Enter the velocity pressure reading recorded at each traverse point, in order across the duct.
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How to Use the Pitot Traverse Calculator
Choose round or rectangular duct and enter the diameter, or width and height, so the calculator can determine cross-sectional area.
Match this to how many readings you actually took in the field, laid out as your traverse method requires. ASHRAE 111 calls for at least 25 points on a rectangular duct.
Fill in the velocity pressure recorded at each point. Add the site elevation and air temperature if conditions differ from standard air, and the calculator corrects air density for the pressure-to-velocity conversion.
Click Calculate Airflow to see average velocity, total airflow, the air density used, and the spread between your highest and lowest point velocities.
Understanding Pitot Tube Traverses
Air moving through a duct doesn't travel at a single uniform velocity across the whole cross-section. Friction against the duct walls slows air near the perimeter, while air near the center moves faster. A single-point velocity reading, especially one taken near the duct wall or center, can significantly overstate or understate the true average, which is why a proper traverse takes multiple readings at defined points and averages them.
Why the Log-Tchebycheff Method
Rather than spacing traverse points evenly across the duct, the log-Tchebycheff rule, which sits within ISO 3966, the velocity-area standard for pitot tube measurements, places them at fixed fractions of the diameter or side length, weighted toward the wall where velocity changes fastest. On a round duct, each traverse diameter takes 6 points at 0.032, 0.135, 0.321, 0.679, 0.865, and 0.968 of the diameter, measured from the inner wall. On a rectangular duct, a line of 5 points sits at 0.074, 0.288, 0.500, 0.712, and 0.926 of the side length, and a line of 7 at 0.053, 0.203, 0.366, 0.500, 0.634, 0.797, and 0.947. This calculator does not place the points for you. It takes the velocity pressure at each point you measured and averages the velocities, which works for log-Tchebycheff and equal-area layouts alike.
Round vs. Rectangular Ducts
Round ducts are typically traversed along 2 perpendicular diameters, with points placed symmetrically from each duct wall inward. Rectangular ducts use a grid pattern across both width and height, since velocity can vary independently along each axis rather than radially. ASHRAE 111 calls for at least 25 points on a rectangular duct. For a side under 18 inches (450 mm), readings are taken at the centres of equal areas no more than 6 inches (150 mm) apart, with at least 2 points per side. The straight-run guidance below is written in equivalent round diameters, so a rectangular duct is first converted, which the rectangular to round calculator in duct design handles.
From Velocity Pressure to Airflow
Each traverse point gives a velocity pressure reading, which converts to velocity through the same relationship used in a single-point pitot tube measurement: velocity depends on the square root of velocity pressure divided by air density. Averaging the velocities from all points, rather than averaging the pressures directly, gives a more accurate mean velocity because velocity pressure and velocity are not linearly related. Multiplying that average velocity by the duct's cross-sectional area produces total airflow. Air density matters in that conversion. This calculator assumes standard air at 1.204 kg/m³ unless you enter the site elevation and air temperature, in which case it corrects the density. In Calgary, at about 1,045 m, skipping that correction would read velocity roughly 7 percent high, and the air density calculator in psychrometrics shows the density for any site.
Checking Traverse Quality
A well-executed traverse in a straight duct section, away from fittings and turns, should show a reasonably smooth velocity profile from wall to wall. Common field guidance drawn from ASHRAE Standard 111 and ISO 3966 puts the traverse plane about 10 equivalent duct diameters downstream of the last disturbance and 3 upstream of the next, although a minimum of 5 and 1 may give adequate results. This calculator reports the spread between the highest and lowest point velocities. Some spread is normal because air near the walls moves more slowly, so look instead for abrupt jumps between neighbouring points, a reading of zero, or a lopsided profile, which usually point to an elbow, damper, or fan outlet too close to the traverse plane.
Where This Fits in Commissioning
A pitot tube traverse gives one of the most reliable airflow numbers available in the field, which makes it a common reference point for verifying design airflow during test, adjust, and balance work, checking coil performance, or confirming duct leakage test setups are drawing the expected system airflow.
Frequently Asked Questions
A pitot tube traverse is a set of velocity pressure readings taken at multiple defined points across a duct's cross-section, rather than a single point. Averaging these readings accounts for the uneven velocity profile that exists across real duct airflow, giving a more accurate total airflow than a single-point reading from the airflow measurement calculator.
The log-Tchebycheff method is a point-placement rule for duct traverses, described in ISO 3966, that positions measurement points at fixed fractions of the duct diameter or side length, weighted toward the walls where velocity changes fastest. A round duct, for example, uses 6 points per diameter at 0.032, 0.135, 0.321, 0.679, 0.865, and 0.968 of the diameter from the wall. The average of the point velocities then represents the mean velocity in the duct.
It depends on the duct and the method. Log-Tchebycheff uses 6 points per diameter on a round duct, so 2 perpendicular diameters make 12 points. On a rectangular duct, ASHRAE 111 calls for at least 25 points, and a line of 5 or 7 points follows the log-Tchebycheff positions. A side under 18 inches (450 mm) uses the centres of equal areas no more than 6 inches (150 mm) apart, with at least 2 points per side.
In a straight duct section, ideally with about 10 equivalent duct diameters of straight duct upstream of the traverse plane and 3 downstream, although a minimum of 5 upstream and 1 downstream may give adequate results. Readings taken too close to an elbow, transition, damper, or fan produce an uneven, unreliable profile. Convert a rectangular duct to its equivalent round diameter to apply these distances.
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