Thank you everyone, Russel, for the responces. I built an air flow measuring device that measures pressure drops on one side of the device being tested. While at first I was happy with just knowing if there was any changes in flow rates, I began to wonder if I could actually calc the cfm instead of just detecting changes. When I pass air through a 1.12" orifice, I get a pressure drop of about .725psi from ambient (let's say barrometric pressure is 30.0) ----- Original Message ----- From: "Russell McMahon" To: Sent: Tuesday, October 01, 2002 3:47 PM Subject: Re [EE]: Calc. CFM from pressure drops > ** Is it possible to calculate, or estimate, CFM through an orifice of a > known size by the pressure, psi, on each side of the orifice? This would be > ambient air at about 75 degrees F and, say, 50% relative humidity. ** > __________________ > > There is an easy way to set an UPPER limit - the maximum flow velocity is > sonic in the "throat". While sonic velocity will vary with temperature and > pressure, this fact will give you a first approximation. Sonic flow occurs > for pressure differentials above about 30 psi AFAIR. Searching for "De Laval > nozzle" will give you leads to rocket nozzle design which is what you are > ending up with (even though it doesn't look much like one :-) ). > > RM > > > Somewhat technical intro (but it's going to be) > I think this probably will allow you to work out what you want to know > > http://msowww.anu.edu.au/~geoff/AFD/De_Laval_Nozzle.pdf > > Another > > http://astron.berkeley.edu/~jrg/ay202/node100.html > > Flow through an orifice - just about spot on (maybe :-) ) > (See 'Sonic flow from the end of a pipe') > > http://www.optimal-systems.demon.co.uk/appendix-e.htm > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.