Table of Contents
What is flow units?
Continuity of move equation
Common flow unit metering methods in open channels
Eyeball methodology
Flow depth (Manning)
Main tools
Surface Velocity Meters
Transport time meters
Flow unit measurement methods in full tube functions
Venturi meters
Magnetic flowmeter
Turbine flowmeter
Conclusion
What is move units?
Flow fee is the volume of fluid that passes via a unit of time. In water assets, flow is commonly measured in cubic ft per second (cfs), cubic meters per second (cms), gallons per minute (gpm), or a variety of other items. The measurement of water useful resource move is essential for purposes similar to system control, billing, design and heaps of different functions. There are several ways to measure move in a water resource system. This article outlines a variety of the more widespread methods of flow measurement and supplies some useful details about move units measurement.
Continuity of circulate equation
For water flowing in a pipe under steady-state situations (i.e., not various with time), continuity signifies that water flowing into one finish of the pipe must move out of the opposite end. This additionally implies that the move in the pipe is the same at any level alongside the length of the pipe. The continuity equation may be expressed as
Flow = Velocity * Area
The concept of continuity in steady state conditions leads to the product of velocity * area being equal to a constant at any level within the pipe. This is a helpful principle for making move measurements, as proven beneath.
This is an instance of using the continuity equation to calculate flow. Velocity is measured at 10 feet per second and the cross-sectional space of the move is measured at 10 square toes. Flow rate = 10 ft per second * 10 square ft = a hundred cubic feet per second.
Common circulate unit metering strategies in open channels
Eyeball method
It is usually helpful to estimate the move velocity and cross-sectional space by eye after which multiply the move velocity by the world to acquire the circulate velocity (continuity equation). A ruler or tape measure can be utilized to improve the accuracy of cross-sectional area measurements, and a stopwatch can be utilized to improve velocity measurements by timing floating debris moving a set distance. The eyeball methodology can be utilized to estimate flow when only an “order of magnitude” of circulate is required or when the circulate price is simply too low to be measured with a move meter.
Flow depth (Manning)
When the channel cross-sectional area and channel slope are identified and uniform move circumstances exist, the Manning’s equation can be used to calculate circulate by measuring depth solely. The Manning’s equation is an empirical equation that describes the relationship between move fee in an open channel underneath uniform circulate circumstances and depth, slope and channel friction coefficient (Manning’s n). Uniform circulate implies that depth does not vary with the size of the conduit or channel. Flow measurements using Manning’s equation for depth are not applicable to steadily changing move situations, corresponding to backwater situations upstream of a dam or weir.
The depth circulate method of circulate measurement is more correct than the “eyeball” method. The main problem with depth-only flow measurements is the potential for inaccurate Manning’s n estimates, cross-sectional areas, and non-uniform move circumstances. This method is usually used with ultrasonic circulate meters to estimate the move of a river by measuring only the water stage of the river. Often in these causes, the river level/flow relationship is developed with the help of complicated river hydraulic fashions to account for complex channel geometry and channel friction situations.
Main gear
The major gadget is used to measure flow in open channels, utilizing constructions corresponding to flumes, weirs or dams, to measure flow by measuring depth. The measured depth can then be transformed to a move fee utilizing an equation or rated curve equation.
Primary units work by forcing the circulate by way of a path of critical depth, for instance on the prime of a weir or at the throat of a flume. In technical phrases, the important depth is outlined because the depth of the minimal specific power state that leads to a specific discharge. In apply, this minimum state of vitality signifies that only one circulate corresponds to the critical depth. Therefore, measuring only the depth produces a measurement of the corresponding flux and is subsequently called a “primary” system.
Primary units are a very convenient technique of flow measurement as a end result of the depth may be measured from above the circulate without the necessity to insert a sensor within the water. This makes primary flow meters extra dependable and easier to hold up. A drawback of primary gadgets is that they’ll trigger head loss and backwater in the system. Primary units are sometimes considered to be probably the most accurate methodology of measuring open channel circulate.
