Traverse calculator

How to use

Pick the method. For round duct, always use equal-area. For rectangular duct, equal-area is preferred, and use Log-T only when the spec or your supervisor calls for it. Choose a spot in a straight run where the readings are steady. A good rule of thumb is about 7.5 duct diameters downstream of a fitting, damper or fan and 3 diameters upstream of the next one. If you can't get that much straight duct, use the best spot you have and note it on the report.

Mark the probe. Every mark is measured from the inside wall of the duct. If you mark the probe from the outside, add the thickness of the duct wall and any insulation you're reaching through. The size you type is the metal inside dimension. The lining field is the insulation on the inside, and the marks start at the face of that lining. On round duct, drill two holes 90° apart and run one traverse line through each. Use 6 points per line on 6–9 in. duct, 8 points on 10 and 12 in. duct, and 10 points on anything over 12 in. Duct 5 in. and under can use one centerline reading × 0.9. On rectangular duct, drill the holes along the long side no more than 6 in. apart. A side under 12 in. needs at least 2 reading lines and a side 12 in. or more needs at least 3. In the grid, each row is one test hole and each column is one probe depth, so read straight down the row as you push the probe in.

Enter the readings. Pick FPM if your meter shows velocity, or VP if you're reading velocity pressure in in. w.c. from a Pitot tube and manometer. The calculator turns each VP into fpm with V = 4005 × √VP and then averages the velocities. Never average the VPs themselves. Enter a zero or negative reading as 0 and make a note. CFM is the average fpm times the duct's free area in ft². If fewer than 75% of your readings are above 10% of the highest reading, the airflow is too uneven, so move to a better spot and traverse again. Print form prints the numbers you entered so you can save a PDF and email it.