Measuring Airflow with a Pitot Tube
How to Accurately Measure Airflow with a Pitot Tube
Pitot tubes are well suited for mid to high velocity airflow measurement, and are considered the standard by which all other airflow devices are compared. Accurate measurement requires density correction and a careful traverse. Because the accuracy is dictated by the pressure measurement device attached to the Pitot tube, there are often more economical ways (hot wire and vane) to measure airflow in low flow applications. For high flow or high temperature applications the Pitot tube is ideal.
A Pitot tube measures total pressure and static pressure to determine the velocity pressure from which air velocity can be derived. The Pitot tube is inserted into the duct with the tip pointed toward the airflow. The positive port of the manometer is connected to the total pressure port (Pt) and the negative to the static pressure port (Ps). The manometer will then display velocity pressure which can be converted to velocity.
Photo courtesy Dywer Instruments
In modern Pitot tubes, proper nose or tip design - along with sufficient distance between nose, static pressure taps and stem - will minimize turbulence and interference. This allows use without correction or calibration factors. All Dwyer Pitot tubes are built to AMCA and ASHRAE standards and have unity calibration factors to assure accuracy.
To insure accurate velocity pressure readings, the Pitot tube tip must be pointed directly into (parallel with) the air stream. If the Pitot tube is correctly aligned, the velocity pressure indication will be at its maximum.
Accurate readings cannot be taken in a turbulent air stream. A Pitot tube should be inserted at least 8-1/2 duct diameters downstream from elbows, bends or other obstructions which create turbulence. To insure precise measurements, straightening vanes should be located 5 duct diameters upstream from the Pitot tube if used.
Air velocity is not laminar or equal in across sectional area of a duct so a traverse of the duct needs to be performed to determine an average velocity. Friction closer to the walls of the duct will slow down the airflow as the are scrubs the duct walls. To assure accurate measurement, a defined pattern needs to be followed.
Photo courtesy Dywer Instruments
Dwyer offers a great primer to measuring airflow with a Pitot tube and other methods here.