Types of Water Flow Meters

Accurately tracking the flow of water is a critical need in many industrial and agricultural operations. The capacity to reliably obtain accurate water flow measurements is so important that it is often the deciding factor between turning a profit and taking a loss and in agricultural applications can mean the difference between having enough water for every need and shortages. In other cases, inaccurate water flow measurements or, worse yet, failure to obtain readings at all can cause serious shortfalls or overages in supply which could potentially send entire commodities markets into a spin.

So how is the flow of water measured? Water flow rate is determined inferentially by measuring the liquid¡¯s velocity, or the change in kinetic energy. Velocity is determined by the pressure differential forcing the liquid through a conduit. Because the pipe¡¯s diameter is known and remains constant, the average velocity indicates the flow rate. The basic formula for determining liquid flow rate in cases where the conduit diameter is known and constant is:

Q = VA
where
Q = liquid flow through the pipe
V = average velocity of the flow
A = area of a cross section of the pipe or conduit

Other factors that affect liquid flow rate, and should be factored into calculations include the viscosity and specific gravity of the liquid, as well as the friction substance in contact with the pipe.

Direct measurements of liquid flows can be made with positive-displacement flow meters. These units divide the liquid into specific increments and move it on. The total flow is an accumulation of the measured increments, which can be counted by mechanical or electronic techniques.

Device Types

There are several types of water flow meters available for closed channel (conduit) applications. The different types of meters can be classified as:

differential pressure (orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters) positive displacement (piston, oval-gear, nutating-disk, and rotary-vane) velocity (turbine, vortex shedding, electromagnetic, sonic mass (Coriolis and thermal), open channel (weirs, flumes) and ultrasonic flow meters

Differential Pressure or Head Meters

Differential pressure flow meters are the most common devices in use today. At least fifty percent of all liquid flow measurement applications use this type of unit.

Like most flow meters, differential pressure flow meters have two main elements. The primary element causes a change in kinetic energy and that causes the differential pressure in the conduit. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.

Positive-Displacement Meters

Positive-Displacement meters separate liquids into defined segments before allowing them to pass. Each segment is then counted by a connecting register. Because every increment represents a discrete volume, positive-displacement units are popular for automatic batching and accounting applications. Positive-displacement meters are also recommended for measuring the flows of viscous liquids or when a simple mechanical meter system is needed.

Velocity Meters

Velocity based flow meters operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), they have a greater range. Velocity meters are minimally sensitive to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines.

Open Channel Meters

An open channel is any conduit through which liquid flows with a free surface. This would include tunnels, non-pressurized sewers, partially filled pipes, canals, streams, and rivers. Depth measuring methods are the most frequently used means of determining flow rate in open channels. Depth measuring flow meters assume the flow rate at any given instant can be determined from a measurement of the water depth, or head. Weirs and flumes are the oldest and most widely used primary devices for measuring open channel flows.