How to Read a U-tube Manometer

Updated February 21, 2017

The pressure difference between two places is measured by physicists using a U-shaped manometer. A manometer consists of a U-shaped tube connected at one end to a closed reservoir, with the other side open to the surrounding atmosphere. By placing a liquid of known density, or compactness, into the U-shaped portion, you can calculate the pressure difference between the closed reservoir and the air outside the manometer by measuring the height difference between the liquid in the left and right parts of the U-tube. In science, pressure is measured in pascals, so metric measurements must be made.

Measure the vertical distance in inches between the top of the liquid in the left and right portions of the U-shaped tube. As an example, assume a vertical distance of 16.0 inches.

Convert the vertical distance to meters by dividing by 39.37, since a single meter contains 39.37 inches. Using the example, you have 16.0 inches divided by 39.37 inches per meter, which equals 0.41 meters. Metric units must be used since scientific equations utilise the metric system.

Multiply the vertical distance by 9.8 meters per second squared and by the density of the liquid in kilograms per cubic meter to obtain the pressure difference in pascals between the closed reservoir and the surrounding air. The quantity 9.8 meters per second squared is the acceleration due to gravity on the earth's surface and represents how rapidly a falling object accelerates. For the sample calculation, for the liquid use mercury, which has a density of 13,600 kilograms per cubic meter. Concluding the example computation leads to 0.41 meters times 9.8 meters per second squared times 13,600 kilograms per cubic meter, resulting in a pressure of 54,645 pascals.


Change a pressure measurement in pascals to pounds per square inch by dividing by 6,894.8.

Things You'll Need

  • Tape measure
  • Calculator
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About the Author

William Hirsch started writing during graduate school in 2005. His work has been published in the scientific journal "Physical Review Letters." He specializes in computer-related and physical science articles. Hirsch holds a Ph.D. from Wake Forest University in theoretical physics, where he studied particle physics and black holes.