The open-tube manometer is another device that
can be used to measure pressure. The open-tube manometer
is used to measure the pressure of a gas in a container.
The pressure of the gas is given by h (the
difference in mercury levels) in units of torr or mmHg.
Atmospheric pressure pushes on the mercury from one
direction, and the gas in the container pushes from the
other direction. In a manometer, since the gas in the bulb
is pushing more than the atmospheric pressure, you add the
atmospheric pressure to the height difference:
Pgas > Patm
Gas pressure = atmospheric
pressure + h (height of
Pgas < Patm
pressure = atmospheric pressure - h (height of
You could be asked about a closed-tube manometer
manometers look similar to regular manometers except that
the end that’s open to the atmospheric pressure in a
regular manometer is sealed and contains a vacuum. In
these systems, the difference in mercury levels (in mmHg)
is equal to the pressure in torr.