The pitot
tube was invented by the French engineer Henri Pitot in the early 18th century
and was modified to its modern form in the mid-19th century by French scientist
Henry Darcy. It is widely used to measure air velocities and pressures in HVAC
applications.
There are
main types of pitot tubes used in HVAC applications; static pressure pitot
tubes, total pressure pitot tubes, and velocity pressure pitot tubes.
Each has its use in a different application. The velocity pressure
tube is a combination of the first two and requires two connections.
Today we will demonstrate the differences between each of these three
types.
Static Pressure Pitot Tubes
Static Pressure Pitot Tube
The
static pressure only pitot tube is used to measure static pressure in a duct or
air stream. Note the small ports drilled into the side of the tube and
the closed end. In a typical application the tube is pointed directly
into the air stream. When the tube is pointed directly into the air
stream there is no component of the velocity pressure entering the static ports
the the tube will only measure static pressure. To implement this in a
live application the tube should be inserted into the airstream and connected
to a manometer. The tube should then be rotated until the manometer
indicates a minimum. It is at this point that the pitot tube is
theoretically reading only static pressure. Lock the tube into position
and proceed with calibration of any electronic sensors and transducers.
Total Pressure Pitot Tubes
Total Pressure Pitot Tube
The total
pressure pitot tube is used to measure total pressure, a combination of static
and velocity pressure in the duct or airstream. When the tube is pointed
directly into the airstream it will obviously measure velocity pressure.
In addition you need to recall that static pressure is also acting in
this direction and is indicated on the gauge as well. To install in a
live system you should insert the tube and connect the tube to a manometer.
The tube is then rotated until the manometer reads a maximum value.
It is at this point, the tube is capturing 100% of the velocity pressure
plus the static pressure. It should be noted that if the tube were
rotated until a minimum value were indicated, it would be measuring static
pressure only.
Velocity Pressure Pitot Tubes
Velocity Pressure Pitot
Tube
The
velocity pressure pitot tube differs from the static and total pressure pitot
tube in that it requires two connections. In reality, this tube design is
just a combination of the two previous tubes in one engineered package.
One of the ports reads static pressure and the other port reads total
pressure. If you recall from our last discussion on static pressure in
duct work, the velocity pressure is derived by subtracting these two values. To
install one of these, insert the tube into the air stream and connect a
manometer to just the total pressure side of the pitot tube. Rotate the
tube until the manometer is reading a maximum value. At this point lock
the tube in place and reconnect the tube so that the manometer is reading the
differential across the pitot tube. This is the velocity pressure.
The physical arrangement of the tube and the connections handles the
subtraction of the two physical measurements.
