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Another way to create a pressure difference in a flow of fluid is place a weir directly in the flow. A weir is a narrowing of an open channel to create an obstruction that is placed in the flow to cause a pressure drop as fluid flows through it (see Figure below).
When the fluid flow increases, the pressure drop through the weir will increase. One example of this type of application would be a weir placed in a canal used to move cooling water to a pond. Many industries use ponds for cooling water or too store waste water while it's being treated. it's important to get an estimate of the amount of water that is flowing into the pond if water-treatment chemicals are added, so a weir is used to measure flow. The pressure of the water when it's obstructed by the weir will cause the velocity of the flow to change slightly, which will cause a slight pressure increase. When the water exits the weir, its pressure will decrease slightly. This small amount of pressure difference can be used to calculate flow. Weirs are also used extensively in irrigation ditches to measure the amount of water flow. In these applications the amount of flow is totalized so the water can be sold.
ABOVE: Weirs are used to create a pressure
drop so flow can be calculated.
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