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Thread: Heat mat or heated propagator

  1. #25
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    I use a home made propagator made from a plastic tray about 500mm * 1000mm by 50mm deep. It is filled with sharp sand and has a 50W heater cable embedded in the sand. It is important to keep the sand very moist to give good heat conductivity. The temperature is controlled by a small computer from a probe in one of the pots. You don't have to use a computer - a thermostat will do. It's just that I control lots of other things (such as heated beds) and thermostats and timers get expensive when you have a lot of them.

    I aim for IP44 Ingress Protection. Exposed metal is earth bonded and the whole greenhouse is behind an RCP


    Propagator - without its fleece tent

    It is very heavy: about 30 odd kg sand and 10l water.

    Once seed starting off is out of the way I use it for three 30cm pots with auberines and a couple of smaller pots for chilies and it seems very successful. The roots spread right out of the pots into the sand. I replace the sand each year.


    Oops - mistake. The green lines below refer to a bed not the propagator, which is not currently recorded but runs about 18C
    Here are the sort of temperatures I get with 50W - blue is the air temperature in the greenhouse and green is the temperature in the bed. This is with the fleece tent in place. Lowest air temp in greenhouse so far has been about 2 C and lowest in-pot temp about 10 C. Last few days have been clear so big diurnal variations in temperature.


    Temperature over last few days. Blue greenhouse air. Green [s]in-pot[/s] in-bed compost
    Last edited by quanglewangle; 22-01-2020 at 07:32 AM. Reason: typo
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  2. #26
    Babru is offline Rooter
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    Wow, quangle, I bet the moon landing wasn't as well organised as this.

  3. #27
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    Quote Originally Posted by Babru View Post
    Wow, quangle, I bet the moon landing wasn't as well organised as this.
    I grow stuff too
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  4. #28
    Can the Man is offline Tuber
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    Seems very complicated for the amateur gardener Quangle, I bet you work in electronics or automation industry.

  5. #29
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    Quote Originally Posted by Can the Man View Post
    Seems very complicated for the amateur gardener Quangle, I bet you work in electronics or automation industry.
    Yes, I am quite interested in the way commercial growers operate and, by training, I am averse to wheel re-invention.

    I used to design and program low level stuff - down where the actual binary bits live. Retirement process took some years and involved a spell teaching undergraduates. Now there is no-one, apart from Mrs quanglewangle, to frown at me when I pick up a soldering iron.
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  6. #30
    Logunner is offline Rooter
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    Is

    Quote Originally Posted by quanglewangle View Post
    low level stuff

    code for I was a spook at GCHQ?
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  7. #31
    mrbadexample's Avatar
    mrbadexample is offline Cropper
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    Heat mat or heated propagator-dscf2345.jpg

    I've stripped out the old lining which was brittle and nasty, so I know I need to re-line it.

    I've got plenty of sand - what depth does it need please?

    How deep should the cable be buried? Should it be on the bottom of the box?

    Please assist, if you know what you're doing.
    Can the Man likes this.
    Our England is a garden, and such gardens are not made
    By singing-'Oh how beautiful!" and sitting in the shade,
    While better men than we go out and start their working lives
    At grubbing weeds from gravel paths with broken dinner-knives. ~ Rudyard Kipling

  8. #32
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    Quote Originally Posted by mrbadexample View Post
    Attachment 89814

    I've stripped out the old lining which was brittle and nasty, so I know I need to re-line it.

    I've got plenty of sand - what depth does it need please?

    How deep should the cable be buried? Should it be on the bottom of the box?

    Please assist, if you know what you're doing.
    "If you know what you are doing" That would be a first for me, but here is advice anyway.

    It's worth standing the propagator on some insulation board. I know theory says not much heat should go downwards, but I found it made a difference. Off-cuts can be scrounged from building sites.

    Depth of sand: remember purpose of sand is to even out heat distribution; to provide some "thermal ballast" so temperature doesn't vary to much; and support the pots. You don't need any more than will achieve that. Mine is about 40mm deep.

    How deep should cable be? Mine is about 2/3 the depth of the sand. I would be worried about putting it on the very bottom, since you want the heat to go up.

    Remember to keep it very moist. It's the water that conducts the heat not the sand.

    Don't over think it. It will be fine.
    Last edited by quanglewangle; 22-01-2020 at 09:09 PM. Reason: grammar (don't laugh)
    I live in a part of the UK with very mild winters. Please take this into account before thinking "if he is sowing those now...."

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