The pitot tube traverse is the most accurate and reliable method for measuring airflow through a duct. Traverse readings should be taken on all fans, supply and exhaust. When finding a location for your traverse make sure the duct is straight and about 6 to 10 duct diameters downstream from any elbows, transitions, or branches. The pitot tube tip should always be pointing into the airstream. The pitot tube traverse is also a good way to compair two different methods of airflow measurement. A good example is reading a sidewall diffuser with your flow hood and comparing that reading with a duct traverse taken upstream. If the readings do not match then an "Correction Factor" may be needed. Now lets talk about how a pitot tube traverse works. There are three pressures that exist in a duct that has air flowing through it.