Leaving airspace in the tank does provide some reassurance of agitation by sound and feel, but may also result in frothing and air bubbles in the developer, as well as increased possibility of streaks near the sprocket holes caused by rapid movement of solution.Seele said:If you use inversion agitation, a tank should not be used to its full capacity. For instance, I develop a single reel in a two reel tank, with the reel held down with a clamp or another reel, and use just enough solution to cover the film. When I invert the tank, the reel is drained of the developer, the solution mixex completely to disperse the resultant bromide, and then as a homogenous whole, plonked back into the film. This ensures completely even development free from bromide streaks...
There's no physical reason that lack of airspace in the tank locks the solution motionless and prevents agitation. It is a fluid, after all, and moves easily in relation to the tank and reels. We just don't hear it gurgling and splashing! It's a simple matter of inertia... you move the tank, the mass of fluid "wants" to remain where it is, therefore there is motion relative to the tank/reel; agitation.
By way of demonstration: What are these things called... a glass globe on a supporting base, inside is a winter alpine village scene... When you pick it up and twist it around, the "snowflakes" are stirred up as the fluid is agitated. See? No airspace needed! 🙂