The accurate measurement of air flow is an important factor in controlling many processes that affect our daily lives. In vehicles with electronic fuel injection, or oil or gas fired home heaters, incoming combustion air must be precisely measured to determine the right amount of fuel to atomize into it. Air flow allocation in central air conditioning systems must be carefully considered in large buildings. These are just a few applications among many requiring reliable and repeatable air flow metering systems.
Flow Parameters
Air flow is usually expressed in engineering units that give us the quickest assessment of what we may need to do about it. For example, expressing wind speed in cubic feet per minute is meaningless to a woman just leaving her hairdresser, but wind speed expressed in miles per hour velocity is an immediate indicator that she may want to protect her investment. Therefore the three basic ways that air flow may be expressed, velocity, volume or mass are usually with respect to the best or easiest use of the information in its application.
Stoichiometric Mass Flow Metering
Most modern automobiles have mass air flow sensors built right into a flow section right before the the throttle and intake manifold of the engine. Here the differential pressure across the flow tube is measured, corrected for atmospheric temperature and pressure readings, yielding instantaneous mass flow in pounds per minute at that throttle setting. From this and several other parameters, the engine computer further calculates the precise amount of fuel each fuel injector should meter into the cylinders at that moment. he same basic technique is used in all forms of combustion-related processes, such as furnaces, heaters, boilers, incinerators and power plants. Stringent metering makes possible highly precise emissions, environmental and pollution controls on exhausts and stack gases.
Volumetric Measurement
Modern heating, ventilation and air conditioning systems incorporate volumetric standard cubic foot per minute air flow sensors right into the air ducts to facilitate optimization of air flows to each area in the building. This is vital to the ever-increasing efficiency requirements of these systems.
Varied Technologies
Velocity, volumetric and mass air flows may use one or a combination of technologies to perform air flow metering. Calibrated hand-held propellers and turbines are among the easiest to use for open and ducted velocity measurements. Pitot tubes and calibrated piccolo tubes can be used to measure differential pressures, which vary proportionately as the square root to air flow.
Sophisticated Technologies
Another sophisticated technology is calorimetric flow measurement, which measures mass flow by comparing the instantaneous heat transfer coefficient of a slightly heated probe to the moving air to that of still air.
Velocity Averaging
Cross sectional air flows tend to be highest at the center of a duct or pipe, and lowest near the walls so measuring velocity at these points and averaging over the cross sectional area can be a good estimate of overall air flow.
