That was typing error which somehow got stuck in my brain and repeated over and over.   Posts have been edited to correct.  RHS should have never been typed, all references should have been RDS, which is the primary company who has marketed a yield monitor based on a photo eye viewing the elevator to determine how full it is. 

The other question....who can flow remain constant if RPM falls?  Simple, if the same amount of material continues entering the elevator, then the flow must remain constant or else stop with a plugged elevator.  If speed slows down, the elevator must run more full to transport the material.  The photo eye is only looking to see how full the elevator is.  In order to actually determine how much is being transported, it must also know how fast the elevator is moving.   The elevator could be stopped and full, and the sensor would be showing 100% darkness, implying a very high flow rate when actually it is zero.    RDS, as far as I remember does not have a sensor measuring elevator speed, so therefore must be assuming it to be a constant.  Correct me if I am wrong, because it has been several years since I had their system, but I do not recall any sensor for speed or any attachment to existing combine sensors.