of my temperature loggers, which are not waterproof ... :D

A 1C rise on 1 cu.m. of water is a bit more than 1kWh, which is probably enough to heat a 12' x 10' greenhouse overnight at this time of year (i.e. I'm thinking that a 2kW fan heat on for 30 minutes, total, for the night would keep a greenhouse at 10C ish?)

I've got my loggers measuring inside temperature of a 4-shelf blowaway, zipped up in my Greenhouse, another in the greenhouse, and a third outside, to see what the lag / heat-store value of the blowaway is, if any.  Going to be a few more days before I have enough data to publish.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on February 23, 2015, 15:52
Where do you buy the logging thermometers? Had a search and can't find them.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 23, 2015, 17:13
Lidl sell them at around 3 from time to time, they are supposed to be for cars and have an inside and outside reading and save the max and min, so you reset to zero each day.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on February 24, 2015, 00:20
Where do you buy the logging thermometers? Had a search and can't find them.

(http://static.rapidonline.com/catalogueimages/Product/S51-5408P01WL.jpg)
http://www.rapidonline.com/test-measurement/voltcraft-dl-101t-usb-temperature-data-logger-51-5408

Sometimes on offer at Clas Ohlson, Conrad and probably Maplin/
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 24, 2015, 06:27
A quick search on ebay using "Temperature Data Logger" and these are half the price of the one above and one comes with a probe that could be used for soil temperature
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Markw on February 24, 2015, 08:15
Cadalot
I am not very far away from you. I use Tinytag Talk 2 data loggers, I would be only to happy to pop over and set some loggers up and get you some information.
I have used them loads of times, I have even buried them in the soil to record soil temperature. so it is easy to record outside, inside, and soil temperature you can then download all the data and print all the information in graft form
http://www.geminidataloggers.com/software
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 24, 2015, 09:16
Hi Markw
Thank you for the offer but I have just purchased a couple of data loggers (the one in the post above with the probe) and some additional cheap small max min temperature & Hygrometer gauges at 3 picture below that run on a single AAA battery

What were your findings when you set them up in your greenhouse?

I have found that now I have installed reflective panels in the top of the space saver greenhouse there is a significant difference from one side of the greenhouse to the other, and top to bottom
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Markw on February 24, 2015, 11:43
Hi Cadalot
I think they are a must have bit of equipment. Not just for the greenhouse but for other monitoring situations.
I have quite a few of them some with probes. they work in the range from -40c to +125c .
They can be set to take a reeding every second or even up to once a day
I have used them in fridges and freezers to see if they are working correctly. in the house for balancing rads etc.
For growing the ones with the probe can be used for soil temperature so you can set up your propagator correctly. In a greenhouse they can be for setting your heating at night time or on a cold day. in the summer you can use them for cooling  if you have an opener installed.
If you were using say 4 of them you can overlay the data on one graph this makes it easy to see what is going on. I have used them for nearly 10 years now and I would be lost without them now. If you were using them for setting up your house heating they pay for them self very quickly.
I have some saved charts somewhere I could send you if you like.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 24, 2015, 13:48
Hi Cadalot
I have some saved charts somewhere I could send you if you like.

Yes please I will PM you with my email address
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on February 24, 2015, 21:22
I've toyed with the idea of buying one of these to see what's going on...but now realise that all I need to know is it's blooming freezing and nothing's going to grow.

If there's any merit in them it's just the fun of seeing what's going on temp wise.  Is that wrong?

Adri
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 24, 2015, 21:53
What has amazed me is that there is a recordable lag with between levels of enclosure reaching the same temperature but that also appears to work in reverse in the morning when heating up. By taking off the propagators lids the temperature equalised more quickly.

And yes when not heating a greenhouse over night, it isn't an awful lot of use for bringing most seeds on early, than when in full sun over the last week the temperatures in side rocket to 40C and then drop as the sun goes behind shielding buildings.

based on these extremes of heating, it's a wonder anything grows and stays alive. But isn't nature a wonderful thing?     
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on February 24, 2015, 21:59
Did I write 'blooming' freezing?