Surface Velocity Meters
An area velocity meter is an open channel move meter that measures flow by making two separate measurements of depth and velocity. The depth is transformed to a cross-sectional space utilizing the geometry of the pipe or channel. The move fee is then calculated by multiplying the move area by the speed using the continuity equation, therefore the identify “AV meter”. Velocity is usually measured using a Doppler sensor, which displays ultrasound waves again from particles in the fluid and uses the Doppler shift in the mirrored sound signal to estimate velocity. Some AV meters measure floor velocity optically to estimate velocity.
AV meters are often used to measure open channel flow in sewers as a end result of the probes are comparatively small and they can be put in in current sewer pipes without inflicting important head loss within the pipe. This also permits them for use for momentary or short-term flow metering applications for sewer research. one drawback of AV meters is that the sensor must be put in within the fluid. In sewers, this requires frequent upkeep to scrub the sensor. AV meters are sometimes considered much less accurate than main circulate meters as a result of main units solely need to measure depth and depth measurements are extra accurate than velocity measurements.
Transport time meters
Transport time meters had been developed in the oil industry to accurately measure circulate in giant pipelines. They have been used with some success for open channel move in water metering functions. Transport time meters additionally use ultrasound like Doppler meters, but instead of bouncing the sound waves off particles within the water like Doppler circulate meters, they ship ultrasound waves between two sensors separated from each other by a sure distance alongside the length of the pipe and uses the transmission time of the sound waves to calculate the rate of the water circulate. Because the speed of sound within the water is known, the velocity of the water may be calculated based mostly on the offset in ultrasonic wave transmission time that happens because of the pace of the water.
Transmission time meters can be expensive relative to Doppler move meters as a result of many sensors and complex installation involved. They can be extra accurate as a result of ability to split the flow into horizontal cross sections and measure the speed of each section.
Flow unit measurement strategies in full tube functions
Venturi meters
Venturi flow meters use the Venturi impact to measure move in a full or pressurized pipe through the use of the converging part of the pipe to limit the flow. According to the continuity equation, the cross-sectional area of the converging section is small and due to this fact the speed is higher in the throat. Due to power conservation and Bernoulli’s precept, higher velocities within the throat lead to a drop in throat stress. The move fee can then be determined by measuring the pressure drop in the convergent section and calculating the circulate fee using Bernoulli’s equation. Venturi meters are extra common in water metering applications as a result of the stress measurement ports can turn into clogged in wastewater purposes.
pressure gauge by applying a magnetic area to the fluid passing through the pipe. This causes a small electron potential distinction that could be measured by the electrode sensor (due to Faraday’s law and electromagnetic induction). The magnitude of the electron potential difference is proportional to the velocity of the water, and the continuity equation can then be used to calculate the circulate price.
An benefit of the magnetometer is that the metering part is similar diameter as the adjacent pipe, so the magnetometer causes no further head loss. For probably the most part, magnetometers are used for full (pressure) pipe applications, but these days open channel magnetometers can be used.
Turbine flowmeter
A turbine circulate meter is a mechanical move meter that makes use of a rotating turbine in flow to measure the flow of water in a pipe. The velocity of the turbine is proportional to the velocity and the circulate fee can then be calculated using the continuity equation. Turbine circulate meters are only used for water applications as a end result of potential problems with wastewater solids assortment and clogging generators.
Conclusion
There are some ways to measure circulate. Each technique has totally different benefits, disadvantages and accuracy in several purposes.
It is important to know the characteristics of assorted move measurement techniques to assist select the proper sort of flow metering for your software or to correctly interpret the move measurements of present circulate meters. Tools like Apure (IoT-based water knowledge analysis) assist to look at measurements collected by flow meters and perform diagnostics to know circulate meter efficiency and shortly course of and analyze the data. Contact us for technical or product service assist.
More articles on move meters:
Mass move fee vs volumetric move fee
Relation between flow and strain
Ultrasonic circulate meter working principle
Difference between circulate meter and flow transmitter
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Table of Contents
What is move units?
