I just purchased th dafm2 thermoanemoeter to test my airflow my question is
1) It ask me to deternine the free area of the air source I am measuring and enter it
2) IT ALSO SAYS FREE AREA IS PUBLISHE BY THE GRILL AND REGISTER MANUFACTURE YOU ARE SERVICING
3) NOT SURE WHAT THERE WANTING HERE BUT THE DEFAULT NUMBERS ON THE METER ARE 1.111 NOT SURE IF THE WANT THE SIZE OF THE BOOT BECAUSE OF THE DECIMAL AND THE NUMBERS BEHIND IT.
4) ANY IDEAS?
Every grill is different, in the back of the heart and cooley book (catolog) is a fairly good explination of free area, along with tons of numbers and I believe graphs.
Basically... free area... ='s free... area (hang tight) a 4x12 boot has a free area of 48". Throw a grill on that and it'll go down by 25% (throwing a number out, but it's a fairly common number). Because of the vaines and dampers that block the free area (area with out stuff in it) you have to have this divisor in there to accurately determine air flow.
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I've done many cfm tests using a vane anemometer and although it isn't the most accurate (a flow hood is the only 100% accurate way) you can get pretty close if you average many points across the register. You need to get the book for the registes you are measuring and go to the engineering data and find the Ak factor for the exact register you are measuring and input it in your meter then the meter will read direct cfm.
If you are looking for accuracy perform a traverse of the pipe feeding the grille you are testing with a hot-wire anemometer.
A hood is the next best in means of accuracy, if you have supply & return penetrations that are not sealed the hood readings will be way off at times.
The AK factors should be found in manufacturers literature but keep in mind these numbers were determined in a laboratory & can be subject to change in the field due to varying velocities.
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I don't like to measure duct discharge air flows at the grill as often I'll have no ides who made the grill and have no idea of the Ak factor.
Other factors if the adjustment is only obd's (outboard dampers) and not in line dampers the air flow is often deflected in such a way I doubt the readings.
The formula is ez. Measure the grill in inches, divide by 144 ( # of sq inches in a square foot because you want to know cfm cubic feet. ) multiply by Ak factor. Cfm = area x velosity.
Traversing the duct you wouldn't need to know the Ak factor as you would be measuring the velosity directly. Everything being measured is going through the grill.
I know a lot of guys buy these tools and without a TAB certifacation won't know what's required for an accurate measurment but it's really my last choice for accuracy. Not the instrument but the method.
Tracers work both ways.
The Ak factor definition from Hart & Cooley is this
Ak = net area in square feet. This is the lab measured area across the face when air is mechanically forced through the opening.
Therefore CFM= Ak x Velocity
We carry the H&C cateloges to look up the Ak factors when we are measuring airflows.
Several years back, Hart Cooley did a monthley Tech Letter. I sure miss it.
Below are attachments I have saved, and make a pretty good short read on the subject.
Thanks for posting those excellent articles.
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