| Types of Water Flow Meters | | | | open channel (weirs, flumes) and ultrasonic flow |
| Accurately tracking the flow of water is a critical | | | | meters |
| need in many industrial and agricultural operations. The | | | | Differential Pressure or Head Meters |
| capacity to reliably obtain accurate water flow | | | | Differential pressure flow meters are the most |
| measurements is so important that it is often the | | | | common devices in use today. At least fifty percent |
| deciding factor between turning a profit and taking a | | | | of all liquid flow measurement applications use this |
| loss and in agricultural applications can mean the | | | | type of unit. |
| difference between having enough water for every | | | | Like most flow meters, differential pressure flow |
| need and shortages. In other cases, inaccurate water | | | | meters have two main elements. The primary |
| flow measurements or, worse yet, failure to obtain | | | | element causes a change in kinetic energy and that |
| readings at all can cause serious shortfalls or | | | | causes the differential pressure in the conduit. The |
| overages in supply which could potentially send entire | | | | secondary element measures the differential pressure |
| commodities markets into a spin. | | | | and provides the signal or read-out that is converted |
| So how is the flow of water measured? Water flow | | | | to the actual flow value. |
| rate is determined inferentially by measuring the | | | | Positive-Displacement Meters |
| liquid¡¯s velocity, or the change in kinetic | | | | Positive-Displacement meters separate liquids into |
| energy. Velocity is determined by the pressure | | | | defined segments before allowing them to pass. Each |
| differential forcing the liquid through a conduit. | | | | segment is then counted by a connecting register. |
| Because the pipe¡¯s diameter is known | | | | Because every increment represents a discrete |
| and remains constant, the average velocity indicates | | | | volume, positive-displacement units are popular for |
| the flow rate. The basic formula for determining liquid | | | | automatic batching and accounting applications. |
| flow rate in cases where the conduit diameter is | | | | Positive-displacement meters are also recommended |
| known and constant is: | | | | for measuring the flows of viscous liquids or when a |
| Q = VAwhere | | | | simple mechanical meter system is needed. |
| Q = liquid flow through the pipe | | | | Velocity Meters |
| V = average velocity of the flow | | | | Velocity based flow meters operate linearly with |
| A = area of a cross section of the pipe or conduit | | | | respect to the volume flow rate. Because there is no |
| Other factors that affect liquid flow rate, and should | | | | square-root relationship (as with differential pressure |
| be factored into calculations include the viscosity and | | | | devices), they have a greater range. Velocity meters |
| specific gravity of the liquid, as well as the friction | | | | are minimally sensitive to viscosity changes when |
| substance in contact with the pipe. | | | | used at Reynolds numbers above 10,000. Most |
| Direct measurements of liquid flows can be made | | | | velocity-type meter housings are equipped with |
| with positive-displacement flow meters. These units | | | | flanges or fittings to permit them to be connected |
| divide the liquid into specific increments and move it | | | | directly into pipelines. |
| on. The total flow is an accumulation of the | | | | Open Channel Meters |
| measured increments, which can be counted by | | | | An open channel is any conduit through which liquid |
| mechanical or electronic techniques. | | | | flows with a free surface. This would include tunnels, |
| Device Types | | | | non-pressurized sewers, partially filled pipes, canals, |
| There are several types of water flow meters | | | | streams, and rivers. Depth measuring methods are |
| available for closed channel (conduit) applications. The | | | | the most frequently used means of determining flow |
| different types of meters can be classified | | | | rate in open channels. Depth measuring flow meters |
| as:differential pressure (orifices, venturi tubes, flow | | | | assume the flow rate at any given instant can be |
| tubes, flow nozzles, pitot tubes, elbow-tap meters, | | | | determined from a measurement of the water |
| target meters, and variable-area meters) positive | | | | depth, or head. Weirs and flumes are the oldest and |
| displacement (piston, oval-gear, nutating-disk, and | | | | most widely used primary devices for measuring |
| rotary-vane) velocity (turbine, vortex shedding, | | | | open channel flows. |
| electromagnetic, sonic mass (Coriolis and thermal), | | | | |