Sounds like rbeech uses something like I do. It's like a 20"by 48" plastic place mat. I swiped it from my wife's gardening set up. I used to use a thermostat sans/temp- scale so I had to fidget around to find the correct setting. That said, it's easy enough w/out the thermostat. Watch the thermometers in your developing trays and just plug it in a minute or two every 15-20 mins. It depends on the temp of your room I guess. I've tried it underneath a water bath tray to smooth temp swings but found that not necessary (for me), even with 55 degree room temps.
I just put an electric oil-filled radiator heater under the counter that holds the trays.
thanks for the advice guys. Unfortunately I don't think matts would be the best option for me and in a 100 year old farmhouse basement, getting the room temperature up would cost me over $100 a month just in heat let alone drywall and insulating the brick walls -but I did toy with that idea in the beginning of setting it up. I was looking at submersible heaters for aquariums, but i was hard pressed to find one without lights on it or that would fit flat in a tank, but I guess I'll just have to look harder.
Submersible heaters for aquariums don't reach the temperature required for colour processing (38 °C) because fishes would die. If you develop colour that means you'll have to "modify" your heater (break it so that the knob turns freely beyond the maximum temperature).
I think for your case the best is one of those food-warmers with a thermostat. I assume you only need one tray with really correct temperature. You will probably be able to find some in a second-hand store or used on the internet.
Something like this:
make sure there is a thermostat.
No water, no light (or easily maskable) and no great expense either.
Don't food warmers have too high a minimum temp? Seems like you would want to keep food temps pretty high to avoid health issues. :confused:
You could use a water bath thermostat, as is done in the chemistry and molecular biology labs. They are expensive when bought new, but with some luck and patience you can get them used at very moderate prices (but not below 20$ :( ). The simple technical principle is always the same, but the lab thermostats are rather precise, in contrast to any home-built solutions like e.g. aquarium heaters, and far better built than the comparatively cheap ones you in former times could buy for amateur photo labs. Once you have calibrated the built-in thermostat, you can trust it, sometimes to a tenth of a grade. You can just put your chemistry bottles into the water bath, if they float, put some weights on them. There are special ring-formed weights (lead coated with plastic) available for this purpose from lab suppliers, but of course they are not cheap (almost no lab supplies are :pouty: ). Either you make some yourself (rings from (stainless) steel or so) or again try to find used ones.
The bigger ones (heavy, not sooo cheap) of these machines can even be fitted with an external heating circuit, like a very small central heating. So you can heat almost anything (within certain size limits ;) ) with it by connecting some tubing etc.
So you could put your tank into the water bath if heating the chemicals in their bottles before use is not enough, but there will be some splashing around, when you move it or take it out. Perhaps you can live with that. Or you wrap some suitable tubing very closely around your tank, basically building an outer jacket, and connect it to the "mini central heating" mentioned above. Then you can keep your bottles inside the reservoir which the bigger lab thermostats (with external circuit) invariably have. A double-walled developing tank would be great, but probably does not exist :unsure:
Try a battery blanket from Cdn Tire on your 8x10 tank. You can control the temp with a standard dimmer type wall switch.
I've tried a lot of heating ideas, and aquarium heaters work, but their thermostats are not very accurate. However, they are intended for submersion, and are rated and certified for that application. Black and white processes should not be too fussy about temperature. For fussier color processing (film development) I had to purchase a lab heater from Cole-Parmer for about $175. It is accurate, although a little fussy, and the least expensive reliable solution I could find. Be sure to observe all electrical-code regulations and safety procedures.