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So recently I conducted an experiment to measure the pressure using pitot tube and use that to find the velocity profile which would be used to find mass flow rate. The whole experiment is rather long so I will only consider the first pressure. The pressure measured in pitot is in mmH2O and atmospheric pressure is 763.25 mmHg.

Experiment Setup:

Air pump blowing air at an unknown constant velocity into a horizontal large tube which narrows down to a tube with a smaller diameter(venturi). Pitot is attached to the small tube in the middle about 15cm away from where the small tube starts. A digital display is used to display the pressure from pitot. All of the apparatus is at the same level(horizontal), no elevation. the other end of the small tube is open.

Now stagnation pressure = dynamic + static pressure. So stagnation pressure must be the pressure measured by the pitot which appears on a digital display and the static pressure must be the atmospheric as it is 90 degrees to the flow. Am I right or wrong ?

So what I am doing is taking the pitot pressure 80 mmH2O and converting it to Pascals which gives me 784.5 Pa. Now when I convert the atmospheric pressure 763.25 mmHg to Pascals which gives me 101767 Pa which I sub back into my equation to find dynamic pressure gives me 784.5 - 101767 = Dynamic Pressure. To my understanding I should now be able to find the velocity using 0.5*desnity of air*v^2 = 784.5 - 101767 but as you can see it gives me a negative number which cant be square rooted. So I am thinking I am wrong can anyone explain to me how I might go about finding the velocity using the data from the pitot tube because really all I have is the pitot pressure, temperature of the room (18 Degrees Celcius) and the atmospheric pressure.

Thank you very much for any replies.