How to measure airflow in PVC piping requires careful measuring of airflow with a pitot tube or calibrtated anenometer to determine CFM

Measuring Airflow in Piping

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Accurately measuring Airflow in piping is never trivial. This page will review some of the ways it is done. But first let me describe the easiest way to use a pitot tube to make this measurement in common radon piping. This requires a magnehelic or digital micro-manometer and of course a pitot tube. The pitot tube and magnehelic can be purchased from Infiltec or directly from Dwyer.
Airflow in a pipe is not uniform. When air enters a turn (elbow) airflow is most disrupted coming out of the turn. The general rule is make airflow measurements at least 2 pipe diameters before an elbow and at least 8-10 pipe diameters after an elbow. To most accurately measure airflow in a pipe that has no airflow vanes to make the airflow more uniform you would average multiple pitot measurements using the following pattern. Not trivial as you can see.

In my lab I have carefully calibrated a flow grid using a lo-flow balometer to determine the actual airflow. I then made pitot tube measurements in the center of the pipe and obtained the following calibration figures for different common radon piping sizes.

Center Pipe - Pitot Tube - Calibration Factors

Pipe Size Calibration Factor

2" 91

3" 210

4" 294

6" 735

Drill a small hole in the side of the pipe in a location that will have the straightest airflow. Connect the side port of the pitot tube into the reference port of the digital micro-manometer or the magnehelic. Connect the bottom port of the pitot tube to the Inlet port of the digital micro-manometer or magnehelic. Be sure to aim the pitot tube directly into the air stream right in the middle of the pipe. I often wrap a small strip of duct tape around the pitot tube at the half pipe size distance on the pitot so I can more easily place it half way into the pipe. Set your Digital Micro-manometer to read in inches of water column. This is the velocity pressure. If it reads in Pascals take that measurement and divide by 249 to get the velocity pressure in inches of water.

Enter the velocity pressure into a calculator and press the square root function. Multiple this result times the calibration factor listed for the pipe size you are measuring from the table above. This is the approximate CFM flowing through your pipe.

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