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Encyclopedia > Flow measurement

Flow measurement is the quantification of bulk fluid or gas movement. It can be measured in a variety of ways. A fluid is defined as a substance that continually deforms (flows) under an applied shear stress regardless of the magnitude of the applied stress. ... This article does not cite any references or sources. ...


Dependent on the quantity measured different symbols are used. The volumetric flow rate is usually given the symbol Q and the mass flow rate the symbol  dot m. In fluid dynamics, the volumetric flow rate, also volume flow rate and rate of fluid flow, is the volume of fluid which passes through a given volume per unit time (for example gallons per minute or squeaks per parsec). ... Mass flow rate is the movement of mass per time. ...

Contents

Units of measurement

Gas

Gas volumetric flow rate is sometimes measured in "standard cubic centimeters per minute" (abbreviation sccm), a unit acceptable for use with SI except that the additional information attached to the unit symbol. The SI standard would be m3/s (with any appropriate prefix, with temperature and pressure specified). The term "standard" indicates that the given flow rate assumes a standard temperature and pressure. Many other similar abbreviations are also in use, such as standard cubic feet per minute or per second. Look up si, Si, SI in Wiktionary, the free dictionary. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ...


Liquid

For liquids other units used depend on the application and industry but might include gallons (U.S. liquid or imperial) per minute, liters per second, bushels per minute and, when describing river flows, acre-feet per day.


Mechanical flow meters

There are several types of mechanical flow meter


Piston Meter

Due to the fact that they used for domestic water measurement Piston meters, (also known as Rotary Piston, or Semi-Positive displacement meters) are the most common in the UK and are used for almost all meter sizes up to and including 40 mm (1 1/2"). The piston meter operates on the principle of a piston rotating within a chamber of known volume. For each rotation, an amount of water passed through the piston chamber.


Woltmann Meter

Woltman meters, commonly referred to as Helix meters are popular at larger sizes. Jet meters (single or Multi-Jet) are increasing in popularity in the UK at larger sizes and are commonplace in the EU.


Venturi Meter

Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure that results across the constriction. This method is widely used to measure flow rate in the transmission of gas through pipelines, and has been used since Roman Empire times. A fluid passing through smoothly varying constrictions experience changes in velocity and pressure, as described by Bernoullis principle. ... Motto Senatus Populusque Romanus (SPQR) The Roman Empire at its greatest extent. ...


Dall Tube

A shortened form of the Venturi. Lower pressure drop than an orifice plate.


Orifice Plate

Another simple method of measurement uses an orifice plate, which is basically a plate with a hole through it. It is placed in the flow and constricts the flow. It uses the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's principle). An orifice plate is a device which measures rate of fluid flow. ... -1...


Pitot tube

Measurement of the pressure within a pitot tube in the flowing fluid, or the cooling of a heated element by the passing fluid are two other methods that are used. These types of sensors are advantageous in that they are rugged, so not easily damaged in an extreme environment. A Pitot tube is a measuring instrument used to measure fluid flow. ...


A pitot tube is an L shaped tube which is also able to measure fluid flow.


Paddle wheel

The paddle wheel translates the mechanical action of paddles rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The paddle tends to be inserted into the flow.


Pelton wheel

The Pelton wheel turbine (better described as a radial turbine) translates the mechanical action of the Pelton wheel rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The Pelton wheel tends to have all the flow travelling around it.


Turbine flow meter

The turbine flowmeter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow travelling around it.


Thermal mass flow meters

Thermal mass flow meters generally use one or more heated elements to measure the mass flow of gas. The gas temperature is also measured and compensated for. They provide a direct mass flow readout, and do not need any additional pressure temperature compensation over their specified range.


Thermal mass flow meters are used for compressed air, nitrogen, helium, argon, oxygen, natural gas. I fact, most gases can be measured as long as they are fairly clean and non-corrosive.


Vortex flowmeters

Another method of flow measurement involves placing an object (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder in two rolls, alternatively from the top or the bottom of the cylinder. This vortex trail is called the Von Kármán vortex street after von Karman's 1912 mathematical description of the phenomenon. The speed at which these vortices are created is proportional to the flow rate of the fluid. Inside the shedder bar is a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created. The frequency of this voltage pulse is also proportional to the fluid flow rate, and is measured by the flowmeter electronics. This is an article on the real vortex phenomena. ... Von Kármán vortex street off the Chilean coast near the Juan Fernandez Islands A Von Kármán vortex street is a repeating pattern of swirling vortices. ... Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. ... FreQuency is a music video game developed by Harmonix and published by SCEI. It was released in November 2001. ...


With f= SV/L where, � f = the frequency of the vortices � L = the characteristic length of the bluff body � V = the velocity of the flow over the bluff body � S = Strouhal Number and is a constant for a given body shape


Magnetic, ultrasound and coriolis flow meters

Modern innovations in the measurement of flow rate incorporate electronic devices that can correct for varying pressure and temperature (i.e. density) conditions, non-linearities, and for the characteristics of the fluid.


