The speed or velocity called be a bear is measured in feet per minute or “FPM”. Air volume flow rate call Q is expressed in cubic feet per minute or “CFM”. We can directly major Q. It’s equal to the product at the average velocity and the cross-sectional area through which the air flows. This is Q equals V times A. There is pasteurized by gravity the barometric pressure at sea level. The barometric pressure about 29.92 inches of mercury and measured with a barometric. The barometric pressure is also called the absolute static pressure. Air starts moving because there is a difference in pressure between 2 points. Air moves from an area up higher pressure to an area of low pressure. The lower pressure in exhaust system is generated by the fan in is call the static pressure. The static pressure created by the fan is less than barometric pressure on the suction or inlet side on the fan. And greather then barometric pressure on the discharge or outlet side index . Equally in all directions in can be measured using a water manometer. Static pressure is expressed in inches of water. You might here the fan generates two inches of negative static pressure. Which means that the pressure inside the doc on the suction side at the fan is to inches of water less the pressure outside the dot. The pressure encountered in ventilation applications are relatively small