Flow measurement is quantification of bulk liquid movement. Streams can be measured in various ways. The positive-displacement flow meter collects the fluid volume and then calculates how many times the volume is filled to measure flow. Other flow measurement methods depend on the strength generated by the flow that flows by overcoming the known constriction, to calculate the flow indirectly. The flow can be measured by measuring the velocity of the liquid over a known area. For very large flows, the tracer method can be used to infer the flow rate from changes in dye concentrations or radioisotopes.
Video Flow measurement
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Both gas and liquid flows can be measured in volumetric or mass flow rates, such as liters per second or kilogram per second, respectively. This measurement is related to material density. The fluid density is almost independent of the conditions. This is not the case for gas, the density is heavily dependent on pressure, temperature and to a lesser degree, composition.
When gases or liquids are transferred for their energy content, as in natural gas sales, flow rates can also be expressed in the form of energy flows, such as gigajoules per hour or BTU per day. The energy flow rate is the volumetric flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. The energy flow rate usually comes from the mass flow rate or volumetric by the use of a flow computer.
Dalam konteks teknik, laju alir volumetrik biasanya diberi simbol , dan laju aliran massa, simbol .
Untuk cairan yang memiliki densitas , laju aliran massa dan volumetrik mungkin terkait dengan .
Gas
The gas can be compacted and change the volume when placed under pressure, heated or cooled. The gas volume under a set of pressure and temperature conditions is not equivalent to the same gas under different conditions. References will be made to "actual" flow rates through meter flow and "standard" or "bottom" flow through meters with units such as acm/h (actual cubic meters per hour), sm 3 /sec (standard cubic meters per second), kscm/h (thousand standard cubic meters per hour), LFM (linear feet per minute), or MMSCFD (million standard cubic feet per day).
The gas flow rate mass can be measured directly, independent of pressure and temperature effects, by thermal mass flow meters, Coriolis mass flow meters, or mass flow controllers.
Liquid
For liquids, various units are used depending on application and industry, but may include gallons (US or imperial) per minute, liters per second, bushels per minute or, when describing river currents, cumecs (cubic meters per second) or acre- feet per day. In the oceanographic general unit to measure the volume of transport (the volume of water transported by the stream for example) is a sverdrup (Sv) equivalent to 10 6 m 3 /s.
Maps Flow measurement
Mechanical flow meter
The positive displacement meter can be compared with the bucket and stopwatch. Stopwatch begins when the flow starts, and stops when the bucket reaches its limit. Volume divided by time gives flow rate. For continuous measurements we need a continuous bucket filling and discharging system to split the flow without letting it out of the pipeline. This constantly shaping and collapsing volumetric shaping can take the form of a redistributed piston in a cylinder, a tooth that is united with an internal meter wall or through a progressive cavity made by twisting an oval or a helical screw.
Piston meter/rotary piston
Because they are used for domestic water measurements, piston meters, also known as rotary pistons or semi-positive displacement meters, are the most common flow measuring devices in the UK and are used for almost any meter size up to and including 40 mm ( 1 < span> 1 / 2 at). Piston meters operate on the principle of rotating pistons in known volume chambers. For each round, some water passes through the piston chamber. Through gear mechanisms and, occasionally, magnetic drives, needle dial and odometer type forward display.
Dental oil gauge
An oval gear meter is a positive displacement meter that uses two or more long gears that are configured to rotate at right angles to one another, forming a T shape. Such meters have two sides, which can be called A and B. No fluid past the meter center, where the teeth of both teeth are always threaded. On one side of the meter (A), the gearshift closes the fluid flow because the elongated tooth on the A side protrudes into the measuring chamber, while on the other side of the meter (B), a cavity holds a fixed liquid volume in the measuring chamber. When the liquid pushes the tooth, it will rotate it, allowing the fluid in the measuring room on the B side to be released to the outlet port. Meanwhile, the liquid entering the entrance will be pushed into the side A measurement chamber, which is now open. Teeth on the B side will now close the liquid from entering the B side. This cycle continues as the teeth rotate and the fluid is measured through alternating measuring chambers. Permanent magnets in spinning gears can transmit signals to an electric reed switch or current transducer for flow measurements. Although claims for high performance are made, they are generally not as precise as the design of shear propellers.
