Air pressure check problem

[TomBoh]

I am trying to measure the pressure of air flowing through the tube. Air flow is created by a small fan with a controlled speed attached to one end of the pipe. The other end of the pipe is restricted (not fully open). I have my digital pressure gauge attached at certain point on the pipe (small pipe attached parallel to the main pipe) and I adjusted the speed of fan to get to the value of 50Pa.

Now I have noticed that with different diameters of pipes linking main tube with pressure gauge I get different results. Generally the smaller the diameter of the hole where I attach my sensor the smaller the value of measured pressure. I confirmed this by installing reducer between the pressure gauge and main pipe - as I slowly close the reducer the pressure reading drops.

Why is it happening? I must be missing some fundamental rule on pressure measurement.

Thanks in advance

 

[FACS]

Yes, you are missing a fundamental rule.
Study the operation/theory of a Venturi pump, and the light should go off.



Charlie

http://www.facsco.com

 

[TomBoh]

damn I've created a Venturi nozzle.... thank's man