The present inventors have recognized that a pressure gradient or differential in a product distribution line for conveying granular particulate material, such as seed or fertilizer, in an air flow to an agricultural field consistently decreases as air speed (velocity) in the product distribution line decreases until a critical air speed is reached. Below the critical air speed, the particulate material may become susceptible to falling out of the air flow to potentially cause a blockage in the system. Accordingly, pressure sensors arranged in the product distribution line can be monitored for effectively controlling an air source producing the air flow. In one aspect, a diameter of the product distribution line can increase between upstream and downstream locations for monitoring with even greater sensitivity.