Wheel gauge
The gear meter is different from the oval gear meter in the measurement space which consists of a gap between the teeth of a tooth. This opening divides the fluid stream and when the gear rotates away from the inlet port, the inner wall of the meter closes the chamber to hold the fixed amount of liquid. The outlet port is located in the area where the gears will come back together. The liquid is forced out of the meter as a mesh tooth and reduces the available pockets to almost zero volume.
Helical gear
Helical gear flow meter gets its name from the shape of its teeth or rotor. This rotor resembles a helical shape, which is a spiral-shaped structure. When the liquid flows through the meter, it enters the compartment in the rotor, causing the rotor to rotate. The length of the rotor is sufficient to allow the inlet and outlet to be always separated from each other thus blocking the liquid free flow. The helical mating rotor creates a progressive cavity that opens to receive fluid, closes itself and then opens to the downstream side to release the liquid. This happens continuously and the flow rate is calculated from the rotational speed.
Connect the disk meter
This is the most commonly used measurement system for measuring water supplies in homes. The liquid, most commonly water, goes to one side of the meter and attacks the nutating disc, which is mounted eccentrically. The disc should "vibrate" or grate on the vertical axis, because the bottom and top of the disc remain in contact with the mounting chamber. A partition separates the inlet space and outlet. As a disk disk, it provides a direct indication of the volume of liquid that has passed through the meter as the volumetric flow is indicated by the gearing and register settings, which are connected to the disk. This is reliable for flow measurements in 1 percent.
Pengukur aliran turbin
The turbine flow meter (better described as the axial turbine) translates the turbine's mechanical action rotating in the liquid stream around the axis to the user-readable flow rate (gpm, lpm, etc.). Turbines tend to have all the streams that surround them.
The turbine wheel is set in the fluid flow path. The fluid that flows over the turbine blades, gives force to the surface of the blade and regulates the rotor in motion. When a stable rotational speed has been reached, its velocity is proportional to the fluid velocity.
Turbine flow meters are used for measuring the flow of natural gas and liquids. The turbine meter is less accurate than displacement and jet meters at low flow rates, but the measuring element does not occupy or severely limit the entire flow path. The flow direction is generally straight through the meter, allowing for higher flow rate and pressure loss less than the displacement-type meter. They are the choice meters for large commercial users, fire protection, and as key gauges for water distribution systems. The filter generally needs to be mounted in front of the meter to protect the measuring element from the gravel or other debris that can enter the water distribution system. Turbine gauges are generally available for 4 to 30 cm ( 1 1 / 2 -12Ã, di) or higher pipe size. Body turbine meters are generally made of bronze, cast iron, or ductile iron. Internal turbine elements can be either plastic or non-corrosive metal alloys. They are accurate under normal working conditions but strongly influenced by flow profile and fluid conditions.
The fire meter is a special type of turbine meter with the approval for the high flow rate required in the fire protection system. They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) or similar authorities for use in fire protection. Portable turbine meter can be installed temporarily to measure the water used from the fire hydrant. Meters are usually made of aluminum to be light, and typically 7.5 cm (3 inches) in capacity. Water utilities often require them for water measurements used in construction, pool filling, or where the permanent meter has not been installed.
Woltman Meter
The Woltman meter (invented by Reinhard Woltman in the 19th century) consists of a rotor with a helical blade that is axially inserted in the flow, like a dispensing fan; it can be considered as turbine flow meter type. They are usually referred to as helix meters, and are popular in larger sizes.
Single jet gauge
A single jet meter consists of a simple impeller with radial propellers, thrown by a single jet. They are increasingly popular in the UK at a larger size and are commonplace in the EU.
Rowing wheel gauge
This is similar to a single jet meter, except that the impeller is small against the width of the pipe, and the project is only partially into the flow, like a rowing wheel on the Mississippi river.
