Differences Between a Pitot Tube And a Static Pressure Tube

One of the questions I get asked a lot from those who are new to airflow measurement is in regards to the differences between a Pitot tube and a static pressure tip. When I first started learning to measure pressures and airflow, this question was at the top of my list too. Depending on where I looked, there wasn’t a lot of information on this subject. This led to a lot of unanswered questions on my end. It’s my hope that if you have some of the same questions that I did, you’ll have a better understanding of the differences and similarities of these two pressure measuring accessories by the end of this entry.

The Static Pressure Tip

The static pressure tip is going to be the most common accessory used for measuring static pressure in an HVAC system. They are typically 4 inches long (depending on the manufacturer) and have a magnetic base to keep the tip from falling out of a duct system once it has been inserted. Pay special attention to the magnet as it falls off the base of the tip easily unless it’s permanently attached.

 

Static Pressure Tip

The end of the static pressure tip is completely closed off ensuring the accessory only picks up a static pressure reading. This pressure reading is obtained through four small openings drilled into the sides of the static pressure tip. When measuring, the end of the static pressure tip should be placed where it is parallel with the direction of the airflow in the duct.

The Pitot Tube

Pitot tubes have been a mainstay in the air balancing profession for decades. Although the origin of the Pitot tube dates back more than 200 years, they are still widely used by air balancers today due to their rugged dependability. The Pitot tube is constructed of two tubes, with one inside the other. They typically range in length from 12” to 60” for use in a wide variety of duct sizes. The Pitot tube directly measures total pressure and static pressure. If both ports of the accessory are hooked to a manometer, velocity pressure will be directly displayed. Let’s look at the pressures capable of being measured with a Pitot tube.

Total Pressure

Total pressure is the combined pressure of static pressure and velocity pressure created from air moving through a duct system. The end of the Pitot tube, which is inserted into the airstream, has an opening that feeds total pressure into the inner tube. Total pressure is then capable of being read by attaching a hose to the opposite straight end of the Pitot tube and attaching it to a manometer.

 

The Inner Workings of a Pitot Tube

Static Pressure

Static Pressure is an outward pressure created as air moves through a duct system that is similar to blowing up a balloon. Static pressure enters the outer portion of the Pitot tube through a series of small openings drilled several inches back from the insertion end of the Pitot tube that are perpendicular to the airflow. Static pressure is then fed to the right angle port by traveling through the outer portion of the Pitot tube. When a hose is attached to the right angle end of the Pitot tube, static pressure will be read.

Velocity Pressure

Velocity Pressure is a moving pressure created by the force of air moving through a duct system that is similar to releasing the pressure from a balloon. Velocity pressure is what propels the balloon. Velocity pressure is not measured directly with the Pitot tube but is calculated by subtracting static pressure from total pressure. When a Pitot tube is inserted into the airstream and both ports of the Pitot tube are connected to a manometer, Velocity pressure will be displayed directly on the manometer. Velocity pressure can then be used to calculate air velocity in a duct system.

If you’ve never had the opportunity to experiment with a Pitot tube, I would highly recommend it. If you have never had the opportunity to use a static pressure tip, what are you waiting for? Today is the perfect day to start testing.