It's actually pretty simple. They work perfectly as a tempering bath.
I'm no engineer myself but have gotten it to work perfectly. I fried up a relay doing something stupid, and another one stopped working for no apparant reason. Glad they're under $5. Heh. Maybe because I only paid under $5?
I just increased my 8x10 test bed size to a large garbage pail that I think will be my tank. I want to be able to do 8x10 sheet hangars and a basket of reels. That's the plan anyhow, and with this size it works great. Saves me around $100 for a custom tank which is what I would like, but it isn't really necessary.
It doesn't appear that I will be using the 4kw heaters. A test with it showed it raising a full casserole dish of water 4 - 5 degrees in less than 10 seconds. Within 2 seconds of power, water was sizzling off the element. With the small tank sizes, it's overkill and if the relay ever fails and keeps it open, it will melt any film in it.
I ordered a 200W aquarium heater that I hope I can set past the 32C limit. The dials are usually just stopped by a plastic piece that can easily be bypassed. That's what I have done with a couple 50W heaters. 29C was the limit but they'll go over 40C no problem now.