Multiple jet meters
A double or multijet meter jet is a type of meter speed that has an impeller that rotates horizontally on a vertical axis. The impeller element is in a housing where several inlet ports direct the flow of fluid to the impeller causing it to rotate in a particular direction according to the flow velocity. This meter works mechanically like a single jet meter except that the port directs the flow to the impeller evenly from several points around the circumference of the element, not just one point; this minimizes uneven wear on the impeller and its axis. So this type of meter is recommended to be mounted horizontally with a roller index pointing towards the sky.
Pelton wheel
The Pelton wheel turbine (better described as a radial turbine) translates the mechanical action of the Pelton wheels which rotates in the liquid stream around the axis to the user-readable flow rate (gpm, lpm, etc.). The Pelton wheel tends to have all the streams that surround it with inlet streams focused on the blades by jets. The original Pelton Wheel is used to generate power and consists of a radial flow turbine with a "reaction glass" that not only moves with the strength of water on the face but returns the flow in the opposite direction using this fluid change direction to further improve the efficiency of the turbine.
Current meter
The flow through large penstocks such as those used in hydroelectric power can be measured by an average flow rate across the entire area. The vane flow meter (similar to pure mechanical flow meter, but now with electronic data acquisition) can be passed over the penstock area and the average velocity to calculate the total flow. This may be on the order of hundreds of cubic meters per second. The flow must remain stable during the flow meter. The method for testing hydroelectric turbines is given in IEC 41 standards. Such flow measurements are often commercially important when testing the efficiency of large turbines.
Pressure-based meter
There are several types of flow meters that rely on the Bernoulli principle, either by measuring differential pressure in narrowing, or by measuring static pressure and stagnation to lower the dynamic pressure.
Venturi meter
A Venturi meter narrows the flow in several modes, and the pressure sensor measures differential pressure before and within the constriction. This method is widely used to measure the flow rate in gas transmission through pipes, and has been used since the time of the Roman Empire. The coefficient of Venturi meter release is 0.93-0.97. The first large-scale Venturi meter for measuring liquid flow was developed by Clement Herschel who used it to measure the flow of small and large water that began in the late 19th century.
Orifice plate
Orifice plate is a plate with a hole through it, placed perpendicular to the flow; it narrows the flow, and measures the pressure difference along the constriction giving the flow rate. This is basically a rough shape of the Venturi meter, but with higher energy losses. There are three types of holes: concentric, eccentric, and segmental.
Dall tube
The Dall tube is a shortened version of the Venturi meter, with a lower pressure drop than the orifice plate. Like this flow meter, the flow rate in the Dall tube is determined by measuring the pressure drop caused by in-line restrictions. Pressure differences are usually measured using diaphragm pressure transducers with digital readings. Since this meter has a much lower permanent pressure drop than the orifice gauge, the Dall tube is widely used to measure the mass flow rate of pipeworks. The differential pressure generated by the Dall tube is higher than the Venturi tube and the nozzle, all of which have the same neck diameter.
Pitot-tube
A Pitot-tube is used to measure the fluid flow velocity. This tube points into the flow and the difference between the stagnation pressure at the tip of the probe and the static pressure on its side is measured, producing a dynamic pressure from which the fluid velocity is calculated using the Bernoulli equation. The volumetric flow rate can be determined by measuring the velocities at different points in the flow and resulting in a speed profile.
Multi-hole pressure check
Multi-hole pressure probes (also called impact probes) extend pitot tube theory to more than one dimension. A typical impact probe consists of three or more holes (depending on the probe type) on the measuring end arranged in a particular pattern. More holes allow the instrument to measure the direction of flow velocity in addition to magnitude (after appropriate calibration). Three holes arranged in a single line allow the pressure probe to measure the velocity vectors in two dimensions. Introduction of more holes, e.g. five holes arranged in a "plus" formation, allowing the measurement of three-dimensional velocity velocity.
