Measuring air flow

[WDwonder]

What tool and brand are you using to measure the CFM? I have looked at an anemometer but everything I look at looks to be too small to measure the flow of a 2 HP dc. I have the HF doc.  Thanks for your help.

[retired2]

I purchased a Kestrel 1000, and I believe Phil also uses a Kestrel, but a different model.  The 1000 is good for 89 MPH.  It will go higher, but the accuracy is not guaranteed above that level.  I followed Phil's advice for taking readings.

You are correct, most of the affordable anemometers are at the low end of the range for DC measurements.  That problem can be solved with one of Phil's tricks, i.e just increase the line size where you plan to take your readings.  As the line size increases the velocity will decrease. 

To make sure you won't suck the vanes out of the instrument you need to do a little math to make sure the line size where you are measuring will have a velocity low enough to be within the operating range of your meter.  Start by using the rated CFM of your DC.  It will almost certainly be overstated, so it is a good safe number to use.  Convert that CFM to FPM for the pipe size you are going to use for measuring.

If you are considering a HF unit, I can almost guarantee you the Kestrel 1000 would be very comfortable taking measurements at the end of a 6" pipe.  The Kestrel will give you velocity in FPM which you can then convert to CFM.

Phil suggests taking readings at the edge of the pipe and in the center, and then average them.  I would add to that, repeat this about three times and then average.  You will find the instrument is very sensitive, and consequently you will see some variation from one reading to the next.

[WDwonder]

With a 6" pipe and 10' long?

[retired2]

With a 6" pipe and 10' long?

I'm afraid you are going to have to ask your question with a complete sentence because I'm not sure I understand what you are asking.

[WDwonder]

You said to take the measurement at the end of a 6" pipe. I wanted to know how long that pipe needs to be?

[retired2]

You said to take the measurement at the end of a 6" pipe. I wanted to know how long that pipe needs to be?

I would think a foot or two of straight pipe would be adequate.  I don't think it would be advisable to take readings close to any fittings that might disrupt the air flow.  Actually air entering a straight pipe is somewhat turbulent, that is the reason for two readings, one at the wall and one in the center.

[WDwonder]

Thank you.