Don't think so.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 25, 2015, 07:08
Did I write 'blooming' freezing?

Don't think so.

Tis the power of Aunty she is everywhere and see everything, plus some word are pre programed to alter on the forum especially around Christmas time  :lol: :D :nowink:
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen 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!
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot 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
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko 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

Thus ends the ramblings of lunatic
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko 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:
Title: Re: Monitoring Temperature in a Greenhouse
Post by: JayG 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!
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko 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!
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on February 26, 2015, 16:21
the staging supports are concrete breeze blocks (long story  ::)) ...

Dense blocks would have been a good choice :)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: JayG 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.)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen 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 :)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko 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
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko 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.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 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.


Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on February 26, 2015, 18:28
I wonder if one butt, rather than several, connected to some old radiators (to increase area-to-volume collection) painted black to absorb heat, would create a good enough convection thermal current to circulate the water butt's water and heat it more efficiently then just having the sun shining on the surface of the water butt(s) itself?

That said if the hot water returns to the top of the water butt I'm not seeing how, at night, there would be a thermal current to get the heat to circulate from water butt to radiators, and thus radiate to heat the greenhouse. Perhaps it is as simple as the water in the radiators cooling so flowing downhill back to the water butt and thus warm(er) water flowing out of the top of the water butt to the radiators.

Still struggling with what happens on a realy cold night, or during a spell of dull overcast and inclement weather - but I suppose if electric (or whatever) heater available, but the hot water store surficies some/much of the time its an overall saving.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko on February 26, 2015, 18:38
@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.

Maybe re read my post  :nowink:

The rads might work as the water plus the metal makes for a better thermal mass
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on February 26, 2015, 20:32
But I think that the whole point of thermal mass for a PT or Greenhouse is to smooth out the extremes of either heat or cold.  My Black water barrels (IMHO) served as good a purpose at keeping down the temperature during heat extremes as keeping the temp up when it was cold. 

If you went for the black radiator linked to the barrels you'd soon want a way of disconnecting it when summer came.  I once made a DIY solar hot water heater and it had to be drained several times a day in summer as it was in danger of melt down.  All that heat wouldn't be good for the plants..
 
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko on February 27, 2015, 14:18
Just been to the plot.  There's a metal water tank of around 700l.  Right next to it are 7x 150kg reinforced concrete slabs.  They both get the sun for as long as it's out, which is all day today. 

I wasn't able to check actual temps but the concrete was air temp, so 6deg or so.  The water?  Significantly colder, and that's from 700kg of water.

Conclusion? At low temps concrete absorbs more heat than more than 4x the mass of water.  A smaller body of water would heat up faster but would cool down faster too.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko on February 27, 2015, 17:07
Been reading up on water as a thermal mass. It is used, but requires large amounts of space. At this time of year I'd just heat your greenhouse or if you can't use another sealed 'grow house' inside your greenhouse
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 on February 27, 2015, 20:21
Just been to the plot.  There's a metal water tank of around 700l.  Right next to it are 7x 150kg reinforced concrete slabs.  They both get the sun for as long as it's out, which is all day today. 

I wasn't able to check actual temps but the concrete was air temp, so 6deg or so.  The water?  Significantly colder, and that's from 700kg of water.

Conclusion? At low temps concrete absorbs more heat than more than 4x the mass of water.  A smaller body of water would heat up faster but would cool down faster too.

Hmmmm. You may (or may not) be right. Checking a temperature by feeling it with the skin is notoriously inaccurate. Water will damp the skin and evaporation will make it feel colder.