Magnetic flow meters

Industrial magnetic flowmeter

The most common flow meter apart from the mechanical flow meters, is the magnetic flow meter, commonly referred to as a "mag meter" or an "electromag". A magnetic field is applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the flux lines. The physical principle at work is Faraday's law of electromagnetic induction. The magnetic flow meter requires a conducting fluid, e.g. water, and an electrical insulating pipe surface, e.g. a rubber lined non magnetic steel tube. Image File history File links Size of this preview: 675 × 599 pixelsFull resolution (962 × 854 pixel, file size: 239 KB, MIME type: image/gif) [Category: Instrumentation]] [Category: Automation]] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Size of this preview: 675 × 599 pixelsFull resolution (962 × 854 pixel, file size: 239 KB, MIME type: image/gif) [Category: Instrumentation]] [Category: Automation]] File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... The most common flowmeter, apart from the mechanical flow meters, is the magnetic flow meter. ... Magnetic field lines shown by iron filings In physics, a magnetic field is a solenoidal vector field in the space surrounding moving electric charges and magnetic dipoles, such as those in electric currents and magnets. ... Faradays law of induction (more generally, the law of electromagnetic induction) states that the induced emf (electromotive force) in a closed loop equals the negative of the time rate of change of magnetic flux through the loop. ... For magnetic induction, see Magnetic field. ...


Ultrasonic flow meters

Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in and against flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times tup and tdown and the distance between receiving and transmitting transducers L and the inclination angle α one can write the equations: The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...


v = frac{L}{{2;sin left( alpha right)}};frac{{t_{up} - t_{down} }}{{t_{up} ;t_{down} }} and c = frac{L}{2};frac{{t_{up} + t_{down} }}{{t_{up} ;t_{down} }}


where v is the average velocity of the fluid along the sound path and c is the speed of sound.

Schematic view of an flow sensor.

Measurement of the doppler shift resulting in reflecting an ultrasonic beam off the flowing fluid is another recent innovation made possible by electronics. By passing an ultrasonic beam through the tissues, bouncing it off of a reflective plate then reversing the direction of the beam and repeating the measurement the volume of blood flow can be estimated. The speed of transmission is affected by the movement of blood in the vessel and by comparing the time taken to complete the cycle upstream versus downstream the flow of blood through the vessel can be measured. The difference between the two speeds is a measure of true volume flow. A wide-beam sensor can also be used to measure flow independent of the cross-sectional area of the blood vessel. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... The Doppler effect is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. ... Ultrasound is sound with a frequency greater than the upper limit of human hearing, approximately 20 kilohertz. ... Human blood smear: a - erythrocytes; b - neutrophil; c - eosinophil; d - lymphocyte. ...


Coriolis flow meters

Using the Coriolis effect that causes a laterally vibrating tube to distort, a direct measurement of mass flow can be obtained in a coriolis flow meter. Furthermore a direct measure of the density of the fluid is obtained. Coriolis measurement can be very accurate irrespective of the type of gas or liquid that is measured; the same measurement tube can be used for hydrogen gas and peanut butter without recalibration. A mass flow meter, also known as inertial flow meter and coriolis flow meter, is a device that measures how much liquid is flowing through a tube. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Peanut butter in a jar. ... Calibration refers to the process of determining the relation between the output (or response) of a measuring instrument and the value of the input quantity or attribute, a measurement standard. ...


Laser doppler flow measurement

Laser-doppler flow meter.

Blood flow can be measured through the use of a monochromatic laser diode. The laser probe is inserted into a tissue and turned on, where the light scatters and a small portion is reflected back to the probe. The signal is then processed to calculate flow within the tissues. There are limitations to the use of a laser doppler probe; flow within a tissue is dependent on volume illuminated, which is often assumed rather than measured and varies with the optical properties of the tissue. In addition, variations in the type and placement of the probe within identical tissues and individuals result in variations in reading. The laser doppler has the advantage of sampling a small volume of tissue, allowing for great precision, but does not necessarily represent the flow within an entire organ. The flow meter is more useful for relative rather than aboslute measurements. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Something which is monochromatic has a single color. ... A packaged laser diode with penny for scale. ...


See also

Mass flow rate is the movement of mass per time. ... In fluid dynamics, the volumetric flow rate, also volume flow rate and rate of fluid flow, is the volume of fluid which passes through a given volume per unit time (for example gallons per minute or squeaks per parsec). ... A residential gas meter of the usual diaphragm style A gas meter is used to measure the flow of fuel gases such as natural gas and propane. ... A typical residential water meter A water meter is a device used to measure water usage. ... An orifice plate is a device which measures rate of fluid flow. ... Automatic meter reading, or AMR, is the technology of automatically collecting data from water meter or energy metering devices (water, gas, electric) and transferring that data to a central database for billing and/or analyzing. ... Airspeed Indicator in a light aircraft The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the crafts airspeed, typically in knots, to the pilot. ... An air flow meter, also know as air consumption meter, is a device that measures how much air is flowing through a tube. ... A mass flow meter, also known as inertial flow meter and coriolis flow meter, is a device that measures how much liquid is flowing through a tube. ... Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. ...

External links


  Results from FactBites:
 
A Flow Measurement Orientation (3246 words)
It is for this reason that a flow restriction causes an increase in the flowing velocity and also causes a drop in the static pressure of the flowing fluid.
One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive.
When measuring the flow of compressible materials, volumetric flow is not very meaningful unless density (and sometimes also viscosity) is constant.
Flow measurement - Wikipedia, the free encyclopedia (1044 words)
Flow measurement is the quantification of bulk fluid or gas movement.
Another method of measurement, known as a venturi meter, is to constrict the flow in some fashion, and measure the differential pressure that results across the constriction.
Measurement of the doppler shift resulting in reflecting an ultrasonic beam off the flowing fluid is another recent, accurate innovation made possible by electronics.
  More results at FactBites »

 
 

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