Jobo re-introduced some of their legendary processing machines and thus rotational development becomes more and more popular. This leads to an increasing amount of new-“rotationalitsts” asking us why we do not offer dedicated developing timetables.
All developing times are starting values which shall be finetuned to the individual lab requirements (your thermometer, your tank and agitation rhythm, your water quality etc.).
Thus the rule of thumb for the converion of an inversion-rhythm to a rotation rhythm is sufficent:
Decrerase the developing time by 10-15% if you rotate constantly
Why aren´t rotational times given as a standard?
Answer: With B&W films the physical-chemical combination-effect of local developer exhaustion paired with the cool down of the developer during the process (especially effective at 24°C) lead to an increased speed utilisation (and increased compensating of the developer).
This effect is limited in rotation because the chemistry is constantly agitated and the temperature is (usually) controled.
If we were to base our technical parameters on rotation the speed utilisation is lowered by about 1/3rd to 1/2 stop and this would mean inferior data in comparison to other manufacturers values. This is why no one does it.
The effect can be compensated with a slight overexposure of the film and the advantages of rotational development are less chemical usage and better reproducable results.