A gas manometer measures gas pressure using a U-shaped tube filled with liquid, with one end connected to a pressurized gas container. Open-tube gauges use atmospheric pressure to measure gas pressure, while closed-tube gauges avoid this. Digital gauges are also available.
A gas manometer is a device used to measure the pressure of a gas. Pressure gauges can be closed-tube or open-tube and can use water, mercury, or another liquid to measure pressure. The simplest laboratory gas manometer consists of a U-shaped glass or plastic tube connected to a pressurized gas container at one end and partially filled with liquid. Digital pressure gauges are also sold, which are widely used for gas pressure control for household appliances.
The pressurized gas exerts a force on its container. For example, a balloon stretches when it is inflated: the internal air pressure becomes greater than the external air pressure and pushes the elastic material of the balloon outward. A gas manometer uses this principle to measure the pressure of a gas in a container against atmospheric pressure or against a vacuum.
An open-tube gas pressure gauge consists of a container filled with pressurized gas and connected to one end of a U-shaped tube, open at the other end. The side of the tube furthest from the gas container is calibrated with measurement markings. Liquid mercury or water is used to fill the bottom of the U-tube.
If there is a pressure difference between the gas and the atmosphere, the liquid in the U-tube will be forced in one direction. For example, if the gas in the container is at a pressure above that of the atmosphere, the liquid will be forced down the U-tube away from the container. It will rise along the calibrated marks toward the open end, providing a measurement that can be used to infer gas pressure. If the atmospheric pressure is higher, this pressure will push the liquid towards the container, causing the liquid level to drop along the calibrated notches.
Since both atmospheric pressure and gas pressure act on the liquid in the pipe, both must be taken into consideration when finding the pressure of the gas contained. When the gas pressure is greater than atmospheric pressure, the gas not only pushes the liquid up the opposite tube, but also pushes the air above it. This means that atmospheric pressure must be added to the liquid height measurement to obtain the true gas pressure. If the gas pressure is less than atmospheric pressure, the reverse is true: the height measurement is subtracted from the atmospheric pressure to find the true gas pressure.
A closed tube pressure gauge uses the same configuration, but with a closed end that holds the vacuum instead of being open to air. This type of gas pressure gauge avoids the emission of atmospheric pressure. Since no other pressure source acts on the opposite end of the tube, the height of the liquid is determined solely by the pressure of the contained gas. The gas will force the liquid some distance down the tube into a vacuum, at which point the pressure can be read.
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