At 18:32 8/18/99 -0600, Anthony L. Mourkas wrote: >So which is it? Does one measure the difference in water levels from one >side of the "U" to the other after application of pressure? Or does one >measure the total distance that the gas has lifted the liquid from a zero >point on one side of the "U"?

The former. In a U-tube manometer (or any siphon application) the total head is the vertical difference btw the upper and lower water surfaces.

Bulley's method, quoted here:

>You make a "U" shape out of the tube, with the 0-11 scale you marked on the >left leg of the U, and the right leg of the U attached to the regulator. >You fill the U with enough water that the base of the meniscus (the >waterline) comes to 5.5", filling the bottom of the U.

will work (if you attend carefully) but is IMO both unnecessarily complicated and subject to measurement errors, as well as obscuring what's actually going on. Simply adding water so that it is at least say 15" deep in each arm, and there is at least 15" of tubing above the water in the arm that's not connected to the gas, then applying pressure and putting a ruler btw the upper and lower menisci is all that's necessary.

I once used a device called the Van Slyke apparatus for determining CO2 content of blood serum, and it had a complicated procedure with turning valves and raising and lowering a big reservoir of mercury so that you could read the answer as an absolute instead of a difference. It was a blast, unless you dropped the globe of mercury, but a lot more than we need for this application.

Incidentally the pros nowadays use a bourdon-tube meter just like an oil-pressure meter for this measurement, but the manometer is actually more accurate as long as you keep it in a one-gee field and measure the *vertical* distance.

david David Beierl - Providence, RI '84 Westy "Dutiful Passage" '85 GL "Poor Relation"