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


"Bob Young" wrote in message
u...
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