The present invention relates generally to a system for dispensing particles.  In particular, the invention relates to an apparatus and method for monitoring and controlling the velocity of particles in a pneumatic product application system. 
Agricultural implements such as air seeders and row crop planters employ pneumatic means to convey product such as seed and fertilizer from a centralized hopper to distribution on the ground.  Controlling particle flow at an acceptable operating speed can be a difficult task Too much air velocity and thus increased particle velocity can result in product damage and product bouncing or blowing out of the ground furrow.  High particle velocity can also result in increased wear on the plastic air lines of an air seeder and excess consumption of fan power.  Too little air velocity can result in plugging of air delivery lines. 
The air velocity is difficult to optimize for a number of reasons.  The minimum carry velocity varies for different product mass flow rates and for different product types, both of which may change during a field operation. 
Another issue adding to the difficulty in optimizing the air flow velocity occurs when the configuration of the cart is altered.  The delivery hoses in the air system may be reconfigured in a number of ways upsetting the balance of the air flow system.  Even during field operation, if the air seeding system operates on a side slope, the resistance to particle flow will become greater in the line which have raised and vice versa for the lower lines. 
Prior art methods of monitoring the particle flow of a system generally count the seeds at some point in the delivery system to ensure the proper number of seeds is being placed into the ground.  Other systems simply monitor the air flow velocity, which provides a general indication of operating levels, but is not accurate since different products have different carry velocities. 
Many air carts are equipped with methods of adjusting the air flow through the delivery lines by means of damping mechanisms and fan speed adjustment.  One such system is disclosed in United States provisional patent application Ser.  No.  60/086,422.  This prior art system requires interaction of the operator to increase or decrease the air velocity. 
The drawbacks of prior art include the need for manual adjustments to damping mechanisms and fan speed, complexity of the manual adjustments, and lost time and money on operation and maintenance of the air carts. 
There are also further difficulties when the air cart is configured for double or triple shoot applications since each product may have a different carrying velocity. 
Thus, there is a need for a system of monitoring and controlling particle velocity automatically in an air delivery system