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Soil heating specification

Discussion in 'Hobby Electronics' started by Bob Young, Jan 2, 2004.

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  1. Bob Young

    Bob Young Guest

    I need help in specifying the type and resistance value of the insulated
    resistance wire to be used in the heating coil for a plant propagating tray.

    The size of the tray in the unit will be 750 x 450 mm and I need to be able
    to control the soil temperature between 10 - 30 degrees Celsius. The coil
    will be embedded in 25mm of moist sand. From what I have read this would
    require 15 - 25 watts of heating.

    The circuit must use a variable soil thermostat in series with the
    coil,which must be insulated due the presence of water. It will run in
    conjunction with a solenoid controlled misting unit (for which I have all
    the specifications), so could run from 240V AC or a DC supply.

    Any assistance would be greatly appreciated.
    Thanks
    Bob Young
    Melbourne
     
  2. Bob Young

    Bob Young Guest

    Sorry,
    I should have said:
    "From what I have read this would require 15 - 25 watts of heating per
    square foot" I estmate that 5 metres of insulated resistance wire would be
    required to adequately cover the tray.

    regards,
    Bob Young
    Melbourne
     
  3. Bushy

    Bushy Guest

    KISS

    Use all those left over water bed heaters! They provide plenty of heat at
    240 volts and would probably give some thing useful at 12 volts. Or even if
    you want to get fancy then see your local furniture store or council tip and
    order enough to set up the farm!

    They come with mechanical contact closure thermostats that dial in the
    temperature and if you add a layer of plastic liner and then a couple of
    inch thick plus layer of damp (or dry) sand then a second layer of plastic
    you can then lay your seedling trays straight on top and roots don't get
    tangled. If you don't have a heatsink type layer to absorb the swings in the
    temperature as the thermostat swings the seedlings can be burnt.

    Fishtank heaters and two inches of water can hold a tray inside a fishtank
    and there are readily available lights. This could work well with your
    misting unit and an overflow could be added to prevent flooding. They can be
    turned on their side and a two inch wide piece of glass or plastic can be
    added with a tube of silicon.

    Other designs have been a 40 to 100 watt light bulb mounted 6 inches below
    the bottom of the seedling tray. Old fridges and stoves have been used to
    provide a warmed chamber. If you control the temperature of the box that has
    a dozen trays it is often cheaper than to provide a dozen individual
    controllers.

    Have a think over the groups (added aus.gardens) about what cheap or
    recycled materials you might want to use and I'll be happy to help. Others
    may be interested in adding ideas.
    Peter
     
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