Environmental Engineering Reference
In-Depth Information
Open end
Free water
surface
Piezometric surface
Piezometer
figure 2.8 A container not under pressure where the piezometric sur-
face is the same as the free water surface in the vessel.
Pressure applied
Piezometric
surface
figure 2.9 A container under pressure where the piezometric surface is
above the level of the water in the tank.
The surface of water that is in contact with the atmosphere is
known as
free water surface
. Many important hydraulic measurements
are based on the difference in height between the free water surface
and some point in the water system. The piezometric surface is used to
locate this free water surface in a vessel, where it cannot be observed
direct ly.
To understand how a piezometer actually measures pressure, con-
sider the following example. If a clear, see-through pipe is connected to
the side of a clear glass or plastic vessel, the water will rise in the pipe to
indicate the level of the water in the vessel. Such a see-through pipe, the
piezometer, allows you to see the level of the top of the water in the pipe;
this is the piezometric surface. In practice, a piezometer is connected to
the side of a tank or pipeline. If the water-containing vessel is not under
pressure (as is the case in Figure 2.8), then the piezometric surface will
be the same as the free water surface in the vessel, just as it would if a
drinking straw (the piezometer) were left standing a glass of water.
In a pressurized tank and pipeline system, the pressure will cause
the piezometric surface to rise above the level of the water in the tank.
The greater the pressure, the higher the piezometric surface (see Figure
2.9). Increased pressure in a water pipeline system is usually obtained
by elevating the water tank.
Note:
In practice, piezometers are not installed on water towers because
water towers are hundreds of feet high, or on pipelines. Instead, pres-
sure gauges are used that record pressure in feet of water or in psi.
