On Sunday 19 January 2003 11:25 pm, John wrote: > A am measuring static air pressure. I need to know the static pressure in a > 4" pipe that has been reduced to 2". The air in the 4" pipe is too turulent > to measure accurately because of the transition to 2". So I would like to > measure the static pressure in the 2" pipe where the flow is much more > stable. However, I have found that the static pressure in the 2" pipe > decreases quite rapidly compared to the pressure in the 4" pipe as velocity > increases. Is there an equation that will let me measure the static > pressure in the 2" pipe and let me convert it to the equivalent pressure in > the 4" pipe if I know the velocity in the 2" pipe? Barry Michels wrote: > The velocity in the 4" section would be 1/2 what it is in the 2" pipe. No. Dave King wrote: > The pressure should be about ~1/4 in the 4 inch section as it has about > 4 times the area of the 2 inch. No. Barry Michels wrote: > I don't know if it applies to such a small diameter pipe, but the calculation > for air in ductwork is: > > CFM = Duct Sq Ft * 4005 * SQRT(Velocity Pressure inwc) Perhaps, but not relevant. This formula is for calculating the velocity by measuring the pressure change along a certain length of *constant-area* ductwork. The area of the 4" pipe is four times that of the 2" pipe. Assuming the same mass flow in each, the velocity in the 4" will be approximately 1/4 that of the 2" pipe. The pressure in both will be similar, except that it will be slightly lower in the 2" pipe because of the Venturi effect. Google on that final term to find formulae. -- Dave Tweed -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body