When calculating the velocity of a gas or liquid through an object, scientists use a simple equation in which the volumetric flow rate is divided by the cross-sectional flow area. In some instances, however, the object that the gas or liquid moves through has some obstructions in it, which makes it more difficult to calculate the actual velocity. In these instances, a superficial velocity is often calculated in which the velocity is measure as if the obstruction were not there.
Measure the cross-sectional area of the object that the vapour moves through. For example if the object is a circle (such as a pipe) simply multiple pi, or 3.14, by the radius of the circle squared. Or A=(pi)(r2).
Measure the volumetric flow rate of the gas with your flow meter. The flow meter detects the flow and transmits a flow measurement signal.
Divide the volumetric flow rate by the cross-sectional area that you calculated in the first step.