It comes back to what we are trying to achieve. Only a heat source will do anything but slow down the rate at which the mass of air in the greenhouse cools. So now, when the evening temperatures are cool and the nighttime temperatures are cold, nothing but a heater will make enough difference to keep the temperature in any part of a greenhouse above 10C. I honestly think that the answer is that any mass (water or concrete) will be better than nothing at all. Sealing and insulating will probably make more difference though.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Baldy on February 28, 2015, 14:11
It was the song, "Sunday blooming Sunday", by U2 on the song thread was where I first came to truly understand the power of the all seeing Aunty.
I now try to be (at least) better behaved than Cadalot to stay out of trouble...  ;)

Pip pip,
Balders
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on February 28, 2015, 17:43
Balders where did that come from?  :ohmy:

What the bleeping hell have I done this time!  :unsure:

It's a ploy I tell you, a ploy to take attention off your antics  :D :D :nowink: 
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Baldy on February 28, 2015, 17:57
It doesn't matter what you have (or possibly haven't) done Cadders...

The point is, you've done something... and you are guilty (we all are) - its just a matter of time ' til Aunty works out what you are guilty.

(Its like a slightly more feminine/gardening version of George Orwell's 1984)  ;)

Pip pip,
Balders
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on March 01, 2015, 14:31
I've got my loggers measuring inside temperature of a 4-shelf blowaway, zipped up in my Greenhouse, another in the greenhouse, and a third outside, to see what the lag / heat-store value of the blowaway is, if any.  Going to be a few more days before I have enough data to publish.

OK, some results ... bit of a cockup, but perhaps more useful data as a result.

I put the logging thermometers:
All covered with a pot so that they were not in direct sun.

Result: Blow-away fell to a LOWER temperature than the greenhouse! However, I think this is because its thermometer was up (relatively high) on the shelf, and the one on the pot, stood on the greenhouse border, had the benefit of warmth rising from the soil.

But I think it shows that the temperature in the Blow-away falls just as quickly as the greenhouse itself, and (by itself) provides no additional insulation benefit.

(https://kgarden.files.wordpress.com/2015/03/greenhouseblowawaytemperaturelog_feb2015_800.gif) (https://kgarden.files.wordpress.com/2015/03/greenhouseblowawaytemperaturelog_feb2015.gif)
(Click for larger image)

Some additional thoughts:
More detailed article on my blog with a break-out image for one day and tables of falling temperatures / lag to outside etc. etc.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: jrko on March 01, 2015, 15:41
Blog?

Linky please  ;)
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on March 01, 2015, 16:29
Not allowed to post a link, as per the forum rules.

There's an icon next to my name.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on March 02, 2015, 14:20
Bought one of these of ebay.  10.39 inc postage brand new.

http://pcsensor.com/pcsensor-new-double-sensor-thermometer-pc-laptop-thermometers-temper2.html (http://pcsensor.com/pcsensor-new-double-sensor-thermometer-pc-laptop-thermometers-temper2.html)

Will test the PT as soon as it arrives.

Title: Re: Monitoring Temperature in a Greenhouse
Post by: Kristen on March 02, 2015, 14:35
Bought one of these of ebay.  10.39 inc postage brand new.

I've had a number of gadgets from China, always a bargain :)  Helps if Mr Customs doesn't get tempted to add VAT on the way in ... he's frequently busy, looking the other way, though :D
Title: Re: Monitoring Temperature in a Greenhouse
Post by: cadalot on March 05, 2015, 07:22
From the web page Precision:  +-2C  so a 4 degree C accuracy in reading the temperature in the first place! But having the temperature wrong it will at least report variations in temperature accurately     
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on March 05, 2015, 09:59
Well I can calibrate it and compensate if it turns out not to be too accurate.  I couldn't justify 40 for one of the better ones.  They'll all be made in the same factory more than likely...
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Elm street on March 08, 2015, 16:31
These units will only work when connected to a computer/device so are not stand alone!
Therefore you will need a long USB cable or leave your device in the greenhouse  :(
Title: Re: Monitoring Temperature in a Greenhouse
Post by: adri123 on March 08, 2015, 21:15
Thx.  I thought that might be the case but fortunately I have a long USB cable and/or a crappy old laptop that can stay in the Pt overnight.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 on May 09, 2016, 18:39
I know this topic has been quiet for a long time but I'be been entertaining myself by creating my own temperature monitoring system using a Raspberry Pi and the DS18B20 waterproof sensors. I've now got to the point where I have run the system for three days with two sensors measuring the temperature every 10 seconds or so and created a graph of the results of the temperature indoors and outdoors.

