Essentially correct, yes. There are a variety of PID controller configurations. Some require an external relay, some have an internal one. The ones with an internal relay can be directly wired to the heating device, or more conveniently, to a power socket into which you would plug in the heating element. You can usually find a ton of them on ebay or other online sites. They are very common. I found one that was already built into a little instrument box, prewired, with the probe. That was so handy I jumped all over it.
Originally Posted by WayneStevenson
"PID" stands for Proportional, Integral, Differential controller. The heating algorithm is quite sophisticated. The PID controller can either auto tune or be manually tuned to the bath and heating element's capabilities. The result is that at least when tuned, the temperature does not overshoot the set value. It arrives at it and sticks, usually to within .1 degree C. It is quite impressive for such a small, relatively cheap box.
I've not found aquarium heaters to be sufficiently powerful for my water bath. I tried, gave up. Just not enough energy introduced quickly enough to get the bath and soup up to temp in a timely fashion, and I'm not certain it is powerful enough to hold the temperature stable while processing. I'm quite certain the PID controller and the heating element I have (which I think is a 1200 watt element, you can get various sizes) is sufficient. I do use a good pump. It works for aquariums or fountains. I don't know how well my pump will work if I try to do something like sous vide cooking in the same tank, which would require temps more like 80 degrees C minimum. That's hot enough I don't want to put my hands in the water, and I don't know if the submerged heater would operate in those temperature ranges. But I don't need that for photography, and if I do burn the pump out, it was only $10 and replacement is trivial. It just sticks to the bottom of the tank with suction cup feet.
I do plug everything into a ground fault circuit, just in case.
For the tank, it is a simple plastic storage bin from a home supply place. To get the heating element in, and supported horizontally, I made a large hole, cut a chunk of plastic (out of a plastic cutting board, about 1/4" thick) to reinforce the mount, glued it all in place, and then sealed it with silicone sealant. The result is that I don't need the water to be very deep to fully cover the heating element. Do note that these simple heaters will burn out almost instantly if powered up in the open air, so they must be under water. The downside to my system is that if I do burn it out, I have a goopy mess to cut through to extract and replace it.
The design on which I based mine had the heating element going in vertically from the top, attached to a separate unit that would clamp onto the bin. I didn't like that one, because it required the bin to be filled with water nearly to the top. If I'm using the Jobo 2509 (?) tank to develop sheet film, the water would be too high. I did consider it, and considered putting in a little submerged platform just for that tank, but decided it wasn't worth it. Better to mount the element low to begin with.
The size and depth of your tank depends on your need. If I was doing tray based processing, I'd construct a much larger container for the tempering bath, and build some sort of submerged shelf or platform for the trays, so that while the water would be deep enough for the heating element, it wouldn't be too deep for the trays.
(A larger volume might require multiple heating elements too.)
I don't have enough room in my bathroom for such a contraption, so this is it for me. I have enough room for Patterson - based roll film development, and for lights out dip and dunk using a combi-plan based system. One problem to solve with that system is the fact that the PID has very bright lights, as does the power switch. I'll rig an opaque cover for it.