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.
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| 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.
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| 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.
