Velocity Flow Meters

A velocity flow meter measures how fast a fluid moves through a pipe and calculates the flow rate by multiplying that velocity by the cross-sectional area of the pipe. The family includes electromagnetic, ultrasonic, turbine, paddle wheel, vortex, target, variable area and differential pressure meters. They differ only in how they sense the velocity.

Velocity meters are the most common flow meters in water, wastewater, HVAC, steam and large process lines, because they scale to any pipe size and most have no moving parts in the flow. Their weakness is the other end of the range: at low flows, in viscous fluids, and where flow starts and stops, the velocity is too small or too unsettled to measure well.

How do velocity flow meters work?

Every velocity flow meter senses the speed of the fluid at one point or across the pipe and reports flow rate as velocity times pipe area (Q = v x A). The pipe must run full, and the flow profile must be settled, so most types need several diameters of straight pipe upstream and downstream. The meter’s calibration assumes that profile; elbows, valves and pumps close to the meter distort it and add error.

The velocity itself is sensed in different ways: the voltage a conductive liquid induces in a magnetic field, the time sound takes to cross the pipe, the speed of a rotor, the frequency of vortices behind a bluff body, the force on a target, the height of a float, or the pressure drop across a restriction. Accuracy is quoted either as a percentage of reading or as a percentage of full scale. A full-scale specification looks similar on paper but doubles the relative error at half flow and triples it at one third of flow.

Velocity units of measurement

Velocity is distance per unit of time. The SI unit is meters per second (m/s); feet per second (ft/s) is common in North America, and air velocity is often quoted in feet per minute. Flow rate is velocity multiplied by pipe area and is reported in volume per unit of time, such as liters per minute, gallons per minute or cubic meters per hour.

For example, water moving at 1.5 m/s through a 2 inch schedule 40 pipe (inside diameter 52.5 mm, area 0.00216 square meters) is a flow rate of about 195 liters per minute, or 51 gallons per minute. Liquid lines are usually designed for 1 to 3 m/s; gas lines run faster, 10 to 30 m/s. Most velocity meters need at least 0.3 m/s (1 ft/s) to read reliably, which is why they struggle at low flow.

Instruments that measure velocity

The instrument depends on whether the fluid is in a closed pipe, an open channel or free air.

Instrument	Measures the velocity of	Typical use
Electromagnetic flow meter	Conductive liquids in a full pipe	Water, wastewater, slurries, chemical lines
Ultrasonic flow meter, inline or clamp-on	Liquids and gases in a pipe	Large pipes, retrofits without cutting the line, portable surveys
Turbine or paddle wheel meter	Clean liquids and gases in a pipe	Fuels, water, irrigation, batch counting
Vortex meter	Steam, gas and clean liquids	Steam and utility measurement
Pitot tube and averaging pitot (annubar)	Air and gas at a point or across a duct	HVAC balancing, stack and duct surveys, aircraft airspeed
Anemometer (vane or hot wire)	Free air	Ventilation, clean rooms, weather
Current meter or water velocity meter (propeller, electromagnetic or acoustic Doppler)	Open channel water	Streams, rivers, canals, stormwater

A portable velocity meter for pipes is normally a handheld clamp-on ultrasonic unit: the transducers strap to the outside of the pipe, and the meter reports velocity and flow rate without any process connection. For open water, a water velocity meter is a wading rod or cable-mounted sensor that reads velocity at set points across the channel; multiplying by the measured cross-section gives the discharge.

Types of velocity flow meters

Electromagnetic (magnetic) flow meters

Magnetic flow meters operate on Faraday’s law of electromagnetic induction, which states that a voltage is induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube. Electromagnetic flow meters can only be used for electrically conductive fluids, such as water. They cannot measure deionized water or most hydrocarbon based liquids. The output voltage produced by the electromagnetic induction is directly proportional to the flow rate. Electrodes mounted in the pipe wall detect the voltage, which is measured by a secondary element.

Electromagnetic flow meters have no moving parts. They are selected for their ability to measure difficult and corrosive liquids and slurries with a very low pressure drop, and are ideal for wastewater applications or any dirty liquid which is electrically conductive. Most meters offered have accuracies between 0.3% and 1% of reading. Flow ranges of up to 100:1 are available. Insertion style meters are typically in the range of $400 to $1,000. The majority of inline versions cost $1,200 to $5,000.

Ultrasonic flow meters

Ultrasonic flow meters use very high frequency sound waves to measure the velocity of particles in the process fluid or the time it takes for the sound waves to pass through the fluid. Since ultrasonic signals can penetrate solid materials, the transducers can be mounted onto the outside of the pipe. Ultrasonic flow meters use one of two principles: the Doppler effect, or time of flight measurement.

The effect of motion on a sound source, relative to the frequency shift of the sound, was first observed and described by Christian Johann Doppler. A practical application for flow measurement is made by sending bursts of sound waves through a fluid filled pipe. The measurement of flow is based on the principle that sound waves travelling in the direction of the fluid flow travel faster than sound waves travelling against the fluid flow. At zero velocity, the transit time differential, or delta T, is zero.

