Reproducibility of heterogeneous gypsum scale development on a reverse osmosis membrane was investigated using real-time membrane surface imaging by light microscopy. Temporal scale development in nine identical experiments using virgin membrane samples followed reproducible trends with decreasing rate of detected crystals, linear increase of fractional surface coverage and decreasing rate of crystal growth. Significant scatter, however, was observed in between individual experiments with respect to absolute values for determined scaling parameters. Using an intentionally higher scale-forming ion concentration (+ 4.1%), scaling behavior lay within the previously observed scatter. Contrarily, stainless steel particles led to significantly higher crystal number densities and smaller mean crystal diameters, while surface coverage remained unchanged. Scale development on a repeatedly scaled and cleaned membrane sample showed increased crystal number densities, i.e. nucleation affinity, and smaller mean crystal diameters in each consecutive scaling experiment. Gypsum crystals physically altered the membrane surface and assumingly initiated additional hetero-nucleation sites. Our observations demonstrate that individual membrane samples exhibit significantly different overall and local nucleation affinities, which may be enhanced by deposited particulate matter or physical membrane surface alterations. Nucleation affinity, however, did not predominantly determine the overall propensity of gypsum scaling.