Calculating CFM hole size and delta P known

[David Meiland]

I'm looking to fabricate a small version of this device http://energyconservatory.com/products/products7.htm , which is used to measure airflow through a typical residential bath fan. It adjusts to three different known hole sizes, and typically connects to a manometer that has algorithms built in for displaying CFM directly. Low end of the range for this device is 10 CFM, which is too high. What I want to do is get approximate measurements of air leakage through recessed can lights when a house is depressurized to a specific level (-50 pascals with respect to outside is standard). I can use this information to determine how much improvement can be expected if the fixture is modified or replaced.

Is there a good formula to use for this purpose? I am expecting to drill a small hole (maybe 3 square inches) and measure in pascals.

Thank you for your help!

 

[256bits]

Wow
Laptop Computer Stand $ 130.00
Re-Usable Fan Shipping Box. $ 50.00
Kinda pricy

Their system has a problem of accuracy, which is only my gut instinct, or rather they do on this one. If you notice in the manual the formula for CFM from the pressure reading matches the chart only at the low end.

Try this
http://woodworking.com/forum/showthread.php?20658-Measuring-CFM

It actually is the same thing as the damper on the stove pipe for a wood burning stove or the interior oil stoves i- e space heater. The cfm is fairly low and may suit your purpose.