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The piezometer, a simple device, provides a measure of the pressure at a location in a fluid being that fluid moving or not moving.
The "piezometer" is a pressure measuring device of antiquity (and with a few applications now). How it works is our interest.
The image depicts water flowing through a long pipe. The pipe elevation is above sea level; the water flows "to the sea." The hydrostatic principle can be used to determine the pressure of the "flowing" water in the pipe.
Use the piezometer reading to calculate the pressure within the flowing water.
♦ Again, we start where the pressure is known; the surrounding atmosphere. The hydrostatic path will follow the center-line of the piezometer to the center of the water flow.
Outside of the pipe (at Z=0) the pressure of air is Standard. Defined there to be 14.7lbf/in².
patm - ρairgo(12 ft) + ρwatergo(12 ft) = pwater
An interesting aspect of the piezometer is that, in effect, the height of a standing column of water tells the pressure of a flowing stream. Both the water in the column and the flowing stream are "static" (recall the meaning of "static?").