All it is is 10 meters of 0.3" clear plastic tube slung over a curtain rail with a bit of food-dyed water in it. To get it into the corner of my mouth while still being able to play a clean note, the rubber teat off an eye dropper works perfectly. Readings are simply the difference in water levels in the two legs of the tube loop for each pitch.
My readings were taken on the Yamaha 6335HS II with Wick 2W mouthpiece.
C3 - 5.1" WG
G4 - 8.0" WG
C4 - 11.4" WG
E5 - 14.0" WG
G5 - 17.7" WG
C5 - 21.1" WG
I'll see if there's any difference with my other instruments tomorrow. Let's put some numbers on 'stuffy'!