A water manometer can be used to measure air velocities too.
This thing is called a velocity rake. It is used to measure the
thickness of the boundary layer. Very handy if you are doing a 
P51 type scoop at the back of your canard. If the boundary bleed is
too high it is too much drag if too low the scoop won't work as well
as it should. If in doubt about the boundary layer thickness this
is the tool to use. Note this is on the middle of the hood of
a first gen RX7 so the boundary layer is very thin. All the readings
are pretty much the same given that there are slightly different 
amounts of water in each U tube. Note the red food dye in the 
water so it can be photographed and seen easier. Also the first
one on the right is the pressure tap in front of the rad.

Mazda used the sledge hammer approach and used a huge 
rad air opening under the front bumper. This proves
the the air pressure in front of the rad is 100% of
dynamic pressure. Unfortunately the pressure in the engine
compartment is also rather high.

I made this mostly from drip irrigation elbows, straight connectors,
drip tubing and embedded it in hot glue on the sheet aluminum 
mounting bracket.

Paul Lamar
 
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