# Commission Delegated Regulation (EU) 2017/654 of 19 December 2016 supplementing Regulation (EU) 2016/1628 of the European Parliament and of the Council with regard to technical and general requirements relating to emission limits and type-approval for internal combustion engines for non-road mobile machinery, Division 2.5.2: Critical flow venturi (CFV)

Extent: E+W+S+N.I.

Calibration of the CFV is based upon the flow equation for a critical venturi. Gas flow is a function of venturi inlet pressure and temperature.
To determine the range of critical flow, K V shall be plotted as a function of venturi inlet pressure. For critical (choked) flow, K V will have a relatively constant value. As pressure decreases (vacuum increases), the venturi becomes unchoked and K V decreases, which indicates that the CFV is operated outside the permissible range.
The airflow rate (qV CVS) at each restriction setting (minimum 8 settings) shall be calculated in standard m3/s from the flowmeter data using the manufacturer's prescribed method. The calibration coefficient K V K × m 4 × s ∕ kg shall be calculated from the calibration data for each setting by means of equation (7-72):
K V = q V CVS × T p p
(7-72)
Where:
qV SSV air flow rate at standard conditions (101,325 kPa, 273,15 K) [m3/s]
T temperature at the venturi inlet [K]
p p absolute pressure at venturi inlet [kPa]
The average K V and the standard deviation shall be calculated. The standard deviation shall not exceed ± 0,3 % of the average K V.

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Source: https://www.legislation.gov.uk/eur/2017/654/annex/VII/division/2/division/2.5/division/2.5.2 (the official version)
Page: https://openlegislation.uk/eu-regulation-2017-654/annex-VII/division-2/division-2.5/division-2.5.2
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.