Response to agitation: I do it different; it works, so why is it the oddball way?

When inversion tanks first came up, I read in a book that this would probably just be some kind of short-lived fashion which would soon be overcome again by the tanks where you rotate the reels at intervals. Well, the fact is, inversion came to stay. Personally, I can understand why one would prefer inversion to rotation. This (and a couple of the posts above) shows that this is probably as much an issue of belief as of physics and chemistry. I have been using "normal" inversion for all my darkroom life, and I will surely stick with it, BECAUSE IT WORKS FINE - FOR ME. Shawn wrote that this is not the kind of answer he needs, and being a physicist myself, I' curious, too. So let's look at it from the scientific perspective:

You can develop w/o agitation. The developer is then depleted near the film, and especially at places where a lot of density builds up. There is plenty of developer in the rest of the solution, so following nature's desire to keep everything in equilibrium, the unused developing agent will diffuse to the emulsion, the oxidised stuff will diffuse away, but VERY SLOWLY. That's why stand development tends to compensate contrast (see also below). If development were taken to complete equililbrium and there were enough developing agent, i.e. if we let the film simmer in the soup until it does not react any more, but the reaction does not stop because there is no developing agent any more, agitation would probably have no significant effect.

However, development is a process far from equilibrium. We won't leave the film in the soup forver, because even unexposed silver would then react increasing (at the very least) the base fog. Also, we cannot usually just wait until diffusion (a notoriously slow process) brings enough developing agent to the emulsion. So we have to help diffusion by agitating. This is supposed to reduce the concentration gradient created by the reaction, thus bringing the solution (not the development as such) to equilibrium. However, we don't want full equilibrium, as some of the effects of concentration gradients are desired (acutance-enhancing edge effect, a slight compensating effect, to name just two).

That said, what is the best method of agitation? We want the solution to be properly mixed, but there must not be any "ordered" flow patterns. Where these occur, they give rise to streaks and non-uniform development. The answer to this is turbulence. It mixes, but there is no ordered flow pattern. So why not just agitate as chaotically as possible? Because we have a third requirement, namely that we want repeatability.

I essence, that's why we have to use some kind of standardized agitation method. (It does not mean that we all have to use the same standard, but that each of us should develop his standard method, and stick with it unless there is a serious reason to change it.)

After this lengthy discussion (which mainly served to bring order to my thoughts) back to the original question: I think there is evidence that one can find a lot of different agitation methods that work. If you have shown that yours works with your equipment, there is reason to believe it does, provided that someone uses equivalent equipment. Most of today's tanks are built with inversion in mind, so inversion will work with them. I wouldn't want to guarantee that a non-standard agitation method will instantly work with a different tank system, too. It might be that the innards of your tank work fine with a specific exotic method, but that another tank system works better with inversion (or yet another method).

Sorry I wound up so long.

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