Editing Flow measurement (section)

==Calibration==
Even though ideally the flowmeter should be unaffected by its environment, in practice this is unlikely to be the case. Often measurement errors originate from incorrect installation or other environment dependent factors.<ref>{{cite journal |last=Cornish |first=D |date=1994 |title=Instrument performance |journal=Measurement and Control |volume=27 |issue=10 |pages=323–328|doi=10.1177/002029409402701004 }}</ref><ref>{{cite book|last=Baker |first=Roger C. |date=2016 |title=Flow Measurement Handbook |publisher=Cambridge University Press |isbn=978-1-107-04586-6}}</ref> [[In situ]] methods are used when flowmeter is calibrated in the correct flow conditions. The result of a flowmeter calibration will result in two related statistics: a performance indicator metric and a flow rate metric.<ref>{{Cite book|chapter-url=http://eu.wiley.com/legacy/wileychi/hbmsd/pdfs/mm269.pdf|title=Handbook of Measuring System Design|chapter=Calibration and Standards in Flow Measurement|last=Paton|first=Richard|publisher=Wiley|access-date=2017-09-26|url-status=live|archive-url=https://web.archive.org/web/20170829024615/http://eu.wiley.com/legacy/wileychi/hbmsd/pdfs/mm269.pdf|archive-date=29 August 2017}}</ref>

===Transit time method===
For pipe flows a so-called transit time method is applied where a radiotracer is injected as a pulse into the measured flow. The transit time is defined with the help of radiation detectors placed on the outside of the pipe. The volume flow is obtained by multiplying the measured average fluid flow velocity by the inner pipe cross-section. This reference flow value is compared with the simultaneous flow value given by the flow measurement to be calibrated.

The procedure is standardised (ISO 2975/VII for liquids and BS 5857-2.4 for gases). The best accredited measurement uncertainty for liquids and gases is 0.5%.<ref name="Finnish Accreditation Service">[https://archive.today/20130213110628/http://www.mikes.fi/Scopes/K028_M09_2008_k01.htm Finnish Accreditation Service]</ref>

===Tracer dilution method===
The radiotracer dilution method is used to calibrate open channel flow measurements. A solution with a known tracer concentration is injected at a constant known velocity into the channel flow. Downstream the tracer solution is thoroughly mixed over the flow cross-section, a continuous sample is taken and its tracer concentration in relation to that of the injected solution is determined. The flow reference value is determined by using the tracer balance condition between the injected tracer flow and the diluting flow.
The procedure is standardised (ISO 9555-1 and ISO 9555-2 for liquid flow in open channels). The best accredited measurement uncertainty is 1%.<ref name="Finnish Accreditation Service"/>