Question

The accompanying sketch is that of a closed-end manometer. Describe how the gas pressure is measured. Why is a measurement of   not necessary when using this manometer? Explain why the closed-end manometer is more suitable for measuring low pressures and the open-end manometer more suitable for measuring pressures nearer atmospheric pressure.

Answer 1

When the container is evacuated, the heights of both mercury columns are equal. As the gas pressure in the container increases, the heights of the mercury columns will change. Because the system is closed, Pbar is not needed. One drawback of this system is that gas pressure much larger than one atmosphere cannot easily be handled unless a very long column is used. (> 1 meter). Otherwise, the column of mercury would be pushed up to the closed end of the manometer at a critical pressure, and remain unchanged with increasing pressure.

Equal heights when evacuated. With increasing gas pressure, heights change. Drawback: High pressures need long columns to avoid critical point.