How to read a manometer

Written by allan robinson | 13/05/2017
How to read a manometer
Difference of Height in a Manometer (Wikimedia Commons)

A manometer is a device that measures pressure. It's typically called by a more descriptive name, such as a barometer, depending on the particular device. The term "manometer" typically refers to an instrument that measures pressure with a column of liquid. A simple manometer consists of a U-shaped tube that contains a liquid. If the pressure on one surface of the liquid differs from the pressure on the other surface of the liquid, the liquid will move away from the source of greater pressure.

Assume the left tube is open. Call the pressure on the left side Po. The right tube is connected to a source of positive pressure, such as a gas line with an airtight seal. Call the pressure on the right side Pa.

Measure the distance (x) between the liquid's current level on one side and its established zero point. Calculate the height (h) of the liquid being supported by the pressure differential Pa -- Po. Remember that if the surface of the liquid on the right goes down by distance x, the surface of the liquid on the left side goes up by x. The height h will therefore be given by h = 2x.

Provide the pressure in water inches. Assuming the liquid in the manometer is water, this is simply the value of height h in inches. The reference pressure Po is assumed to be one atmosphere (atm). Although non-standard, this is a common measure of pressure since it can be determined by a direct measurement.

Convert the manometer reading to standard units of pressure. This is given by the standard formula p = dhg where p is the pressure differential in pascals, d is the density in kilograms per cubic meter, h is the height in meters and g is the acceleration of gravity in meters per second squared. Therefore, the pressure differential Pa -- Po may be given as Pa -- Po = 2xdg.

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