my senior project is about develop the orifice flowmeter and I have to make our own manometer. At the first place we decide to open one side of the rubber tube and connect the other side connect with the pressure hole of the flange. So I have to use 2 rubber tube for the pressure before passing through the orifice and another one for pressure after passing the orifice plate. But when we turn on the pump, the water flow out from the main pipe to the rubber tube and overwhelming the tube so we cannot measure the pressure.
Then we decide to plug the tube instead of open to the atmospheric air and after test again the water change acceptably.
But I still not sure about the pressure calculation. In my calculation
(i will call the rubber tube before the orifice that tube 1 and another that tube 2.)
I measure the height of the air in rubber tubes then I know the volume of air at atmospheric pressure(the tubes already plugged). After run the pump the height of water in tube 1 will increase (tube 2 will decrease due to pressure drop) then I will have the volume of compressed air (expanded air for tube 2) which is now no longer it atmospheric. Then I use the ideal gas law P1V1=P2V2 to get the Pressure of each tube.
I still not sure that my calculation may have some condition that i miss and have to be considered.