I'm not sure what you mean by "time to reach saturation" if you aren't referring to a complete saturation curve. Time to reach saturation is influenced by several factors with shot size being one of the most important. For example, if you compare S-110 and S-230 shot you'll find that you have about 8 times as many particles per pound of S-110 compared to S-230. If you could reach saturation in one minute with the S-110 then it would take you about 8 minutes using S-230 (assuming you adjust air pressure or wheel speed to achieve the same intensity).
I suspect that you are actually referring to arc height readings, not saturation curves, and therefore possibly misinterpreting actual intensity. You might run a complete saturation curve and send me the data points so I can see the shape of the curve etc.
