The tube is filled until both sides are approximately half full. When the pressures are equal, the column of liquid on each side will be at the same height. This is usually marked as zero on a scale. With both sides of the manometer open to the atmosphere, the fluid level on one side will be the same as the level on the other side be Ok… Now suppose one end of the U-tube manometer is connected to an unknown pressure P1 whose value must be determined. The other end is left exposed to the atmospheric pressure, P2.

The difference in the height of the liquid on the two sides of the tube is the differential pressure. In this case, the manometer provides a gauge pressure measurement because it is referenced to the atmosphere. OK… So the total difference in liquid height is 4 units. And this is where the pressure measurement becomes interesting! What does the 4 unit difference represent?

Let’s assume our U-tube manometer is filled with water. That means that our pressure differential is 4 inches of water column.

From our previous article, DP Flow Transmitter Testing and Re-Calibration, you know that inches of water column is a unit of pressure measurement. Using conversion charts, we could easily convert 4 inches of water column to 0.144 psi(g).

As you can appreciate, a U-tube manometer filled with water is only capable of measuring very small pressures.

A manometer is one of the most accurate devices for measuring pressure in the lower ranges.

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