Fluid Dynamics

I decided to go back and give the Lab-Box another chance, using a non-polyester film, and I'm glad to report there was no light leak as occurs with PET film bases. The results, using HP5+ in Diafine got me thinking about something above my pay grade. I know next to nothing about fluid dynamics, but have a basic knowledge of aerodynamics from building and flying model aircraft.
The motorized Rondinax or Lab-Box is giving me results I don't get with a conventional tank when using Diafine. When I first tried it out about thirty years ago I was unimpressed. But I was using a conventional Paterson tank and inverting. It made for very grainy results, and I moved on to other developers for most things. But I kept it around, especially for large format negatives when I needed large volumes of solutions (it was cheap to use over and over again!) I soon discovered that being rather conservative about the assumed speed increase helped a lot. It might be that rolling BTZS tubes in large trays of water was also helping out as I'll postulate below.
You'd think that minimising agitation is probably the way to get good results from Diafine, as it just seems to get really grainy if you agitate roughly. And yet the motorized tanks are giving me super smooth results with continuous agitation. I was wondering if it is possible the continuous rolling agitation is resulting in laminar flow of solution around the cylinder of film on the reel, which means there might be a boundary layer of solution adjacent to the film that isn't getting as much refreshment with new chemicals as you might expect. Turbulent flow, as with inversions, would not permit that at all.

Am I mad or is that possible?

I do not understand how a light leak occuring would be dependant on the base material. Thinking of stiffer base or such seems far fetched to me.
Maybe instead of light leak you mean light fogging as in light entering and fogging through an undyed base.

Light-piping is an issue with PET films, but there is also an effect from the fact those film bases are thicker and tougher. The rotating cylindrical shutter of the 120 tank's temporary storage spot for the film as it is stripped off the paper doesn't seem to make a good seal on the film. So while you can get wild and woolly effects from light travelling through the base material, there tends to be fogging beginning just before the first frame, most strongly just where the ridge on the outside of the cylinder is supposed to press against the film and keep light from getting in. It's hard to describe, but if you haven't seen one, the ridge on the outside of this cylinder

is supposed to rotate anticlockwise and push up against this ridge:

This deforms the film into an s-bend to a small degree and discourages light from getting past the chicane it has created. Thick and tough film bases don't deform as easily, and are springy enough to hold the cylinder open slightly sometimes.

drmoss_ca said:

Light-piping is an issue with PET films, but there is also an effect from the fact those film bases are thicker and tougher.

Click to expand...

Thank you very much!
I was not aware of this latter effect. Thus my wild guess, which I even considered far-fetched, actually applies on this tank.

Thank you for the detail photos too.