Cone Meter
Cone meters are the newer differential pressure gauges which were first launched in 1985 by McCrometer in Hemet, CA. The cone meter is a generic yet powerful common differential (DP) meter that has proven to be resistant to asymmetric and spinning flow effects. When working with the same basic principles as Venturi and DP meter hole type, cone meters do not require the same upstream and downstream pipes. The cones act as conditioning devices as well as differential pressure manufacturers. The upstream requirement is between a 0-5 diameter compared to a diameter of up to 44 for an orifice plate or 22 diameter for a Venturi. Because cone meters are generally welded construction, it is recommended they are always calibrated before service. The heat effect of welding causes distortion and other effects that prevent tabular data on the discharge coefficients with respect to line size, beta ratios and Reynolds number operations collected and published. The calibrated cone meter has an uncertainty up to/- 0.5%. Uncalibrated meter cones have an/-5.0% uncertainty.
Linear abstraction meter
Linear resistance meter, also called laminar flow meter, measures very low flow in which the measured differential pressure is directly proportional to fluid flow and viscosity. This stream is called the tensile flow or laminar flow, as opposed to the turbulent flow measured by the orifice plate, Venturis and other meters mentioned in this section, and is characterized by the Reynolds number below 2000. The main flow element may consist of a single length of capillary tube , a bundle of such tubes, or a porous long block; such low flow creates a small pressure difference but the longer flow element creates a higher and more easily measured difference. This flow meter is very sensitive to temperature changes that affect the fluid viscosity and the diameter of flow elements, as can be seen in the regulating Hagen-Poiseuille equation.
Region-variable flow meter
A "wide area measurer" measures the fluid flow by allowing the cross-sectional area of ​​the device to vary in response to the flow, causing some measurable effects indicating the rate. The rotameter is an example of a variable area of ​​measure, in which the weighted "float" rises in a pointed tube as the flow rate increases; the float stops rising when the area between the float and tube is large enough that the buoy weight is offset by a fluid flow drag. The type of rotameter used for medical gas is the Thorpe tube flowmeter. Floats are made in various shapes, with the most common rounded ball and ellipse. Some are designed to spin visible in fluid flow to help the user in determining whether the buoy is trapped or not. Rotameters are available for a wide variety of liquids but are most commonly used with water or air. They can be made to reliably measure the flow to 1% accuracy.
Another type is the variable orifice area, where the tapered spring plunger is deflected by the flow through the orifice. Displacement can be attributed to flow rate.
Optical flow meter
The optical flow meter uses light to determine the flow rate. The small particles that accompany natural and industrial gas pass through two laser beams focused on short distances in the flow path in the pipe by irradiating the optics. The laser beam is scattered when the particles pass through the first beam. The detecting optics collect the scattered light on the photodetector, which then generates the pulse signal. When the same particle crosses the second beam, the detecting optics collect the scattered light on the second photodetector, which converts the incoming light into a second electrical pulse. By measuring the time interval between these pulses, the gas velocity is calculated as
A laser-based optical flow meter measures the true particle velocity, a property that is independent of the thermal conductivity of the gas, the variation in gas flow or gas composition. Its operating principle enables optical laser technology to produce highly accurate flow data, even in challenging environments that may include high temperatures, low flow rates, high pressure, high humidity, pipe vibration and acoustic noise.
The optical flow meter is highly stable with no moving parts and provides very high repeatable measurements over the life of the product. Because the distance between two laser sheets does not change, the optical flow meter does not require periodic calibration after initial commissioning. The optical flow meter requires only one installation point, not two installation points normally required by other types of meters. One installation point is simpler, requires less maintenance and is less prone to error.
The commercially available optical flow meter is capable of measuring flow from 0.1 m/s to faster than 100 m/s (turn down ratio of 1000: 1) and has been proven effective for flare gas measurements from oil wells and refineries, atmospheric pollution contributors.
Open channel flow measurement
The open channel flow illustrates the cases in which the liquid flowing has the top surface open into the air; the cross-section of the stream is determined only by the shape of the channel on the lower side, and varies depending on the depth of fluid in the channel. An appropriate technique for fixed cross-section flow in pipes is useless in open channels. Measuring flow in a water channel is an important open channel flow application; Such installations are known as flow meters.
