Tools to measure air flow in a DC system?

[Greg Wandless]

Sometime back on the Pond there was a thread about the equipment needed to measure what the airflow was in a DC system. I have access to a Magnehelic gauge, but as I recall there a special tube that you hooked up to the gauge and inserted into the ductwork to make the measurement. I remember looking it up at MSC, it costed about $40, but I can't remember was its callled. Can someone refresh my memory.

Thanks,
Greg

[Mac McAtee]

It is called a "Pitot Tube".

[Greg Wandless]

You won't by any chance remember the web site that had the calculations etc...

Thanks,
Greg

[Daniel Rabinovitz]

It's been a long time!
Try looking up "Manual J"
See what happens.

[Harry Keller]

Try this:

http://www.dwyer-inst.com/htdocs/pdffiles/iom/airvelocity/160_IOM.pdf

I believe that the Pitot Tubes in the MSC catalog are from Dwyer as well.

Good luck,

Harry

[Dale Sherman]

Greg,
Take a look at pitot tubes at the Dwyer Instruments (http://www.dwyer-inst.com/htdocs/airvelocity/qs160.cfm#CRA) website. I use the 166-6 because it fits into the 2010-AV-port (http://www.dwyer-inst.com/htdocs/airvelocity/qsportmagavgage.cfm) magnehelic gauge kit I have. The 2002-AV is better for measuring flow, the 2010-AV is better for measuring static pressure.

Dwyer also has a webpage that tells all about using a pitot tube (https://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.html) . Actually, more than you probably want to know. I usually stick the pitot tube in 1/3 of the way across the duct. It gives a reading that is close enough to average for evaluating you DC system.

Dale

[Greg Wandless]

Dale,

Your 2010 has a range from 0-10" and 2002 is 0-2" H2O right. What is a typical reading? I just checked the gauge that I have and its 0 to 0.5" H2O and i have a feeling thats not wide enough. I was looking at the inclined manometers from Dwyer with a range of 0-3" H2O or should I go for a wider range?

Thanks,
Greg

[Doug Edwards]

A 1/2" gage will only measure to 2800fpm. Most systems will operate at 4000 - 6000 fpm. To measure this range you will need a 2" gage. If the price is too high, you can measure the velocity pressure using a piece of clear plastic tube with water in it. The reading may not be as accurate (the water is accurate, the measurement is difficult), but it can save you $50.

The calculation is simple: Velocity = 4005 * square root of the gage reading. CFM = duct area*velocity. (4" duct is 0.0873 ft2, 6" duct is 0.1963 ft2)

You should probably get a Dwyer model 167-6 or 167-12 pitot tube. This has a smaller tube diameter than standard. The larger diameter will not fit in ducts less than 6"diameter.

[Greg Wandless]

Thanks for all the info. :)

Greg

[Dale Sherman]

With a 3" gauge and a pitot tube, you can read velocities up to 6937 fpm, more than enough for most applications. An optimum air velocity of 3500 fpm would yield a reading of about .76" on your 3" gauge. The 167-6 pitot tube that Doug suggested has a little shorter nose than the 166-6 which makes it easier to use in small diameter ducts.

Static pressure readings on a DC could get up to 8" or more so, if you want to measure SP (static pressure) too, you'll want a 'U'-tube manometer (or higher-costing magnehelic gauge or digital gauge). As Doug suggested, make your own with a clear tube and add water. Add food dye to make it more visible.

Dale