Doppler ultrasonic meters require that there are reflecting particles in the fluid. This method is not suitable for clear liquids. Since the Doppler flow meter’s performance is so highly dependent on the physical properties of the fluid, such as the sonic conductivity, particle density, and flow profile, this method is only suitable for applications that do not require a high accuracy.

Time of flight ultrasonic flow meters measure the time for the sound to travel between a transmitter and a receiver. This method is not dependent on the particles in the fluid. Two transceivers are located on each side of the pipe. The transmitters send pulsating ultrasonic waves at a predefined frequency from one side to the other. The difference in transit time is proportional to the average fluid velocity.

Ultrasonic flowmeters are chosen for their clear flow path, low pressure drop, corrosion resistance, and relatively low power consumption compared to magnetic flow meters. Typical accuracy is about 1% of flow rate, although multi-beam ultrasonic meters can offer higher levels of accuracy. Clamp-on or bolt-on ultrasonic flow meters are available at very economical prices. Inline styles typically range from one thousand to five thousand dollars.

Reliable operation of ultrasonic flowmeters requires high frequency sound transmitted across the pipe. Liquid slurries with excess solids or with entrained gases may block the ultrasonic signals. Ultrasonic flow meters are not recommended for primary sludge, mixed liquor, aerobically digested sludge, dissolved air flotation thickened sludge and its liquid phase, septic sludge and activated carbon sludge.

Turbine flow meters

Turbine flow meters are a propeller blade that spins within a pipe as fluid moves through it. There are many different manufacturing designs of turbine flow meters, but in general they are all based on this same simple principle. The turbine blade motion is sensed by a magnetic pick-up and an electrical pulse is generated. The number of electrical pulses counted for a given period of time is directly proportional to flow volume.

To achieve a repeatable result, the liquid flow must be fully developed and stable across the cross section of the pipe. To achieve this stable profile, a few diameters of straight pipe on the inlet and the outlet are required. Turbine meters can be used for clean liquids with viscosities up to 50 centistokes. The turndown ratio can be as high as 100:1 if the turbine meter is calibrated for a single fluid and used at constant conditions. The majority of turbine meters have an accuracy of 0.5% to 1.5% of reading.

Paddle wheel flow meters

Paddle wheel flow meters are cost effective devices typically used for water or water-like fluids. These meters, like the turbine meter, require a few pipe diameters of straight pipe on the inlet and on the outlet. Chemical compatibility should be verified when not using water. The rotor of the paddlewheel sensor is perpendicular to the flow and contacts only a limited cross section of the flow. Prices typically are in the range of $100 to $600.

Accuracy is typically 1% to 3% of full scale. An accuracy measured against the full range of the device will double at 50% of full scale and triple at 33% of full scale. Caution should be used when measuring with a device that states a “full scale accuracy” as opposed to an “of reading accuracy”. Read more on accuracy of reading vs. accuracy of full scale.

Vortex shedding flow meters

Vortex flow meters make use of a natural phenomenon that occurs when a fluid flows around a bluff object. Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the fluid flowing past the bluff object. The three necessary components of a vortex flow meter are a bluff body mounted in the flow meter bore, a sensor to detect the presence of the vortex and generate an electrical impulse, and a signal amplification circuit.

The main advantages of vortex meters are their low sensitivity to variations in process conditions and low wear relative to orifices or turbine meters. Initial and maintenance costs are low. Performance with slurries or high viscosity liquids is poor. Used in both gas and liquid applications, vortex flow meters are used abundantly in steam measurement. The majority of vortex flow meters cost $1,000 to $3,000. Accuracies are typically in the range of 0.5% to 1.0% of reading.

Target flow meters

Target flow meters sense and measure the force caused by liquid impacting on a target or disc suspended in the liquid stream. A direct indication of the liquid flow rate is achieved by measuring the force exerted on the target. In its simplest form, the meter consists only of a hinged, swinging plate that moves outward with the liquid stream, and the device serves as a flow indicator. More sophisticated versions use a precision, low-level force transducer as the sensing element, with the force on the target sensed by a strain gauge. Target meters are useful for measuring flows of dirty or corrosive liquids. They are relatively inexpensive and offer 10:1 flow ranges with accuracies of 0.5% to 1.0% of full scale.

Variable area flow meters (rotameters)

Variable area flow meters consist of a vertically oriented glass or plastic tapered tube with a larger inside diameter at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity. The float rises to a level where the area between the float and tube reaches a state of dynamic equilibrium between the upward differential pressure and buoyancy factors and the downward gravity force. The height of the float is an indication of the flow rate.

The tube can be calibrated and graduated in appropriate flow units. Magnetic floats can be used for alarm and signal transmission functions. Similar designs use a spring load instead of gravity to act against the flow, and these designs can be mounted horizontally. Widely used because of their low cost, simplicity, and low pressure drop, these meters typically have a turndown ratio up to 10:1. The accuracy ranges from 1% to 5% of full scale. Prices generally fall in the range of $50 to $1,000.

