CFM Tool?

[Reed Wells]

I dont know what it would be called, but in the next month or so I will be running new dust duct work in my whole shop, ( about 2400 sqft. ) I will have some round about runs and would like to have a meter to show me what CFM I am getting at each drop. Anyone know where I would look and what it is called. Thanks, Reed

[Steven Wilson]

You want an anemometer. Roughly around $300 or more plus you'll need to know how to do the measurements.

[Tim Marks]

Typically people buy a Dwyer Magnahelic DP gauge. You measure the airflow by sticking a pitot tube into the airflow, and measuring several points (it will measure either inches of H2O or ft/min depending on the gauge you buy). I think you want something in the range of 0-2" of H2O or 0-4000 fpm.

You see these types of gauges going fairly cheap on ebay, however the gauge is not going to typically come with the pito tubes (static and dynamic pressure taps). And you will need to figure out a conversion chart for inches of H2O into fpm and then into cfm based upon duct sizing.

Not really very easy to do. You are pretty much just better off just going off of general thumbrules of which pipe sizing to use based upon your DC Hp, and then assuming that your airflow is good.

Don't make this into rocket science, no point in getting things that accurate. You don't NEED to know that you are getting 4500 fpm at a specific spot.

[Tom Veatch]

I dont know what it would be called, but in the next month or so I will be running new dust duct work in my whole shop, ( about 2400 sqft. ) I will have some round about runs and would like to have a meter to show me what CFM I am getting at each drop. Anyone know where I would look and what it is called. Thanks, Reed

I doubt that you'll find anything that will give you a direct readout on CFM, at least not for a price you might be willing to pay for a one-time use. There are too many variables involved.

I would recommend getting a hand held anemometer (example (http://www.weatherconnect.com/product.asp?itmky=441601&cat=20)) and use it to take several velocity readings across the duct opening. With the area of the duct and the average velocity across the duct opening it would be easy to calculate a CFM flow rate that would be a good approximation of the actual flow rate.

You could also rig up a pitot/static system using the same principles used to measure airspeed of aircraft. Done properly, that could be mounted in the inlet of the DC to monitor airspeed at the DC inlet under all usage conditions. That airspeed along with the proper fudge factor and duct area would give you the instantaneous CFM under any operating condition. However, the problem of the pitot tube becoming blocked from the dusty conditions might make a permanent installation impractical.

[Greg Funk]

You need a dwyer 166-6 (http://www.dwyer-inst.com/htdocs/PDFFILES/cat/airvelocity/160_cat.pdf) pitot tube and a manometer. I bought a used digital dwyer 475 model on ebay and there are usually a few for sale. There are different versions depending on the measurement range you need. For air velocity only you will be measuring about 1" WC for 800cfm in a 6" pipe. Model 475-0 will measure 0-10" which also allows you to measure static pressure which will be higher than the differential pressue of the pitot tube.

The meter measures pressure, either differential or static. To convert to CFM you need to account for the density and temperature of the air. They provide tables to allow you to do the conversion.

If you are interested in a Pitot tube let me know as I bought 2 by mistake.

If you want reasonably accurate measurement of CFM you need to put the Pitot tube in a straight section of pipe. There should be no bends for about 8 diameters of pipe in front of or behind the measuring point. So if you are measuring a 6" duct you should ideally have a 96" length of straight pipe with the pitot tube in the middle.

Greg

[Phil Thien]

My Kestrel anemometer correlates very nicely with the Dwyer I borrowed. You wouldn't want to use one of these if you're going to write a peer-reviewed paper on dust collections. But, for relative numbers, they just cannot be beat. You can simply place it over the end of your pipe and take a MPH reading. Use that # along with the diameter of the pipe (with a small adjustment to compensate for the area of the Kestrel) to compute CFM. If you are math challenged let me know and I'll provide more details.

Kestrel meters trace back to NIST.

http://www.nkhome.com/ww/wwindex.html