ABB AquaProbe FEA100 User Manual page 30

AquaProbe FEA100 / FEA200
Electromagnetic flowmeter
The Point of Mean Velocity is on the knee of the curve (the velocity at this point is changing rapidly with
distance) so it is necessary to position the probe extremely accurately in order to measure the correct
velocity. If the probe is inserted accurately to 72.5 mm, it is therefore measuring the mean velocity of
1.722 m/s which, when multiplied by the area, gives a volume flow of 487 l/s. If the probe is inserted to
74 mm instead of 72.5, the velocity measurement is 1.85 m/s instead of the expected 1.722. Multiplying
this figure by the area results in a volume flow of 523 l/sec – an error of 7.4 %.
On-site it can be very difficult to locate a probe exactly, so this sort of error is quite common. With devices
other than AquaProbe, working under any degree of pressure in the line, inserting a probe to within 10 mm
of its intended location is often accepted. Using the calculation above, this produces an error of
approximately 15 %. This can be reduced significantly by using the following method.
Referring to Fig. A.1, in the middle of the pipe, near the centre line, the profile is relatively flat, i.e. the flow
velocity does not change very much with distance into the pipe. Therefore, if the velocity is measured on
the centre line, measurement errors due to positional errors (i.e. not locating the probe where required) are
very small; hence most users will try to use the centre line measuring position. However, as explained
previously, this process gives us the wrong answer, Fortunately there is a mathematical relationship
between the velocity at the centre line and the mean velocity within the pipe – the Profile Factor (Fp). The
value of Fp can be calculated by an equation (below) or obtained from a graph – see Fig. A.2.
Fp is calculated as follows:
= –
F 1

Where:
=
Y r
---------------------------------------
b

n
And:
=
n 1.66
And:
R
e
8
0.875
0.870
0.865
0.860
0.855
0.850
200
Fig. A.2 Profile Factor v Flow Velocity for Pipe Sizes 200 to 2000mm (8 to 78 in.)
28
1
-- -
n
 
–
r Y
b
------------------ -
r
n
2
2n
 2n  
+ +
1 1
 
log R
e
D
---------- -
=

16 24 32
400 600 800
D = Pipe Diameter

= Fluid Density
v = Average Fluid Velocity

= Fluid Viscosity
Pipe Bore in inches
40 48 56
1400
1000 1200
Pipe Bore in mm
64 72
80
1600 1800 2000
OI/FEA100/200–EN Rev. D