Flow rate
The water level is measured at the point determined on the back of the weir or in flume using a variety of secondary devices (bubbler, ultrasonic, float, and differential pressure is a common method). This depth is converted to flow rate according to the theoretical formula of the form where is the flow rate, is the constant, is water level, and are exponents that vary with the device used; or converted according to empirically obtained flow/flow data points ("flow curves"). Flow rate can then be integrated from time to time into volumetric flow. Level-to-flow devices are typically used to measure surface water flows (fountains, rivers, and rivers), industrial dumps, and waste. Of these, the weir is used in low-flow flow streams (usually surface water), whereas flumes are used in streams containing low or high solids content.
Area/speed
The cross-sectional area of ​​the flow is calculated from the depth measurement and the mean flow velocity is measured directly (Doppler and propeller methods are common). The velocity times the cross-sectional area produces a flow rate that can be integrated into the volumetric flow. There are two types of area velocity flow meters: (1) dampened; and (2) non-contact. The wetland velocity sensor should be installed at the bottom of the channel or river and use Doppler to measure the speed of trapped particles. This programmable depth and cross section can then provide flow measurement measurements. Non-contact devices that use lasers or radar are mounted over the channel and measure the speed from above and then use ultrasound to measure the water depth from above. Radar devices can only measure the surface of velocites, while laser-based devices can measure sub-surface speeds.
Dye test
The amount of dye (or salt) known per unit time is added to the flow stream. After complete mixing, concentrations are measured. Dilution rate is equal to flow rate.
Velocimetry Doppler Acoustics
Acoustic Doppler velocimetry (ADV) is designed to record instantaneous velocity components at a single point with a relatively high frequency. Measurements were made by measuring the particle velocity in the remote sampling volume based on the Doppler shift effect.
Thermal mass flow meter
Thermal mass flow meters generally use a combination of heat elements and temperature sensors to measure the difference between static heat transfer and flow to the fluid and summarize the flow with knowledge of specific heat and fluid densities. Fluid temperatures are also measured and compensated. If the density and specific heat characteristics of the fluid are constant, the meter can provide direct mass flow readings, and does not require additional pressure temperature compensation during the specified range.
Technological advances have enabled the manufacture of thermal mass flow meters on microscopic scales as MEMS sensors; this flow device can be used to measure flow rates in the range of nanoliter or microliter per minute.
Thermal mass flow meter (also called thermal dispersion or thermal displacement flowmeter) technology is used for compressed air, nitrogen, helium, argon, oxygen, and natural gas. In fact, most of the gases can be measured as long as they are clean enough and not corrosive. For more aggressive gases, meters can be made of special alloys (eg Hastelloy), and pre-draining gases also help minimize corrosion.
Currently, thermal mass flow meters are used to measure gas flow in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow measurement technologies. This is because the thermal mass flow meter monitors the variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or heat specific) of the gas medium to determine the mass flow rate.
Sensor MAF
In many end car models, the Mass Airflow (MAF) sensor is used to accurately determine the intake air mass flow rate used in internal combustion engines. Many such mass flow sensors use thermal elements and downstream temperature sensors to show the airflow rate. Other sensors use spring-loaded propellers. In both cases, the vehicle's electronic control unit interprets the sensor signal as a real-time indication of engine fuel requirements.
Vortex flow meter
Another method of flow measurement involves placing the bluff body (called a bar shedder) in the fluid path. When the liquid passes through this bar, a disturbance in the flow called vorticity is made. Traces of vortices behind the cylinder, or from each side of the cliff body. This vortex trace is called Von KÃÆ'¡rmÃÆ'¡n vortex street after von KÃÆ'¡rmÃÆ'¡n's 1912 mathematical description of the phenomenon. The frequency at which the alternate sides of the vortices are essentially proportional to the flow rate of the liquid. Inside, above, or downstream the shedder bar is a sensor to measure the frequency of vortex shedding. These sensors are often piezoelectric crystals, which produce small but measurable voltage pulses each time a vortex is created. Since the frequency of the voltage pulse is also proportional to the fluid velocity, the volumetric flow rate is calculated using the cross-sectional area of ​​the flow meter. The measured frequency and flow rate are calculated by electronic flowmeter using the equation
The sonar flow meter is a non-intrusive clamp on devices that measure the flow in pipes that deliver slurry, corrosive liquids, multifase fluids and streams where insertion of the meter flow type is undesirable. The sonar flow meter has been widely adopted in mining, metal processing, and upstream oil and gas industries where traditional technology has certain limitations due to their tolerance to various flow regimes and lowered ratios.