My next step is to increase the number of sensors and to measure the temperature inside the waterbutt to see if the temperature rises and falls and to compare that with the temperature inside the polytunnel, both just in the atmosphere and on top of the waterbutt.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Trikidiki on May 09, 2016, 20:22
HG22, as you are aware I was monitoring soil temperatures using DS18B20s and an Arduino. It worked really well as long as my power source held up. I had a blue-tooth connection to an old phone so I could check the temperatures on the fly rather than having to remove the SD card and transfer the data onto the PC. The blue-tooth was a big battery drain especially if they disconnected for some reason as it constantly tried to reconnect.

I am now playing with a 'solar heating system' in my small greenhouse but have been too busy rebuilding my fruit cage and other real gardening chores, to give it the time. Hopefully get back onto it when spring planting subsides.

Basically it is a black painted radiator feeding a dark green oil drum. The water is pumped during daylight hours for 1 minute every 15 minutes. My problem at the moment is the startup current for the pump motor keeps burning out relays.

The plan is eventually to have one radiator painted black feeding the oil drum, the pump will only run when the radiator temperature reaches a certain threshold above the oil drum temperature, thus saving battery power by only pumping when there is heat to be gained. A second radiator will be my propagator bench with a transparent "sun cloche" over it. At night when the temperature drops to a predetermined level another pump will run the water through the bench radiator to heat the cloche. I could also set it to pump the water to cool the bench radiator during the daytime if the temperature starts to rise too high on a very sunny day.

I had thought of keeping the drum on the sunny side of the greenhouse to add to the solar heating effect but am now thinking that using a plastic drum buried in the ground will insulate the drum well, may provide a little geo-thermal heating effect and also free up the floor space in the greenhouse.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Headgardener22 on May 10, 2016, 14:29
Hi Trikidiki,

We're obviously going in similar directions (I'm just a bit slower). My ultimate plan goes something like this:

A solar panel; a water tank; two 12v pumps; a length of hose around the polytunnel; three temperature sensors (in the solar panel, the tank and the polytunnel; a car battery; and a solar panel to charge the battery.

When the temperature in the solar panel is warmer than the temperature in the tank, the water is pumped round the panel and into the tank;
When the temperature in the tank is warmer than the temperature in the polytunnel, the water is pumped around the pie in the polytunnel.

I think I'll also need a fan which runs if the temperature in the polytunnel is higher than (say) 35C.
Title: Re: Monitoring Temperature in a Greenhouse
Post by: Trikidiki on May 10, 2016, 21:44
I have had the prototype running on occasions and also have another oil drum of water next to it which is currently 'stagnant'. Just using touch there is an appreciable difference in the temperatures in the two tanks when it has been running for a while. My gut feel is that it will not produce enough heat to maintain an adequate temperature in a large volume which is why I am focusing on just heating a propagation bench. If it works better than expected I might try to heat more of the greenhouse.

I just need to get some larger relays. The pumps draw 3A when running but they are burning out the tracks on 10A relay modules so must be producing a huge power surge on start up. It may be that although the relays are rated at 10A the circuit boards cannot cope with the 3A the pumps are drawing.

Title: Re: Monitoring Temperature in a Greenhouse
Post by: steved on May 13, 2016, 07:58
You would need a motor rated relay/contactor. Induction motors can draw upto 8x the running current on start up