The reason is simple to explain. When you drill out the intake and continue to use it as intake, you will get a wrong measurement of the flowrate because it's very unlikely that you can open it exactly(!) to 5.6mmTop Nurse said:Shoggy recommended that if you do this that you should put the intake in the output side. Not sure how that would affect anything, but I assume someone has already done the footwork on that one
But if you use the outlet as intake you can make sure that it has a diameter of exactly 5.6mm. Now you can use a value of 256 impules per minute because that's the value of a flowmeter which is identical in construction and has an intake with a diameter of 5.6mm.