The sonar flow meter has the capacity to measure the velocity of the liquid or gas that is not intrusive in the pipe and then utilizes this velocity measurement into flow rate using the pipe cross section and the pressure and the channel temperature. The principle behind this flow measurement is the use of acoustic underwater.
In an underwater acoustics, to find objects underwater, sonar uses two known ones:
- The speed of sound propagation through an array (ie, the velocity of seawater)
- The distance between the sensors in the sensor array
and then calculate the unknown:
- The location (or angle) of an object.
Likewise, sonar flow measurements use the same techniques and algorithms used in underwater acoustics, but apply them to the measurement of oil and gas well flow and flow paths.
To measure flow velocity, the sonar flow meter using two is known:
- The location (or angle) of the object, which is 0 degrees because the flow moves along the pipe, parallel to the sensor array
- The distance between the sensors in the sensor array
and then count the unknown:
- The propagation velocity through the array (ie the speed of the medium flow in the pipe).
Electromagnetic, ultrasonic, and Coriolis flow meters
Modern innovations in flow rate measurements incorporate electronic devices that can correct various pressures and temperatures (ie density), non-linearity, and fluid characteristics.
Magnetic flow meter
The magnetic flow meter, often called the "mag meter" or "electromag", uses a magnetic field applied to a metering tube, which yields a potential difference proportional to the flow velocity perpendicular to the flux line. The potential difference is felt by the electrode parallel to the flow and the applied magnetic field. The physical principle in the workplace is Faraday's electromagnetic induction law. The magnetic flow meter requires conduction fluid and non-conductor pipe liner. Electrodes should not corrode in contact with process fluids; some magnetic flowmeters have an extra transducer installed to clean the electrode in place. The magnetic field used is pulsed, allowing the flowmeter to cancel the stray voltage effect in the piping system.
Non-contact electromagnetic flow meter
The Lorentz-style velocimetry system is called the Lorentz-style flowmeter (LFF). LFF measures the strength of integrated or bulk Lorentz generated from the interaction between the moving molten metal and the applied magnetic field. In this case the characteristic length of the magnetic field is an order of magnitude equal to the channel dimension. It should be noted that in cases where a local magnetic field is used, it is possible to perform local velocity measurements and hence the term Lorentz-style velosimeter is used.
Ultrasonic flow meter (Doppler, transit time)
There are two main types of ultrasonic flow meters: Doppler and transit time. While they both use ultrasound to make measurements and can be non-invasive (measuring flow from outside tubes, pipes or vessels), they measure the flow with very different methods.
Ultrasonic transit time flow meter measures the difference in transit time of ultrasonic pulses propagating in and against the flow direction. This time difference is the measure for the average velocity of the fluid along the ultrasonic ray path. By using absolute transit time, the average flow velocity and speed of sound can be calculated. Using two transits and and the distance between receiving and transmitting transducers and slope angle One can write an equation:
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di mana adalah kecepatan rata-rata cairan sepanjang jalur suara dan adalah kecepatan suara.
With ultrasound ultrasonography the timing of the beam width beam can also be used to measure the volume flow independently of the cross-sectional area of ​​the vessel or tube.
