23
ECNO: 622
7. Q & A
7.11 What is the approximate accuracy of measurement ?
Specifications:
Formerly, the expression
K K
% of full scale was often used. But, in the recent age of digital
system, it is more frequently expressed in % of the displayed value. Under the condition of low
flow velocity, the absolute value of error is used as a standard of accuracy in consideration of the
threshold of device performance.
7.12 What about error factors ?
On PORTAFLOW X, ultrasonic waves are emitted from the outside of the piping and the time is
measured while the waves are passing through the piping material - fluid - piping material.
Therefore, the flow coefficient is determined according to the piping material, size and the angle of
propagation of sound waves. As mentioned on the previous page, the following points become the
error factors to be considered when evaluating the measured values.
(1)
Piping size
When the value set for piping size is different from the actual size of piping, and if the difference
from the inside diameter is about 1% in size, the error is about 3% of deviation obtained by flow
conversion.
(The following shows an example of 1mm deviation in inside diameter)
Flow velocity
Inside diameter
*1: Example of calculation
Error at 2m/s
?
/
±
0.03
×
100/2 =
±
1.5%
Error at 1m/s
?
/
±
0.03
×
100/1 =
±
3.0%
φ
13 to
φ
50 or less
φ
50 to
φ
300 or less
φ
300 to
φ
6000
±
1.5% of measured flow
±
0.03 m/s*
1
±
1.0% of measured flow
±
0.02 m/s
±
l.0% of measured flow
±
0.01 m/s
2 to 32 m/s
0 to 2 m/s
2 to 32 m/s
0 to 2 m/s
1 to 32 m/s
0 to 1 m/s
Accuracy
100
6
–6
–2
0
–4
4
2
400 500
[%]
300
mm
φ
+1mm
–1mm
200
Inside diameter
Flow error
