A manometer measures the difference in air or liquid pressure by comparing it to an outside source, usually a sample of Earth's atmosphere. There are several types of manometers, the simplest being a piezometer tube, which is a single tube and a base that holds the liquid. More common manometers are U-shaped and have interconnected tubes. Manometers are used in atmospheric surveys, weather studies, gas analyses and research of the atmospheres of other planets. They are usually made of glass or plastic, and while most are scored for measurement, some can measure changes digitally. The single-tube manometer measures only the pressure of a liquid, since there is no alternate place to compare gases. A U-shaped manometer essentially pits two different gas pressures against one another, and measures the strength of the captured gas. The free-flowing gas is usually air at the current atmospheric level.
Mechanics of Manometers
A liquid is placed in the tube, usually a responsive liquid like mercury that is stable under pressure. One end of the U-tube is then filled with the gas to be measured, usually pumped in so the tube can be sealed behind it. The other end is left open for a natural pressure level. The liquid is then balanced in the lower section of the U, depending on the strength of the gas. The atmospheric pressure pushes down on the liquid, forcing it down and into the closed end of the tube. The gas trapped in the sealed end also pushes down, forcing the liquid back to the other side.
Then a measurement is taken to see how far the liquid in the sealed end has been pushed either below the point of the liquid in the open end or above it. If the liquid is level, straight across in both tubes, then the gas is equal to outside air pressure. If the liquid rises above this level in the sealed end, then the air's pressure is heavier than the gas. If the gas is heavier than the air, it will push the liquid in the sealed end below the equal point.
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Since Earth's atmosphere can change based on elevation and temperature, the difference must be computed to reach an average atmospheric pressure. Otherwise, the manometer will show slightly different results at different elevations, making precise studies impossible.