The Ultrasonic Doppler flow meter measures the Doppler shift resulting from reflecting ultrasonic light from the particles in the flowing liquid. The transmitted beam frequency is affected by the movement of particles; This frequency shift can be used to calculate the fluid velocity. In order for the Doppler principle to work, there must be a high enough reflective sonic material density such as solid particles or air bubbles suspended in liquids. This is in stark contrast to ultrasonic transit time flow meters, where bubbles and solid particles reduce measurement accuracy. Because of the dependence on these particles there is limited application for Doppler flow meters. This technology is also known as Doppler acoustic velocimetry.
One advantage of ultrasonic flow meters is that they can effectively measure the flow rate for various liquids, as long as the speed of sound through the liquid is known. For example, ultrasonic flow meters are used for measurements of various liquids such as liquefied natural gas (LNG) and blood. One can also calculate the expected sound velocity for a given fluid; this can be compared with the sound speed measured empirically by the ultrasonic flow meter for the purpose of monitoring the measurement quality of the flow meter. A decrease in quality (a change in measured sound velocity) is an indication that gauges need to be serviced.
Coriolis flow meter
Using the Coriolis effect that causes the distorted lateral vibrating tube, the direct measurement of mass flow can be obtained in the coriolis flow meter. Furthermore, a direct measure of the fluid density is obtained. Coriolis measurements can be very accurate regardless of the type of gas or fluid being measured; the same measuring tube can be used for hydrogen gas and asphalt without recalibration.
The Coriolis flow meter can be used for natural gas flow measurements.
Doppler Laser flow measurements
The laser beam that beats the partially moving particles will be dispersed with changes in wavelength proportional to the particle velocity (Doppler effect). A Doppler velocimeter (LDV) laser, also called a laser Doppler anemometer (LDA), focuses the laser beam into small volumes in a flowing liquid containing small particles (natural or inducible). Particles scatter light with Doppler shift. This shiftable wavelength analysis can be used directly, and with great precision, determine the particle velocity and thus approach the approximate velocity of the fluid.
A number of different device techniques and configurations are available to determine the Doppler shift. All use a photodetector (usually photodiode avalanche) to convert light into electrical waves for analysis. On most devices, the original laser beam is divided into two beams. In one common LDV class, two beams are made to intersect at their focal point where they interfere and produce a set of straight edges. The sensor is then aligned with the flow so that the frinji is perpendicular to the flow direction. As particles pass through the edges, Doppler light shifts are collected into the photodetector. In other common LDV classes, one beam is used as a reference and the other is Doppler-scattered. The two beams are then collected onto a photodetector where optical heterodyne detection is used to extract Doppler signals.
Calibration
Although ideally flowmeter should not be affected by its environment, in practice this is not possible. Often the measurement error comes from a faulty installation or other dependent environmental factor. The in situ method is used when the flow meter is calibrated under the correct flow conditions. The results of flowmeter calibration will yield two related statistics: performance indicator metrics and flow rate metrics.
Transit time method
For a so-called flow pipe the transit time method is applied where the radiotracer is injected as a pulse into the measured stream. The transit time is determined with the help of a radiation detector placed outside the pipe. The volume flow is obtained by multiplying the mean fluid flow velocity measured by the inner portion of the inner pipe. The value of this reference flow is compared with the simultaneous flow value provided by the measurement of the stream to be calibrated.
The procedure is standard (ISO 2975/VII for fluids and BS 5857-2.4 for gas). The best accredited measurement uncertainty for liquids and gases is 0.5%.
Tracer dilution method
The radiotracer dilution method is used to calibrate open channel flow measurements. Solutions with known tracer concentrations are injected at a constant known velocity into the channel flow. The downstream tracking solution is completely blended over the cross-sectional flow, the continuous sample is taken and the concentration of the tracer in relation to that of the injected solution is determined. The stream reference value is determined by using a regulatory balance condition between the injected tracking flow and the dilution stream. The procedure is standard (ISO 9555-1 and ISO 9555-2 for fluid flow in open channels). The best accredited measurement uncertainty is 1%.
See also
- Automatic meter reading
- Flow meter error
- Ford's consistency cup
- Gas meter
- Laser Doppler velocimetry
- Main stream element
- Water meter
References
Source of the article : Wikipedia
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