Manometer
Definition and meaning of Manometer in chemistry.
A manometer is an instrument used to measure the pressure of a gas or liquid, or the pressure difference between two points, by observing the height of a liquid column in a calibrated tube. The vertical displacement of the liquid indicates the magnitude of the pressure difference being measured.
In more detail
Manometers work by applying the principle of hydrostatic pressure balance: a fluid column rises or falls until the pressure it exerts equals the applied pressure. The most common type is the open-tube manometer, which has one end exposed to atmospheric pressure and measures gauge pressure (the pressure above or below atmospheric). Mercury is the standard measurement liquid because of its high density (13.6 g/cm³), which allows for compact instruments, and its negligible vapor pressure at room temperature. Water or other liquids are used for measuring very small pressure differences where greater sensitivity is required.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Primary Liquid | Mercury (Hg) |
| Principle | Hydrostatic pressure equilibrium |
| Typical Range | 0.1 to 2000 mmHg |
A simple open-tube manometer connected to a gas container might show the gas pressure is 25 mm above atmospheric pressure if mercury on the open (atmospheric) side rises exactly 25 mm higher than the gas side; this would be reported as a gauge pressure of 25 mmHg.
Frequently asked questions
Why is mercury preferred over water in manometers?
Mercury's high density and low vapor pressure make it ideal; its density allows for shorter tubes (13.6 times denser than water), and it does not evaporate significantly at room temperature, ensuring measurement accuracy.
What is the difference between open-tube and closed-tube manometers?
An open-tube manometer has one end exposed to the atmosphere and measures gauge pressure (pressure relative to atmospheric). A closed-tube manometer has both ends sealed and can measure absolute pressure or very low pressures near a vacuum.