Differential pressure flow meters

Differential pressure meters infer velocity from the pressure drop across a restriction in the pipe, or from the pressure of the moving fluid against a probe. They are the oldest flow measurement technology and remain common because they are simple, inexpensive and available in any size and material.

Orifice plate flow meters. Output is inferred through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict. Orifice plate meters are simple, cheap, and can be delivered for almost any low viscosity application in any material. The turndown ratio for orifice plates is less than 5:1. Their accuracy is poor at low flow rates. High accuracy requires that the orifice plate be in good shape, with a sharp edge on the upstream side; wear to the plate edges reduces the accuracy.

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications. The flow nozzle is relatively simple and cheap, and available for many applications in many materials. The turndown ratio and accuracy are similar to the orifice plate.

Venturi tube flow meters are often used in applications that require higher turndown rates, or lower pressure drops, than the orifice plate can provide. In the venturi tube, the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area is recovered. With proper instrumentation and flow calibration, the venturi tube can measure over a 10:1 turndown.

The pitot tube is used to measure the local velocity at a given point in the flow stream and not the average velocity in the pipe or conduit. The pitot tube is widely used to determine the airspeed of an aircraft and to measure air and gas velocities in industrial applications. It measures the fluid flow velocity by converting the kinetic energy of the flow into potential energy. The use of the pitot tube is restricted to point measuring. With the annubar, or multi-orifice pitot probe, the dynamic pressure can be measured across the velocity profile, and the annubar obtains an averaging effect. The pitot tube is a very common and inexpensive way to measure fluid flow, especially in air applications for ventilation and HVAC systems. They are easy to install and have a very low pressure loss. Typical turndown is 10:1.

Velocity flow meter comparison chart

Meter type	Accuracy	Turndown	Price range	Fluids	Straight pipe	Moving parts
Electromagnetic	0.3% to 1% of reading	Up to 100:1	$400 to $1,000 insertion, $1,200 to $5,000 inline	Conductive liquids, slurries, wastewater	Short runs	None
Ultrasonic, transit time	About 1% of rate	Wide	$1,000 to $5,000 inline, less for clamp-on	Clean liquids and gases	About 10 diameters	None
Ultrasonic, Doppler	Low, application dependent	Wide	Low	Liquids with particles or bubbles	About 10 diameters	None
Turbine	0.5% to 1.5% of reading	Up to 100:1 on one fluid	Low to moderate	Clean liquids to 50 cSt, gases	Several diameters each side	Rotor
Paddle wheel	1% to 3% of full scale	About 10:1	$100 to $600	Water and water-like fluids	Several diameters each side	Rotor
Vortex	0.5% to 1.0% of reading	About 20:1	$1,000 to $3,000	Steam, gas, clean liquids	About 10 diameters	None
Target	0.5% to 1.0% of full scale	10:1	Low	Dirty or corrosive liquids	Several diameters	Target
Variable area	1% to 5% of full scale	Up to 10:1	$50 to $1,000	Clean liquids and gases	None	Float
Orifice plate, flow nozzle	Poor at low flow	Under 5:1	Low	Low viscosity liquids, gases, steam	Long runs	None
Venturi tube	Good with calibration	About 10:1	Moderate	Liquids, gases, steam	Long runs	None
Pitot tube, annubar	Point measurement	10:1	Low	Air and gas	Long runs	None

When a velocity flow meter is the wrong choice

Velocity meters are the right choice for large pipes, dirty or conductive liquids, steam and air. They are the wrong choice when the velocity is too low or too unsettled to measure. Below about 0.3 m/s most types lose accuracy; a fluid thicker than 50 centistokes stalls a turbine and absorbs ultrasound; a flow that starts, stops or reverses in milliseconds never develops the settled profile the meter’s calibration assumes; and a test stand or dispensing head rarely has ten diameters of straight pipe to spare.

Those are the conditions positive displacement flow meters are built for. Because a PD meter traps and counts fixed volumes rather than sensing speed, it reads accurately from 0.005 cc/min, in any viscosity, with no straight pipe, and responds to every change in flow instantly.

Max flow meters for low, viscous and intermittent flow

Max Machinery makes positive displacement piston, gear and helical meters for the flows velocity meters cannot measure: 0.005 cc/min to 500 liters per minute, 0.2% to 0.3% of reading, viscosities from 0.5 to 1,000,000 centistokes, and response in 1.6 milliseconds.

• Piston flow meters for fuel consumption, injector, servo valve and laboratory measurement, from 0.005 cc/min.
• Gear flow meters for hydraulics, lubricants, adhesives and chemicals from 0.5 cc/min to 240 L/min.
• Helical flow meters for heavy fuels, coatings and resins to 500 L/min.

Not sure whether your application needs a velocity meter or a positive displacement meter? Call +1 (707) 433-2662 or request a quote and an application engineer will walk through the fluid, flow range and installation with you.

Frequently asked questions