Computing CFM

[Larry Blighton]

Ok I am pretty much done setting up my dust collection system and wanted to try and take some CFM measurements for my system. I watched alot of videos and was also able to borrow an anemometer over the weekend and did some readings. The anemometer even had a setting for CFM, However the problem I have now is that that the anemometer in CFM mode gave me a reading in thousands not the hundreds.

My system is completely made up of 4" PVC with minimum flex hose lengths (all less than 5').
So what do I need to do with this number to get my true CFM reading?

[glenn bradley]

These threads have a way of taking off into the twilight zone. At the airspeeds a home shop might normally see, a 4" pipe will move somewhat less than 400 CFM. 395 CFM at 4500fpm is a number I recall but, I am sure folks will start chiming in with more math than you can ever use ;-)

[Rob Luter]

V x A = CFM.

Measure the ID of the 4" PVC. (D x D X .7854)/144 is area in Sq Feet. FPM Velocity x Area = CFM.

[Malcolm McLeod]

... The anemometer even had a setting for CFM, ...
So what do I need to do with this number to get my true CFM reading?

The anemometer can't calculate CFM unless it has the area of the 'duct' (or other measurement area) entered into its configuration/calibration settings. Or, maybe it is designed for only one duct..?? (No idea of its specs.)

V x A = CFM.

Measure the ID of the 4" PVC. (D x D X .7854)/144 is area in Sq Feet. FPM Velocity x Area = CFM.

^+1 - - Skip 'direct to CFM' and use Rob's method. Make sure you measure in FPM, so units match.

See other threads regarding uncertainty introduced by fan anemometers; more better good to use hot wire.

Best of all is dPress from pitot tube set ~2/3 off center of duct (position makes sure you get decent average of velocity profile across duct's diameter. Or carefully transit the pitot across the diameter, recording values to build a real velocity profile, then integrate the area, calcu...zzzssscckkKK!...
....And now return control of this thread to Glenn. Hope you have enjoyed this brief transit of ... The Twilight Zone.

Engineers: Ask 'em the time; they tell you how to build a clock!

[Art Mann]

If the anemometer you are using is a little fan type, then your measurements are probably so far off as to be unusable. When you put the device in the air stream, it partially blocks the air flow. Since the area is constricted, the air velocity near the fan blades could either increase or decrease by a huge margin, depending on how close to the pipe you hold the meter. To make usable measurements, you need what is called a hot wire anemometer and it needs to be inserted into the air stream through a small hole somewhere upstream of the end.

[Mark Carlson]

I have a couple of cheap anemometers that measure mph. I use both to get an average reading, then use an online calculator to convert to cfm. I then write the mph/cfm readings on the port so I can come back in the future and take the same measurement to see how my filter is doing or whether I have a restriction. I'm more interested in the relative reading then whether its supper accurate. It appears to be in the ballpark of what I'd expect given my DC and ducting. This is a link to an online air flow calculator. Enter your flow rate in what ever unit you choose and duct size, hit calc. https://www.engineering.com/calculators/airflow.htm

I dont do this often because I have a manometer hooked to my DC so I can monitor the filter performance and like others have mentioned its not easy to get accurate flow rates with cheap anemometers. Its still an interesting exercise.

[Richard Coers]

Curious about your goal with the calculation? Do you intend to tear down the system and change it if the CFM calculation is low? Time to calculate CFM is in the design stage.

[Doug Garson]

Retired engineer here, yes CFM = velocity x area but the velocity in a pipe is not constant over the full area. The velocity near the walls of the pipe are less than the average (due to friction) and the velocity in the center is more than the average. To get the true CFM you need to measure the velocity at several locations across the diameter (called a traverse) and calculate the average, there are standards as to how many measurements and at what location and how far away from any bends or obstructions upstream or downstream.

[Rob Luter]

Engineer looking forward to retirement here . In a past life I worked in commercial and industrial ventilation. I've conducted a number of multi point pitot traverses. In this instance it would be like measuring a 2 x 4 with a micrometer. An anemometer will be close enough. As an alternative you could measure the pressure drop across the opening with a U -Tube or an inclined manometer. and use the entry coefficient to calculate airflow. This would also be a close approximation. One of the most accurate methods is to compare fan RPM and motor amp draw with an accurate fan curve, but then you need to know RH, Temp, and Barometric pressure to convert from Standard Air. That's one of the challenges of trying to measure a compressible fluid. It's always a close approximation. Air is hateful stuff that way.

[Doug Garson]

Agree with Rob, my airflow measurement experience is more at the industrial level with duct dimensions measured in feet not inches. Multi point traverses in a 4" pipe would be difficult and not worth it. Getting back to the OP, not sure why your anemometer readings are so far off. Accurate readings are difficult to get and probably of limited value. A better approach may be to get a Dylos or other air quality measurement device and monitor your shop air quality with the system in operation.

[Ben Rivel]

Keep an eye out on eBay for one of these (LINK). Sometimes they can be found for very cheap.