There is another one DIY link at http://www.fumblingwithfilm.com/how-...eveloping-c-41. I built one and it works great, very convenient and also cheaper.
Regardless which design will be considered, safety is always the first concern. Always assumes the worst case scenario. I introduced one more grounding wire connected to the metal grate in the design as mentioned above. So if no GFCI is available or it malfunctions, at least the leaking current will find the least resistive path to the ground. Another good idea would be including an inline fuse box with a fuse rated ELT (equal to or less than) the lesser of the SSR (solid state relay) current rating and the gauge of wire connected to the heating element.
Also for the sake of SSR's longevity, it's essential to install it to a heat sink with thermal grease/pad in between. In a pinch, a thin smear of anti-seize compound (the silver color one used for spark plug) will also do. For easy trouble shooting, I also installed a small DTDP (double throw double pole) switch between PID and SSR so that I can fine tune PID without having to power on the heating element. BTW, the thermal couple (k-type) I got has the polarity marked wrong. I was totally thrown off at first by decrementing readings as the real water temperature went up.
This looks quite interesting, I like the link tranquibra posted, it looks a lot simpler to build. I might give it a go at some point but try to neaten it up a bit.
It does feel a bit much paying £200ish for an old Jobo just to heat the chemicals up and agitate the film.
I don't care much about the electronics because I would rather use off the shelf component i.e temperature controller, RTD, thermocouple, etc.. but I am try to look for a way that I can get the temperature of the chemicals the same as the temperature of the water bath especially during processing.
I built a film processor a while back. It used an Auber temp controller, platinum rtd , 300w titanium aquarium heater and a fountain pump to circulate the water in the bath. Once up to temp it varies maybe 0.2 degrees C. I used standard SS drums(either 4 roll or 2 roll) with small bolts soldered on the bottom,which hook into the drive so I could pull it in and out and change solutions.The tank itself runs on a set of plastic rollers. For the drive I used a automobile window motor, and an electro-mechanical system to turn the tank back and forth. This seemed a lot simpler to me than microprocessors and such .
I've run about 70-80 rolls thru the thing, and the only hassles are taping up the top of the tank to make sure it doesn't leak, and a sticky relay. As for keeping the solutions warm I used a wide enough tank(plastic storage box) to let me put 4 soda bottles in it. That covers 1st and color developers and I add a bottle as one is used. also don't over-insulate the tank. I did that and the temp controller couldn't hold a temp. Once I ripped off the foamboard , it worked great
Now I want to try out a version for 70mm film
Look for a Devtec heater, it has a long u-shaped heating element attached to a small controller box. It's totally submersible. Used them before when I processed Tmax 3200 in Tmax dev at 100°F. You just need to calibrate it to the temp you need and if this is the only setting you see using, drop some hot glue or sealant to keep the knob from moving. I have one I planned to use in my sink but it doesn't go down to 70°.