On Wed, 17 Jun 2009 08:30:45 -0700 (PDT), ContestID67
wrote:
[snip]
at the vario/bottle. Then create some slight positive pressure on the
system (ship's tubing to vario to capacity bottle) by lowering the
open end. I would think that the open end would need to be sealed to
create this pressure.

No, the end away from the probe connection should be open. The
pressure is provided purely by the unbalanced weight of water in the
two arms of the U.

Since you have the system pressure on one end and atmospheric pressure
on the other, you're measuring the "gage" pressure, or the difference
between the two. One inch of height difference between the arms
represents about 0.036 psi.

You can make the gage pressure either positive (water level higher in
the outer arm) or negative (higher in the inner arm), but negative is
safer: if the system is indeed leaking under positive pressure, the
water level in the inner arm will gradually rise, and if you aren't
sufficiently careful, it can get pushed into the system.

rj