Editing Flow measurement (section)

==Electromagnetic, ultrasonic and Coriolis flowmeters==
[[File:Tetley's brewery, Leeds (10th May 2010) 008.jpg|thumb|right|A magnetic flowmeter at the [[Tetley's Brewery]] in [[Leeds]], [[West Yorkshire]]]]

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 flowmeters===
[[Magnetic flow meter|Magnetic flowmeters]], often called "mag meter"s or "electromag"s, use a [[magnetic field]] applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the [[flux]] lines. The potential difference is sensed by electrodes aligned perpendicular to the flow and the applied magnetic field. The physical principle at work is [[Faraday's law of induction|Faraday's law]] of [[electromagnetic induction]]. The magnetic flowmeter requires a conducting fluid and a nonconducting pipe liner. The electrodes must not corrode in contact with the process fluid; some magnetic flowmeters have auxiliary transducers installed to clean the electrodes in place. The applied magnetic field is pulsed, which allows the flowmeter to cancel out the effect of stray voltage in the piping system.

===Non-contact electromagnetic flowmeters===
A [[Lorentz force velocimetry]] system is called Lorentz force flowmeter (LFF). An LFF measures the integrated or bulk Lorentz force resulting from the interaction between a [[liquid metal]] in motion and an applied magnetic field. In this case, the characteristic length of the magnetic field is of the same order of magnitude as the dimensions of the channel. It must be addressed that in the case where localized magnetic fields are used, it is possible to perform local velocity measurements and thus the term Lorentz force velocimeter is used.

===Ultrasonic flowmeters (Doppler, transit time)===
There are two main types of [[ultrasonic flow meter|ultrasonic flowmeters]]: Doppler and transit time. While they both utilize ultrasound to make measurements and can be non-invasive (measure flow from outside the tube, pipe or vessel, also called clamp-on device), they measure flow by very different methods.
[[Image:Tttecnology.gif|thumb|150px|Schematic view of a flow sensor]]
Ultrasonic '''transit time''' flowmeters measure the difference of the transit time of ultrasonic pulses propagating in and against the direction of flow. 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 <math>t_\text{up}</math> and <math>t_\text{down}</math> and the distance between receiving and transmitting transducers <math>L</math> and the inclination angle <math>\alpha</math> one can write the equations:
<math display="block">v = \frac{L}{{2\;\cos \left( \alpha  \right)}}\;\frac{t_\text{up}  - t_\text{down} }{t_\text{up} \;t_\text{down}}</math>
and
<math display="block">c = \frac{L}{2}\;\frac{t_\text{up} + t_\text{down}}{t_\text{up} \;t_\text{down}}</math>
where <math>v</math> is the average velocity of the fluid along the sound path and <math>c</math> is the speed of sound.

With wide-beam illumination transit time ultrasound can also be used to measure volume flow independent of the cross-sectional area of the vessel or tube.<ref>{{cite journal|last=Drost|first=CJ|title=Vessel Diameter-Independent Volume Flow Measurements Using Ultrasound|journal=Proceedings of San Diego Biomedical Symposium|year=1978|volume=17|pages=299–302}}</ref>

Ultrasonic '''Doppler '''flowmeters measure the [[Doppler shift]] resulting from reflecting an [[Ultrasound|ultrasonic]] beam off the particulates in flowing fluid. The frequency of the transmitted beam is affected by the movement of the particles; this frequency shift can be used to calculate the fluid velocity. For the Doppler principle to work, there must be a high enough density of sonically reflective materials such as solid particles or [[air bubble]]s suspended in the fluid. This is in direct contrast to an ultrasonic transit time flowmeter, where bubbles and solid particles reduce the accuracy of the measurement. Due to the dependency on these particles, there are limited applications for Doppler flowmeters. This technology is also known as [[acoustic Doppler velocimetry]].

One advantage of ultrasonic flowmeters is that they can effectively measure the flow rates for a wide variety of fluids, as long as the speed of sound through that fluid is known. For example, ultrasonic flowmeters are used for the measurement of such diverse fluids as liquid natural gas (LNG) and blood.<ref>[[American Gas Association]] Report Number 9</ref> One can also calculate the expected speed of sound for a given fluid; this can be compared to the speed of sound empirically measured by an ultrasonic flowmeter for the purposes of monitoring the quality of the flowmeter's measurements. A drop in quality (change in the measured speed of sound) is an indication that the meter needs servicing.

===Coriolis flowmeters===
Using the [[Coriolis effect]] that causes a laterally vibrating tube to distort, a direct measurement of [[mass flow rate|mass flow]] can be obtained in a [[coriolis flow meter|coriolis flowmeter]].<ref>{{Cite book|title=Introductory guide to Flow Measurement|last=Baker|first=Roger C.|publisher=ASME|year=2003|isbn=0-7918-0198-5}}</ref> 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 [[bitumen]] without re[[calibration]].{{Citation needed|date=July 2015}}

Coriolis flowmeters can be used for the measurement of natural gas flow.<ref>[[American Gas Association]] Report Number 11</ref>