Major differences between materials occur because of the combined effect of the 2:1 ratio in material flow rate for a given air flow rate, and the fact that for materials that are capable of being conveyed at low velocity, the slope of the curves in the dense phase conveying region can be very different. Type C materials typically take a downward curve and material flow rate reduces with decrease in air flow rate. Type A materials often continue to give an increase in material flow rate with decrease in air flow rate, but many others give little or no change in material flow rate as the air flow rate is reduced. The phenomena is considered in detail in Chapters 12 and 13.