I thought of that but the water that comes out of the skimmer is not necessarily the dwell time I understand to be of importance. I may be wrong, if so be kind...
Isn't the dwell time considered the amount of time the bubble is inside the skimmer to the riser neck? So once the bubble is created at the impeller (assuming venturi driven) would be the start time of the watch and the end time would be when the riser neck "top" is or has the first bubbles?
