A bent open tube facing upstream in a uniform airflow regime is used to measure the static pressure in the duct. If connected to a vertical capillary tube, it will cause the fluid to rise, as in an inclined manometer. The pressure measured in this way is the static-free stream pressure. This is the static pressure engineers usually evaluate in the design of HVAC ductwork.
A slightly more complicated system can be used to determine the velocity of the air in the duct. A second tap is made in the duct that’s free of burrs and surface irregularities. (These blemishes will distort the airflow along the wall at the tap and consequently produce erroneous readings.) This tap measures the stagnation pressure of the airflow. The relationship between the static and stagnation pressures is the point-velocity at the static pressure measurement site. In most HVAC applications, with fairly high static pressure and low velocity, a wall tap will indicate the static pressure with good enough accuracy and resolution.
The combination pitot tube has two concentric tubes. The central tube measures the static pressure, as before, while the perforated outer tube measures the free-stream pressure. The two pressures can be tapped into a calibrated gauge to give a direct reading of airstream velocity.
There are many applications for these pitot tubes, although they’re sensitive instruments that must be carefully installed and maintained. The tubes must be kept free of dust and dirt. The wall tap must be made carefully and protected from damage and misalignment. Even the slightest alteration in its geometry can have serious consequences for the accuracy of measurement. This, in turn, can have a drastic effect on the energy efficiency of the equipment being controlled.
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Excerpt from Energy Conservation Projects.