My basement darkroom gets down to the 40's during the winter. I use a small electric heater to keep my toes warm, but I finally found a solution to keeping the chemistry warm. I use trays for sheet film and paper. I purchased 3 aquarium heaters that has a remote temperature sensor and a titanium tube ( no glass or little lights) for the heating element. The tube is rated for salt water. I am testing it now and it seems to be keeping the water at a constant 68 or so. +- 1 degree after an hour.
The only drawback I see is the use of more chemistry to keep the tubes submerged.
A couple of precautions are necessary to do this. I am going to connect the heaters to a GFI receptacle and also putting it on my timer in case I forget to turn it off after a late night session. The thermostat has a small light on it which I need to cover with tape.
The cost was $65.00 including postage. This is a lot cheaper than that compensating timer i keep seeing. Besides, does chemistry work at 50 degrees F?