Ok here's the situation. I worked for a company that makes air purifiers, they wanted a way to show the efficiency of their products at trade shows. Which is kinda hard all they had was a device that tells you how many particles per cubic foot of air there were. So I arranged 2 of these devices to be controlled via serial port to sample air inside a clear plastic sealed room and air from the trade show arena. It would then figure out how much cleaner the air was inside the room with the air cleaner than outside.
http://healthway.com/ieqlaunch.html At the bottom of the page is a second generation version they had a professional design.
ImageShack - Best place for all of your image hosting and image sharing needs This is a visualization of the old system shown in the picture. It required 2 particle counters plugged into the computer.
What I want to do is put a device between the 2 tubes from inside the tent and outside that "swaps" the sources of air by turning a knob on a small tube valve. The strength of the turning only needs to be like that of easy finger turning.
Does that help? 