Pressure is defined as a force per unit of area. The most common and accurate way to measure air pressure is using the simplest form of a manometer (as pictured below) – where the balance of a liquid of known weight is balanced against the pressure source, and measured in inches.






Figure 1 – Image from Dwyer Intruments


A U-tube manometer, as featured above, basically works by filling to the halfway point with liquid and exerting air pressure on one of the columns, causing the fluid to be displaced, and one leg of the water column to rise as the other falls. The difference in height “h” which is the sum of the readings above and below the half way point, indicates the pressure in inches of water of the water column.


This simple U-tube manometer can indicate the difference between two pressures (differential pressure) – by pressurising both ends of the tube, or between a single pressure and atmosphere (gage pressure) – by leaving one side open to the atmosphere.


The U-tube manometer is a primary standard because the difference in height between the two columns remains true indication of the pressure regardless of variations in the internal diameter of the tubing – thus, even the Dwyer Slack Tube® roll-up manometer is as accurate as a laboratory instrument. This provides a real convenience to anyone who uses it, who may otherwise have to carry around a 60″ long rigid glass U-tube manometer.

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