AquaProbe FEA100 / FEA200
Electromagnetic flowmeter – insertion-type flow sensors
3 Mechanical Installation
OI/FEA100/200–EN Rev. D
11
3.3.1 International Standard for Flow Measurement
ISO 7145 '(BS 1042) Measurement of fluid flow in closed conduits 'Part 2 Velocity area methods' describes
methods of calculating volumetric flow from velocity measurements.
Section 2.2: 1982 'Method of measurement of velocity at one point of a conduit of circular cross section'
describes the inference of volumetric flow from measurement of velocity at a single point. Several
conditions must be fulfilled to validate the method, that uses calculations based on empirical data.
Where the validating conditions can be met, the method described in Section 3.3, page 10 is the most
practical. It is possible to measure the velocity either on the centre line, which reduces sensitivity to
positional errors, or at the assumed point of mean flow velocity.
Table 3.1 is an extract from ISO 7145 (BS 1042): Section 2.2: 1982 and is reproduced with the permission
of BSI. Complete copies of the standard can be obtained by post from BSI Publications, Linford Wood,
Milton Keynes, MK14 6LE.
3.3.2 Velocity Limitations
All insertion flow sensor devices are susceptible to the vortex shedding effect that can cause severe
vibration of the flow sensor, resulting in damage and/or measurement instability. Electromagnetic devices
with no moving parts, such as the flow sensor, are less susceptible to this effect than mechanical devices.
Fig. 3.4, page 12 shows the maximum permissible velocities, depending on the flow sensor's location.
This information is provided as a guide only. Some installations may experience unwanted vibration
resonance that may further limit the maximum velocity at which the flow sensor can be used.
Note.
Where the above ideal conditions cannot be achieved, the flow profile must be tested for
symmetry in order to obtain reliable flow results.
Minimum upstream straight length*
Type of disturbance upstream from the
measuring cross-section
For a measurement at the
point of mean axial velocity
For a measurement on the
axis of the conduit
90° elbow or a t-bend
Several 90° coplanar bends
Several 90° non- coplanar bends
Total angle convergent 18 to 36°
Total angle divergent 14 to 28°
Fully opened butterfly valve
Fully opened plug valve
50
50
80
30
55
45
30
25
25
50
10
25
25
15
* Expressed in multiples of the diameter of the conduit.
Downstream from the measurement cross-section, the straight length shall be at least equal to five duct diameters
whatever the type of disturbance.
Table 3.1 Straight Pipe Lengths
