Docedison:
You stated thhat there would be a "Dry Tube" and Lefty mentioned that Air will compress but a liquid won't... so your returned data is anyones guess.
I would disagree with Lefty as well; using a dry tube as the OP describes will give a valid reading.
Not sure if I can come up with a good real world example but I'd start by pointing out the millions of homes in the world with private water wells that rely on a pressure tank half-filled with air and read the water pressure from the top of that column of air. Should the air bladder in that pressure tank burst and all air be lost from the tank it doesn't result in any change to the water pressure in the home. Well, the water pump starts cycling more frequently but that's another issue ...