Continuity of flow equation
Common flow unit metering strategies in open channels
Eyeball method
Flow depth (Manning)
Main tools
Surface Velocity Meters
Transport time meters
Flow unit measurement strategies in full tube purposes
Venturi meters
Magnetic flowmeter
Turbine flowmeter
Conclusion
What is circulate units?
Flow rate is the volume of fluid that passes by way of a unit of time. In water resources, flow is usually measured in cubic feet per second (cfs), cubic meters per second (cms), gallons per minute (gpm), or quite so much of other units. The measurement of water useful resource circulate is necessary for applications corresponding to system management, billing, design and lots of different purposes. There are several ways to measure flow in a water useful resource system. This article outlines a number of the extra common methods of flow measurement and supplies some useful details about move units measurement.
Continuity of circulate equation
For water flowing in a pipe under steady-state situations (i.e., not varying with time), continuity signifies that water flowing into one finish of the pipe must move out of the other end. This also signifies that the flow in the pipe is the same at any point alongside the size of the pipe. The continuity equation can be expressed as
Flow = Velocity * Area
The concept of continuity in regular state circumstances ends in the product of velocity * space being equal to a relentless at any point in the pipe. This is a helpful precept for making move measurements, as shown below.
This is an instance of utilizing the continuity equation to calculate flow. Velocity is measured at 10 ft per second and the cross-sectional space of the flow is measured at 10 square toes. Flow rate = 10 ft per second * 10 square feet = a hundred cubic toes per second.
Common flow unit metering methods in open channels
Eyeball methodology
It is typically helpful to estimate the flow velocity and cross-sectional space by eye after which multiply the circulate velocity by the area to acquire the flow velocity (continuity equation). A ruler or tape measure can be used to improve the accuracy of cross-sectional area measurements, and a stopwatch can be utilized to enhance velocity measurements by timing floating particles shifting a set distance. The eyeball technique can be used to estimate move when only an “order of magnitude” of move is required or when the circulate fee is too low to be measured with a move meter.
Flow depth (Manning)
When the channel cross-sectional space and channel slope are identified and uniform move situations exist, the Manning’s equation can be utilized to calculate circulate by measuring depth solely. The Manning’s equation is an empirical equation that describes the connection between circulate rate in an open channel beneath uniform flow circumstances and depth, slope and channel friction coefficient (Manning’s n). Uniform flow signifies that depth doesn’t vary with the length of the conduit or channel. Flow measurements utilizing Manning’s equation for depth usually are not applicable to gradually changing move circumstances, corresponding to backwater situations upstream of a dam or weir.
The depth move method of flow measurement is more correct than the “eyeball” methodology. The primary problem with depth-only circulate measurements is the potential for inaccurate Manning’s n estimates, cross-sectional areas, and non-uniform move circumstances. This method is usually used with ultrasonic circulate meters to estimate the circulate of a river by measuring only the water level of the river. Often in these causes, the river level/flow relationship is developed with the help of advanced river hydraulic fashions to account for advanced channel geometry and channel friction circumstances.
Main gear
The major device is used to measure flow in open channels, utilizing constructions such as flumes, weirs or dams, to measure circulate by measuring depth. The measured depth can then be transformed to a circulate rate utilizing an equation or rated curve equation.
Primary gadgets work by forcing the move by way of a path of important depth, for instance at the high of a weir or on the throat of a flume. In technical phrases, the important depth is defined because the depth of the minimal specific energy state that ends in a selected discharge. In follow, this minimum state of vitality signifies that only one move corresponds to the crucial depth. Therefore, measuring solely the depth produces a measurement of the corresponding flux and is due to this fact called a “primary” device.
Primary units are a really convenient method of flow measurement because the depth can be measured from above the move without the need to insert a sensor within the water. This makes major move meters more dependable and simpler to keep up. A disadvantage of major gadgets is that they will trigger head loss and backwater in the system. Primary devices are sometimes considered to be the most correct methodology of measuring open channel circulate.
