I'd suggest doing more repeats and averaging the results. I'm not an engineer, but my two cents as a printer: on the first graph I'd call the difference in highlight gradation slight at most. Contrast with XTOL is a little lower between x=2.3 and x~2.7, but after that contrast is essentially identical to D316. So I'd be surprised if the divergence at x=2.3 has anything to do with buffering. One would expect less buffering to reduce highlight densities/contrast, but the difference between the curves should keep increasing (at least that's what I'd expect). To me it looks like the occasionally kinked curves I've gotten with XTOL. On the second graph while I'm not surprised the curves are the same for a ~4% change in development time (within normal margin of error in my opinion), I am somewhat surprised you found such a marked increase in grain with such a small difference in development time, and without an apparent increase in density.
Originally Posted by albada