The speed, or velocity, of fluid or gas flow through a pipe or vessel. Also the reservoir flow rate, which is one of the parameters used in flow testing analysis to determine the quantity of gas available in a natural gas field. Even when favorable geological, geophysical and petrophysical data are available for exploration or development activities in a field, there is no alternative of knowing that the wells in the area of interest would actually flow the desired fluid(s) at economical rates. This means that dynamic evidence of recoverable hydrocarbons is needed eventually. Well testing is one of the most effective means of characterizing hydrocarbon reservoirs under dynamic conditions. Such characterization of reservoirs is as accurate as the measured rates and pressures, fluid properties and relevant geological and petrophysical information. It is, therefore, essential to have accurate measurements of both flow rates and pressures due to their complementary nature in given reservoir system. Although in most cases, pressure measurements are accurate, more attention needs to be paid to the flow rate measurements, especially during the early stages of exploration.