by 84-1074663779 » Wed Apr 14, 2004 7:26 pm
Hmmmmm, difficult to put into words.
Any manometer works by pushing fluid up a tube, and it reaches a level depending on the applied air pressure. But the fluid that goes up the tube must come from somewhere.
For instance with a standard U manometer one leg goes up, and the other leg goes down by an equal amount. So you must measure the total difference in fluid height from one tube leg to the other. You could just measure the rise in one leg with a ruler, but the pressure would actually be double what you measure in that one leg, because the other leg has dropped an equal amount. (if you know what I mean).
If one manometer leg was vertical and the other leg sloped at some angle, you would get the same effect, but it would not be so simple to work out what was going on. You cannot just measure the fluid rise along the sloping portion as the total pressure reading. You could make a special scale calibrated in inches of pressure or %flow, but an inch of pressure would not be an inch of rise along the scale. But it could be done that way with a specially corrected scale.
A better way to build any manometer is to make a well type manometer. Here there is a large reservoir with a very large surface area. Imagine a bucket full of water. Dip one end of your manometer tube in the bucket and suck some water up the tube with a negative pressure.
The water in the tube rises, but the level in the bucket falls only a minute distance. So if your well is big enough you can then measure pressure directly with a true inch scale next to the tube.
The same thing can be done with a sloping manometer. If your well has enough surface area you can ignore any change in fluid height change in the well, and assume the zero point does not move. It is not difficult to make your well surface area 1000 times your manometer tube internal area.
But your well might only have ten times the tube area, better than nothing, and you are still going to need a corrected scale. Some of the commercial manometers have a well that looks like a syringe body. If you are making your own manometers, do not be stingy, build a decent well for it, and you can then use a stainless steel ruler or measuring tape at least on the test pressure manometer.
With a sloping manometer the actual pressure rise is going to be referenced back to the fluid height in the other leg or well, and this must be taken into account.