Surface Velocity Meters
An area velocity meter is an open channel circulate meter that measures flow by making two separate measurements of depth and velocity. The depth is transformed to a cross-sectional space utilizing the geometry of the pipe or channel. The circulate price is then calculated by multiplying the move area by the velocity using the continuity equation, therefore the identify “AV meter”. Velocity is typically measured using a Doppler sensor, which reflects ultrasound waves back from particles in the fluid and uses the Doppler shift within the mirrored sound sign to estimate velocity. Some AV meters measure floor velocity optically to estimate velocity.
AV meters are sometimes used to measure open channel move in sewers as a end result of the probes are comparatively small and they are often installed in current sewer pipes with out inflicting significant head loss in the pipe. This also permits them to be used for momentary or short-term circulate metering purposes for sewer research. one disadvantage of AV meters is that the sensor have to be put in within the fluid. In sewers, this requires frequent maintenance to wash the sensor. AV meters are sometimes thought-about less correct than primary circulate meters as a result of major devices solely need to measure depth and depth measurements are more accurate than velocity measurements.
Transport time meters
Transport time meters were developed in the oil trade to accurately measure move in giant pipelines. They have been used with some success for open channel move in water metering applications. Transport time meters also use ultrasound like Doppler meters, however as an alternative of bouncing the sound waves off particles in the water like Doppler move meters, they send ultrasound waves between two sensors separated from each other by a certain distance along the length of the pipe and makes use of the transmission time of the sound waves to calculate the speed of the water flow. Because the velocity of sound in the water is thought, the speed of the water could be calculated based on the offset in ultrasonic wave transmission time that happens because of the speed of the water.
Transmission time meters may be costly relative to Doppler circulate meters due to the many sensors and complex set up concerned. They can be more correct due to the ability to separate the circulate into horizontal cross sections and measure the speed of each section.
Flow unit measurement strategies in full tube functions
Venturi meters
Venturi flow meters use the Venturi effect to measure circulate in a full or pressurized pipe through the use of the converging section of the pipe to limit the flow. According to the continuity equation, the cross-sectional space of the converging section is small and subsequently the speed is larger within the throat. Due to power conservation and Bernoulli’s principle, larger velocities within the throat lead to a drop in throat strain. The flow price can then be determined by measuring the strain drop in the convergent section and calculating the flow price utilizing Bernoulli’s equation. Venturi meters are extra frequent in water metering purposes as a outcome of the strain measurement ports can turn into clogged in wastewater purposes.
Magnetic flowmeter
The electromagnetic circulate meter works by making use of a magnetic field to the fluid passing through the pipe. This causes a small electron potential difference that could be measured by the electrode sensor (due to Faraday’s legislation and electromagnetic induction). The magnitude of the electron potential difference is proportional to the velocity of the water, and the continuity equation can then be used to calculate the flow fee.
An benefit of the magnetometer is that the metering part is identical diameter as the adjoining pipe, so the magnetometer causes no extra head loss. For the most part, magnetometers are used for full (pressure) pipe functions, however these days open channel magnetometers can also be used.
Turbine flowmeter
A turbine circulate meter is a mechanical circulate meter that uses a rotating turbine in circulate to measure the move of water in a pipe. The speed of the turbine is proportional to the speed and the flow price can then be calculated utilizing the continuity equation. Turbine flow meters are solely used for water purposes as a end result of potential issues with wastewater solids assortment and clogging turbines.
Conclusion
There are many ways to measure flow. Each technique has completely different advantages, disadvantages and accuracy in different purposes.
It is important to grasp the traits of varied circulate measurement techniques to help choose the right sort of move metering in your utility or to correctly interpret the circulate measurements of current flow meters. Tools like Apure (IoT-based water knowledge analysis) assist to look at measurements collected by move meters and perform diagnostics to understand circulate meter efficiency and shortly process and analyze the data. Contact us for technical or product service support.
More articles on move meters:
Mass move rate vs volumetric move fee
Relation between circulate and strain
Ultrasonic move meter working precept
Difference between flow meter and circulate transmitter