Why is static pressure measure in “inches of water column”? Why not PSI?

Water column (wc) is related to PSI, or pounds per square inch.

1 psi = 28 in. wc
1 in. wc = 1/28 psi

1/28 psi is a small value. Using “in. wc” helps us represent these smaller pressure values in larger units

The History and Reason for Inches of Water Column
U-tube manometers used in older systems to measure static pressure were installed with water inside a u-shaped portion of the piping. As gas or some other medium filled the piping, it pushed the water through the u-tube and created a column of water. As the pressure of the gas or medium increased, it pushed the water even higher up on that column. This column, as you can imagine, was measured in “inches.”

Here’s a YouTube video you can check out for more information.

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