Quote Originally Posted by octofish View Post
Not sure that a well insulated tank would be that useful. Unless you temper it very accurately - which is a pain to do - you will get a temperature change when you put chems in anyway. Tempering will be impossible from the outside (it's not even all that feasible using a normal plastic tank - takes too long), which pretty much means you need to prewet whether you want to or not.

The other option is estimating the expected temperature change, which is necessarily uncertain. You can always measure inside the tank with a thermometer to work out where you are, but then if you are wrong, how do you get the tank temperature to where it needs to be, if the tank is well insulated? A water bath will be ineffective.

Also I believe some proceses (fixing?) result in a bit of a temperature change by their nature, in which case you're stuck where you end up as a result of that. Could be wrong about that though.

Of course, this is all if you care about that level of precision. If you don't, then a plastic tank is pretty much good enough anyway.
I think that an insulated tank would be so that you would pour in developer at 39C and expect it to stay above 38C long enough to process a roll of film. One of the issues, the developing tanks we know and love, were designed for B&W film, where you usually used room temperature, measured the temperature of the chems, and used a time that matched. If you were to design a tank today for colour use, it would be designed quite differently.

1) It would be able to maintain temperature for the duration of the process.
2) It would not leak, for a steel tank with a steel lid, this is actually easy, put a neoprene ring in the lid, so that when the lid and top of the tank meet, the neoprene fills any voids.
3) Would have a plastic top cap, on a spring over the filler hole. When filling and emptying the tank, you hold the cap open, otherwise the spring holds it closed. If pressure builds, the pressure pushes the cap up, and releases the pressure. We add a small weighted latch, so that when the tank is not sitting flat, the cap is held closed. You can still use inversion agitation, during the 5-10 seconds your agitating the tank, pressure isn't going to build enough, that during the 50 seconds it's sitting flat, it can't be released. Of course when pressure is released, you might burp a little chemical, but you can wipe that off with some paper towel.

The fixing process, could change the temperature slightly, fixing, is not that critical temperature wise, it's development where temperatures are critical.