When we test airflow, PSI is often too broad to show the small pressure differences that matter. That’s why the flowbench works in inches of water. A water column manometer is sensitive enough to show minute changes in pressure or vacuum by how far the water moves. Since 1 in. of water equals about 0.036 PSI, even a small fraction of a PSI creates visible movement in the column. That finer scale gives us a more precise way to measure pressure drop and compare one airflow part against another.
The original flow benches measured pressure drop with a water-column manometer. A manometer is a simple instrument that reveals very small pressure changes by showing how far water is displaced by either air pressure or vacuum.
To demonstrate how it works, a regulator is used to apply vacuum to the manometer. Even when the vacuum is opened only slightly, the water column moves noticeably. At roughly one-tenth of a PSI of vacuum, the displacement is already substantial, which illustrates how sensitive the instrument is to small pressure changes.
That sensitivity comes from the scale used by the manometer. One inch of water corresponds to 0.036 PSI, so relatively tiny pressure differences produce visible movement in the column. This makes the device well suited for measuring the small pressure drops typically evaluated on a flow bench.
The same sensitivity appears when the regulator is switched to positive pressure. At the lowest positive-pressure setting available on the regulator, the manometer still shows a clear response. In the demonstration, that setting was just over half a PSI, again underscoring how effectively a water-column manometer displays minute pressure changes.
