Editing Flow measurement (section)

==Pressure-based meters==
There are several types of flowmeter that rely on [[Bernoulli's principle]]. The pressure is measured either by using laminar plates, an orifice, a nozzle, or a Venturi tube to create an artificial constriction and then measure the pressure loss of fluids as they pass that constriction, or by measuring [[Static pressure|static]] and [[stagnation pressure]]s to derive the [[dynamic pressure]].

===Venturi meter===
A [[Venturi effect|Venturi meter]] constricts the flow in some fashion, and [[pressure sensor]]s measure the differential pressure before and within the constriction. This method is widely used to measure flow rate in the transmission of gas through [[Pipeline transport|pipelines]], and has been used since [[Roman Empire]] times. The [[Discharge coefficient|coefficient of discharge]] of Venturi meter ranges from 0.93 to 0.97. The first large-scale Venturi meters to measure liquid flows were developed by [[Clemens Herschel]], who used them to measure small and large flows of water and [[wastewater]] beginning at the very end of the 19th century.<ref>[[Clemens Herschel|Herschel, Clemens]]. (1898). ''Measuring Water''. [[Providence, Rhode Island]]: Builders Iron Foundry.</ref>

===Orifice plate===
An [[orifice plate]] is a plate with a hole through it, placed perpendicular to the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate. It is basically a crude form of [[Venturi meter]], but with higher energy losses. There are three type of orifice: concentric, eccentric, and segmental.<ref>Lipták, [https://books.google.com/books?id=ju0U2ItY1QsC&dq=orifice%20plate%20meter%20eccentric%20plate&pg=PA85 ''Flow Measurement''], p. 85</ref><ref>{{cite report|publisher =[[American Gas Association]] |title = Report Number 3: Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids|date = September 2012 |url-access = subscription|url = https://global.ihs.com/doc_detail.cfm?document_name=AGA%20REPORT%20%233%20P1}}</ref>

===Dall tube===
The Dall tube is a shortened version of a Venturi meter, with a lower pressure drop than an orifice plate. As with these flowmeters the flow rate in a Dall tube is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is typically measured using diaphragm pressure transducers with digital readout. Since these meters have significantly lower permanent pressure losses than orifice meters, Dall tubes are widely used for measuring the flow rate of large pipeworks. Differential pressure produced by a Dall tube is higher than Venturi tube and nozzle, all of them having same throat diameters.

===Pitot tube===
{{Main|Pitot tube}}
A [[pitot tube]] is used to measure fluid flow velocity. The tube is pointed into the flow and the difference between the [[stagnation pressure]] at the tip of the probe and the [[static pressure]] at its side is measured, yielding the dynamic pressure from which the fluid velocity is calculated using [[Bernoulli's equation]]. A volumetric rate of flow may be determined by measuring the velocity at different points in the flow and generating the velocity profile.

===Averaging pitot tube===
{{See also|Annubar}}
Averaging pitot tubes (also called impact probes) extend the theory of pitot tube to more than one dimension. A typical averaging pitot tube consists of three or more holes (depending on the type of probe) on the measuring tip arranged in a specific pattern. More holes allow the instrument to measure the direction of the flow velocity in addition to its magnitude (after appropriate calibration). Three holes arranged in a line allow the pressure probes to measure the velocity vector in two dimensions. Introduction of more holes, e.g. five holes arranged in a "plus" formation, allow measurement of the three-dimensional velocity vector.

===Cone meters===
[[File:VW8-WN-RF-Cls300 composite.Low Res.jpg|thumb|{{convert|8|in|mm|0|adj=on}} V-cone [[flowmeter]] shown with ANSI 300# ({{convert|300|lb/sqin|bar MPa|disp=output only}}) raised face [[weld neck flange]]s]]
Cone meters are a newer differential pressure metering device first launched in 1985 by McCrometer in Hemet, CA. The cone meter is a generic yet robust differential pressure (DP) meter that has shown to be resistant to effects of asymmetric and swirling flow. While working with the same basic principles as Venturi and orifice type DP meters, cone meters don't require the same upstream and downstream piping.<ref>{{Cite news |url=https://pgjonline.com/2012/07/03/cone-dp-meter-calibration-issues/|title=Cone DP Meter Calibration Issues |work=Pipeline & Gas Journal |language=en-US |archive-url=https://web.archive.org/web/20170927052949/https://pgjonline.com/2012/07/03/cone-dp-meter-calibration-issues/ |archive-date=27 September 2017 |url-status=live |access-date=1 September 2019}}</ref> The cone acts as a conditioning device as well as a differential pressure producer. Upstream requirements are between 0–5 diameters compared to up to 44 diameters for an orifice plate or 22 diameters for a Venturi. Because cone meters are generally of welded construction, it is recommended they are always calibrated prior to service. Inevitably heat effects of welding cause distortions and other effects that prevent tabular data on discharge coefficients with respect to line size, beta ratio and operating Reynolds numbers from being collected and published. Calibrated cone meters have an uncertainty up to ±0.5%. Un-calibrated cone meters have an uncertainty of ±5.0%{{Citation needed|date=July 2015}}

=== Linear resistance meters ===
Linear resistance meters, also called laminar flowmeters, measure very low flows at which the measured differential pressure is linearly proportional to the flow and to the fluid viscosity. Such flow is called viscous drag flow or laminar flow, as opposed to the turbulent flow measured by orifice plates, Venturis and other meters mentioned in this section, and is characterized by Reynolds numbers below 2000. The primary flow element may consist of a single long capillary tube, a bundle of such tubes, or a long porous plug; such low flows create small pressure differentials but longer flow elements create higher, more easily measured differentials. These flowmeters are particularly sensitive to temperature changes affecting the fluid viscosity and the diameter of the flow element, as can be seen in the governing [[Hagen–Poiseuille equation]].<ref>{{cite book|last1=Miller|first1=Richard W.|title=Flow Measurement Engineering Handbook|date=1996|publisher=Mcgraw Hill|isbn=0070423660|page=6.16–6.18|edition=3rd}}</ref><ref>{{cite book|editor1-last=Bean|editor1-first=Howard S.|title=Fluid Meters, Their Theory and Application|date=1971|publisher=The American Society of Mechanical Engineers|location=New York|pages=77–78|edition=6th}}</ref>