This International Standard specifies a method for the determination of the volume rate of flow of a single phase fluid of a substantially constant density (Mach number ≤ 0,25) under steady fully developed flow conditions in a closed conduit of circular cross-section running full by measurement of the flow velocity in a single point.
The method provides for the possibility of placing the primary velocity measuring device either at the point where it is assumed the mean axial velocity prevails, i.e. at a distance of 0,242 R from the wall of the conduit (R being the radius of the conduit), or on the axis of the conduit.
If there are doubts about the symmetry of the flow it is advisable to use at least two measuring points located symmetrically on one circumference at the distance from the wall specified above.
If there are doubts about the symmetry of the flow it is advisable to use at least two measuring points located symmetrically on one circumference at the distance from the wall specified above.
1.2 Field of application
The method specified in this International Standard does not apply unless the following conditions have been fulfilled:
The conduit shall have a straight length sufficiently long so that, in the measuring section, a distribution of velocities corresponding to fully developed turbulent flow can be observed (see 2.3.5).
Hydraulic resistance coefficient λ of the conduit should not exceed 0,06.
- The flow must be turbulent and the Reynolds number, ReD\' should exceed or be equal to the following values:
| λ | ≥ 0,03 | 0,025 | 0,02 | 0,01 |
| ReD | 104 | 3 × 104 | 105 | 106 |
When the velocity is measured on the conduit axis, the flow should be in fully rough turbulent regime (see 2.3.6). The Reynolds number, ReD, should then exceed or be equal to:
| λ | 0,06 | 0,05 | 0,04 | 0,03 | 0,025 | 0,02 | 0,01 |
| ReD | 3 × 104 | 5 × 104 | 105 | 3 × 105 | 5 × 105 | 106 | 5 × 107 |
- The experimental data on which this International Standard is based principally relate to conduits of diameter equal to or greater than 300 mm, but there is every reason to believe that the method can be applied to conduits of smaller diameter.
- In any point of the measuring cross-section, the angle between the direction of local velocity and the axis should not exceed 5°.
This condition can be verified either with the probe used for the measurements, if the design permits this, or with a different type of probe. It can be assumed that if the condition required is verified for a given flow q, then this condition is also met within the range q/3 to 3q.
1.3 Accuracy of the method
As a guide, it can be considered that determination of flow from velocity measurement at a single point, carried out in accordance with the requirements of this International Standard, will lead to an uncertainty (at a confidence level of 95 %) not exceeding ± 3 %. However, the uncertainty on the flow shall be calculated for each individual application of this International Standard depending on the type of primary device, on the method of use and if necessary, on the method of calibration as well as on the measuring conditions.