MacGyvering a tempering bath for color development
There have been various discussions on how to control color chemical temperature during development, without resorting to a Jobo or some complex permanent bathroom plumbing.
The most simple solution is to fill a tub or sink with water at a bit above the target temperature, immerse your chems, let the chems come up to temp, adjusting the water bath as needed until everything is just above 38 degrees C. If you are quick and efficient, you can get one process done before the temperature drops too much. But it is a bit stressful, timing is everything. This is how I started with my current darkroom (I used to have a Jobo years ago, but abandoned it after a home remodel).
Some use a crock pot, rice cooker, or similar to preheat chems. This works, but accuracy may not be perfect.
The ultimate home brew system would have a fully tempered water bath that is maintained within C-41 or E-6 specs. This can be done with a PID temperature controller, a heating element, and a water pump, inside some sort of water container large enough to hold chems and film processing tanks or drums. (I do light's out dip and dunk for my 4x5 sheet film, rather than tray developing. My water container is sized accordingly.)
I have always wanted to build one of these systems, and just finished my first one. I have a plastic storage bin bought from the local home supply place. I found PID controller on the Chinese online shopping site (www.taobao.com) with temperature probe, already assembled into a little box with power switch, power cord, and switched power socket (controlled by the PID). The whole setup, including an immersable water heater, cost me less than $50 US Dollars (I bought it in China, where I'm living. That's the converted cost). With a system like this, you have to keep the water moving. I used a medium sized submersible aquarium pump, which cost about $10 USD, including a good length of tubing so that I could route the flow properly.
Here are photos of the finished device:
It works like a champ, temperatures stay within .1 C very consistently in the bath. All for well less than $100 US. It took an evening to assemble.