A pressure gradient or differential in a product distribution line for conveying granular particulate material, including at least one of seed or fertilizer, in an air flow to an agricultural field consistently decreases as air speed and velocity in the product distribution line decreases until a critical air speed is reached. Below the critical air speed, the particulate material is susceptible to falling out of the air flow to cause a blockage in the system. Pressure sensors arranged in the product distribution line are monitored for effectively controlling an air source producing the air flow. A diameter of the product distribution line increases between upstream and downstream locations for monitoring with greater sensitivity.