Final answer:
To correctly read a classic water-filled manometer or "u" tube to measure gas pressure, you must consider the height difference of the mercury columns in the two arms of the U-tube.
Step-by-step explanation:
To correctly read a classic water-filled manometer or "u" tube to measure gas pressure, you must consider the height difference of the mercury columns in the two arms of the U-tube. In the case of an open-ended manometer, the gas pressure is equal to the difference in the height of mercury in the two arms of the U-tube plus the atmospheric pressure. In the case of a closed-ended manometer, the gas pressure is equal to the difference in the height of mercury in the two arms of the U-tube.