Expert: Paul Soderman - 9/13/2005

Question
Hello, Thank you for you last reply i have found it very useful. I was looking at the diagram of a pitot tube on the Nasa website. I think that i could make one in a very short period of time but one thing is puzzling me. Measuring the air density of the air inside the tunnel? How could i calculate the density with a moving volume of air?

Thank You
Sam Logan

Answer
Hi Sam

At the low speed of most wind tunnels, air is almost incompressible.  So the density in the test section will be the same as outside the wind tunnel.  By measuring barometric pressure and temperature, you can compute air density using the standard gas law;

p = RT(rho)g

where R = gas constant, 53.3 ft lb/(deg F lb)   for air
         T = absolute temperature, deg R = deg F + 459.69
         rho = air density
        g = acceleration of gravity, 32.2 ft/sec^2
        P = air pressure

The above is for an ideal gas.  If you have humid conditions, you should correct for humidity, but I forget where my reference book on that is.  Books on wind tunnel technology or thermodynamics might have it.

Good luck.

Paul